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Critical Zone Science Comes of Age

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Studying the Critical Zone

Critical Agents of Change at Earth’s Surface

Life teems below the surface, linking critical zone water storage and ecosystems, soil signals tell of landscape disturbances, demystifying critical zone science to make it more inclusive, next steps for the critical zone.

It’s time for a critical zone coming out party.

The critical zone—Earth’s thin living skin, which spans from the top of the canopy to the bottom of the groundwater—is still unfamiliar to many scientists.

More than a concept, it’s an interdisciplinary endeavor that draws on multiple fields to piece together the complicated interactions between water, air, life, rock, and soil that support terrestrial life. By understanding and modeling these relationships and how they evolve, scientists can predict how human activities threaten these necessary, life-sustaining systems.

Critical zone science began almost 2 decades ago and developed in the United States through a network of observatories funded by the National Science Foundation (NSF). Now, as a new generation is applying critical zone science to novel questions and sites, the discipline has finally come of age.

The Birth of a New Field

Gail Ashley , a professor emerita at Rutgers University, first coined the term critical zone in a prophetically titled 1998 abstract for the Geological Society of America meeting, “ Where Are We Headed ?” “I called it the critical zone because it’s critical for life,” she said. “Also, it’s critical to know more about it because of the potential for damaging it.”

A sedimentary geologist, Ashley spent most of her career reconstructing the ancient paleo–critical zone of Olduvai Gorge in Tanzania, one of the richest paleoanthropology sites in the world. Her team uncovered evidence of wetlands and freshwater springs that likely supported hominins living at the site more than a million years ago .

The concept of critical zone science gained traction through a 2001 National Academies report , to which Ashley contributed, that recommended the creation of natural observatories for studying the critical zone.

Susan Brantley , a geochemist at Pennsylvania State University, was a driving force in developing critical zone research in the United States. “In the early 2000s, hydrology was funded separately from geochemistry, was funded separately from geomorphology,” said Brantley. “A lot of us were starting to talk about how we…should be looking at the Earth’s surface as one thing and bringing all those subdisciplines together.” In 2003, Brantley led a team that hosted workshops, wrote white papers, and held town halls at national and international meetings to recruit Earth scientists interested in how biological, physical, and chemical processes shape Earth’s surface.

Brantley and her colleagues proposed various sites for laboratories dedicated to the study of the critical zone, and in 2006, NSF put out the call for interdisciplinary watershed-scale projects that would “advance our understanding of the integration and coupling of Earth surface processes as mediated by the presence and flux of fresh water.” The Critical Zone Observatories (CZOs), the first long-term sites supported by NSF’s Directorate for Geosciences, were born.

Data: The Language of Science

In 2007, NSF awarded 5-year grants to establish three CZOs, all, coincidentally, in forest watersheds. Brantley was a co–principal investigator at the Susquehanna Shale Hills site, an eastern forest on weathering shale bedrock in central Pennsylvania. Sites in California’s southern Sierra Nevada and one in the Colorado Rockies also received funding. The American Recovery and Reinvestment Act of 2009 provided funding for three more observatories, with a wider variety of river basins. Finally, in 2013, the number of CZOs grew to nine, with the closure of one site and four additions. Long-term funding was critical for understanding how the critical zone functions normally and how it responds to disturbances, like fires or floods .

It took a few years for collaborators from different fields to understand each other’s science enough that they could help each other out, “and then we really began to be able to do truly interdisciplinary science.”

“One of the challenges early on was just finding a vocabulary,” said Timothy White , a geologist at Penn State University and the national office director of the CZOs. It took a few years, he said, for collaborators from different fields to understand each other’s science enough that they could help each other out, “and then we really began to be able to do truly interdisciplinary science.”

“From the beginning, it was also about data, how to collect all of these data,” said NSF program director Enriqueta Barrera , who nurtured the program from its infancy.

Ultimately, data and new technologies became common tongues in critical zone science. Advanced techniques in isotope geochemistry helped scientists track the movement of elements through the critical zone, estimate the size of fluxes, and calculate the speed of chemical processes on timescales ranging from minutes to millennia. Through remote sensors, researchers monitored weather, soil moisture and temperature, sap flow, and snow depths from the comfort of their labs. Using lidar, they mapped changes to the landscape from planes. The bounty of data were integrated into numerous geochemical, hydrological, and climate models.

Other countries began to replicate the CZO system. In collaboration with CZO scientists, researchers have established observatories around the world, on every continent except Antarctica. In the United States, the Department of Energy modeled some long-term monitoring sites after CZOs, and many critical zone scientists worked independent of the observatories.

How Far Can You Push the Critical Zone?

Since the field’s inception, critical zone scientists have shown how altering Earth’s surface and vegetation leads to subsurface impacts—changing water flow, minerals, and microbial life at surprising depths. Subsurface imaging and drilling have also shown considerable belowground variation and the ways in which that alteration influences surface conditions. These discoveries opened the door to more applied research and better land management practices.

For example, critical zone studies in California let researchers predict how much deep water, stored in pores in weathered bedrock, is available to forests during droughts. Scientists at the Southern Sierra and Eel River CZOs reported the amounts of root-accessible moisture available from year to year , which sustained plant life during the severe 2011–2015 drought . Nearly 150 million trees succumbed to the drought, mainly in the southern Sierra Nevada. These die-offs were a direct result of the gradual depletion of the root-accessible moisture by overstocked forests—caused by long-term fire suppression—according to research by Roger Bales , a hydrologist at the University of California, Merced, and colleagues at the Southern Sierra CZO.

Now Bales is making recommendations to the U.S. Forest Service and other land managers based on this work. “We can provide metrics that tell you where the forest is likely to suffer drought stress,” said Bales, “based not just on precipitation and temperature, which is what traditional drought indices do, but also factoring in over-year storage provided by long-term bedrock weathering.”

In the Midwest, at the trio of agricultural sites in the Intensively Managed Landscapes CZO in Illinois, Iowa, and Minnesota, researchers showed that industrial farming has changed the prairie landscape from one of transformation into one dominated by transport . The prairie ecosystem that evolved after glaciers from the last ice age receded functioned primarily by cycling nutrients in place and holding water in soil and groundwater (transformation). But with industrial farming, added fertilizer is partially taken up and harvested as grain, and the rest is transported away by water through altered surface pathways and subsurface drainage, ultimately fueling algal blooms in the Gulf of Mexico.

“There is a direct link between what we are finding from how these critical zones function and what we can do better in terms of ecosystem services,” said Praveen Kumar , a hydrologist at the University of Illinois and head of the Intensively Managed Landscapes CZO .

Growing the Critical Zone Community

A crucial outcome from the development of the CZOs is a new generation of Earth scientists trained to think within a multidisciplinary framework. As of 2018, the CZOs had trained more than 300 graduate students and more than 100 postdocs. At least 40 have gone on to become assistant professors.

Critical zone science is “this ability to get people to bring in different data sets and different mindsets and perspectives to really challenge the data about what we’re seeing.”

One former trainee is Pamela Sullivan , an ecohydrologist at Oregon State University. As a postdoc at the Shale Hills CZO, she used monitoring wells to study a series of weathering processes that occurred as water flowed across the small catchment. Now she studies how factors like plant roots, bedrock fracture networks, or a shift from snow to rain alter soil structure and subsurface water flow, to better inform climate models. Working in a multidisciplinary team at the CZO was a “transformative experience,” said Sullivan. “It’s this ability to get people to bring in different data sets and different mindsets and perspectives to really challenge the data about what we’re seeing.”

Sullivan acknowledges, however, that some have felt left out of critical zone science because they were not affiliated with an observatory. To encourage collaboration, she serves as a co–principal investigator on a Critical Zone Research Coordination Network focused on synthesizing data and models to investigate the evolution of the critical zone and how it will respond to future human impacts . The group facilitates networking to advance research within the community.

As critical zone science expands, researchers say the field has finally matured, although “it’s not an exaggeration to think of the critical zone as a new science,” said William Dietrich , head of the Eel River CZO. “Once you see the thing, you can’t walk away from it…. I think the critical zone is here to stay.”

The End of the Observatory Era?

Despite, or perhaps because of, the success of the CZO program, NSF will conclude its funding of the program in 2021. NSF program director Richard Yuretich is spearheading a new program that focuses on testing principles across a cluster of sites instead of comprehensive studies at a single observatory. Ideally, the data—aided by high-performance computing—will be integrated into comprehensive models of critical zone processes across diverse environments. The Consortium of Universities for the Advancement of Hydrologic Science Inc. (CUAHSI), a nonprofit that supports interdisciplinary water science, will serve as the critical zone hub. CUAHSI and collaborators will coordinate activities among the clusters, support training and synthesis activities, and provide cyberinfrastructure for collaborative modeling and for sharing and archiving data.

The shift was unwelcome news to some researchers. “I think it’s going to be a great opportunity for the younger generation,” said White, “but I think it was a shame that the literally tens of millions of dollars that were invested into the observatory network that we have is mostly being cast aside.”

NSF has funded nine critical zone network clusters, which began in September 2020. Many clusters include locations at existing CZOs or long-term field sites with legacy data, but are led by a new wave of early-career scientists. “We have a whole new generation of scientific researchers who now have this in their DNA,” said Yuretich. “The new thematic cluster and coordinating hub network is designed to capitalize on these individuals and bring them into the forefront of designing new research in the critical zone.”

Now free from the constraints of the observatories, scientists can ask questions that transcend location. Teams are investigating how the coastal critical zone responds to rising sea levels and saltwater intrusion, what big data can reveal about how the critical zone recovers from disturbances, the limits of human interference, and the many functions of dust as it travels from its source in the Rocky Mountains to the deserts of southern Nevada.

Forecasting the Future of Critical Zone Science

The new program will also help to deepen understanding of critical zone function beyond alpine watersheds. By graduating to a wider variety of environments, the ultimate goal is for researchers to construct more comprehensive models of critical zone processes worldwide.

One new critical zone network team will focus its operations in the Chihuahuan Desert, which receives about 20 centimeters of water each year, mostly during the July monsoon season. Dryland sites like this have been underrepresented in previous critical zone work. Lixin Jin , a geochemist at the University of Texas at El Paso and a former postdoc at the Shale Hills site, leads the project. She is fascinated by the desert environment, despite growing up in frigid Jilin Province, the Minnesota of China. What she discovers with her colleagues, including co–principal investigator and fellow University of Texas at El Paso geochemist Lin Ma , may have important implications for the future of the Chihuahuan and other dryland environments. These cover about 40% of Earth’s landmass and support more than 2 billion people.

Unlike tropical or temperate environments, where precipitation leaches minerals from rocks and soil, limited water in the desert concentrates minerals into a cement-like layer called caliche . This water-impermeable layer builds up over thousands of years. “If you poke it with your finger, it’s as hard as rock,” said Jin. “We’re trying to understand how this unique architecture will govern how the ecosystem and water function in the critical zone.”

To see whether precipitation reaches the subsurface, the team will drill wells more than 100 meters deep to access El Paso’s freshwater source. Despite multiple hydrology studies, the city does not know the size of its reserves or whether surface water is refilling the aquifer. “To be honest, we don’t know what happens to the loose water,” said Ma. “But some of the water may make it all the way down.”

For comparison, the team is looking at younger soils along the Rio Grande, where farmers grow pecans, chilis, and alfalfa. Irrigation from the river carries a lot of minerals, and in less than a century of agriculture, farmlands have already been shown to accumulate salt, which stresses crops. Critical zone researchers will examine how irrigation alters the fate of surface water—whether it enters plants, the atmosphere, or the subsurface—and whether irrigation contaminates drinkable groundwater by mobilizing salt, nutrients, and trace metals.

Jin and her collaborators are also excited to bring a dryland critical zone curriculum to local classrooms and to recruit students from the local Latino population. “Especially in the geology field, or Earth science in general, I just don’t think we have good representation of the minority groups,” said Jin. “We really want to prepare and train the next generation of scientists.”

The ultimate goal of this and other critical zone work is to develop models that transcend the boundaries of individual research sites so we can “Earthcast,” or predict how Earth will respond to a changing climate and more intensive land use.

“We have forecast the weather,” said Jin. “But can we Earthcast the critical zone?”

—Patricia Waldron ( @patriciawaldron ), Science Writer

Waldron, P. (2020), Critical zone science comes of age, Eos, 101 , https://doi.org/10.1029/2020EO148734 . Published on 24 September 2020.

Text © 2020. The authors. CC BY-NC-ND 3.0 Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.

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Critical Zone Research and Observatories: Current Status and Future Perspectives

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Li Guo , Henry Lin; Critical Zone Research and Observatories: Current Status and Future Perspectives. Vadose Zone Journal 2016;; 15 (9): vzj2016.06.0050. doi: https://doi.org/10.2136/vzj2016.06.0050

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The Critical Zone (CZ) is the thin layer of the Earth’s terrestrial surface and near-surface environment that ranges from the top of the vegetation canopy to the bottom of the weathering zone and plays fundamental roles in sustaining life and humanity. The past few years have seen a number of Critical Zone Observatories (CZOs) being developed following the first CZOs established in the United States in 2007. This update summarizes major research findings in CZ science achieved in the past 5 yr or so (2011–2016), especially those obtained from recognized CZOs. A conceptual framework of “deep” science—deep time, deep depth, and deep coupling—is used to synthesize recent CZ studies across a broad range of spatial and temporal scales. This “deep” science concept emphasizes the integration of Earth surface processes that underlies the contributions of CZ science to terrestrial environmental research. We identify some main knowledge gaps and major opportunities to advance the frontiers of CZ science. We advocate that the CZ scientific community work toward a global network of CZOs to link sites, people, ideas, data, models, and tools. We hope that this update can stimulate continuous scientific advancement and practical applications of CZ science worldwide.

CZ, Critical Zone , CZO, Critical Zone Observatory , DOC, dissolved organic carbon , DOM, dissolved organic matter , EEMT, effective energy and mass transfer , ET, evapotranspiration , GPR, ground-penetrating radar , SOC, soil organic carbon , TDR, time-domain reflectometry , WTT, water transmit times

The Earth’s Critical Zone (CZ) is defined as the thin layer of the Earth’s surface and near-surface terrestrial environment from the top of the vegetation canopy (or atmosphere–vegetation interface) to the bottom of the weathering zone (or freshwater–bedrock interface) ( National Research Council, 2001 ). This zone encompasses the near-surface biosphere, the entire pedosphere, the surface and near-surface portion of the hydrosphere and the atmosphere, and the shallow lithosphere ( Lin, 2010 ). This concept of the CZ provides a unifying framework for integrating belowground–aboveground, abiotic–biotic, and time–space in mass and energy flows to holistically understand complex terrestrial ecosystems and offers a fertile ground for interdisciplinary research ( Anderson et al., 2007 ; Lin et al., 2011 ). Thus, the integrated study of the CZ has been recognized as one of the most compelling research fields in Earth and environmental sciences in the 21st century ( National Research Council, 2001 , 2011 ).

Environmental processes within the CZ, such as mass and energy exchange, soil formation, streamflow generation, and landscape evolution are crucial to sustaining biodiversity and humanity ( Lin et al., 2011 ; Field et al., 2015 ). The CZ supplies nearly every life-sustaining resource on which life originates, evolves, and thrives ( National Research Council, 2001 ; Lin, 2014 ). This zone provides diverse services to human society and determines human livelihood ( Lin, 2014 ; Field et al., 2015 ). Knowledge of how the CZ forms, functions, and supports humanity is an increasingly important issue raised by both the general public and the scientific community ( Banwart et al., 2013 ). However, with accelerated socioeconomic development, the CZ is under ever-increasing pressure from human perturbations, such as the rapid growth of human and livestock populations, land use intensification, global environmental changes, and expanding consumption patterns ( IPCC, 2013 ). The rapidly expanding needs for sustainable development give a special urgency to better understand, predict, and manage the complexity and dynamics within the CZ and its interactions with other environmental systems ( Lin, 2010 ; Banwart, 2011 ).

Investigating and understanding the CZ requires a synergistic approach across disciplines, including soil science, hydrology, biology, ecology, geology, geomorphology, geochemistry, geophysics, geobiology, and many others ( Anderson et al., 2008 ; Brantley et al., 2016 ). The spatiotemporal scales of the CZ processes range from the pore scale to the continental scale and from the geological past to the present and into the future ( Brantley et al., 2007 ; Lin, 2014 ).

Such interdisciplinary and multiscale study of terrestrial ecosystem processes may be best accomplished through co-located Critical Zone Observatories (CZOs) where multiple scientific communities study various aspects of the CZ that can lead to synthesized understanding of complex systems ( Anderson et al., 2008 ; Lin et al., 2011 ; Bernasconi, 2014 ; Brantley et al., 2016 ). It is the integration of Earth surface processes (such as landscape evolution, weathering, hydrology, geochemistry, and ecology) at multiple spatial and temporal scales and across anthropogenic gradients that is key to the concept of CZ science and CZOs. However, various conceptions of a CZO exist, and a more concretized concept of a CZO (e.g., how to separate a CZO from a long-term field site) is still an open issue in the scientific community.

In 2007, the first three national CZOs were established in the United States through funding from the US National Science Foundation. Through a decade of efforts, there are now 10 CZOs in the United States, spanning a wide range of climatic, ecologic, geologic, and anthropogenic environments ( Fig. 1 ). Similar efforts were pursued in Europe, including four Terrestrial Environmental Observatories (TERENO) established in Germany in 2008 and the European Union’s four CZOs launched by Soil Transformation in European Catchments (SoilTrEC) in 2009 ( Fig. 1 ). In China, a set of seven CZOs is being developed, among which five were co-funded by a Sino-UK joint program in 2015. In Australia, several CZOs are being established, especially in association with the Long-Term Ecological Research Network sites that are linked to the Terrestrial Ecosystem Research Network (TERN) created in 2009. Based on a summary from Banwart et al. (2013) , a total of 69 CZO-like sites have been registered worldwide that can help forge independently conceived CZOs or CZO-like sites into a global network ( Fig. 1 ).

Given the growing recognition of the significance of CZ research and observatories, the aim of this focus-topic update is to survey the latest CZ-related studies and highlight new accomplishments and emerging concepts in CZ science. We address the following two themes: (i) recent results and emerging concepts in CZ science under a conceptual framework of “deep” science, and (ii) knowledge gaps and major opportunities in advancing the frontiers of CZ science. We are hopeful that such an update can facilitate continuous discussions and stimulate further advancement in CZ science around the world.

Recent Results and Emerging Concepts in Critical Zone Science

The special section titled “Critical Zone Observatories” published in Vadose Zone Journal in 2011 was the first collection of studies on CZOs published in a scientific journal, marking the beginning of broader interests in CZ science. From 2011 to 2015, more than 200 CZ-related studies have been published in Vadose Zone Journal and other journals ( Fig. 2 ) based on the ISI Web of Knowledge. These studies comprise interdisciplinary efforts from soil science, hydrology, biogeochemistry, geomorphology, geophysics, ecology, and other fields, which have advanced our understanding, prediction, and management of the CZ. In the following, we highlight the main findings from these CZ-related studies.

Critical Zone science would be too dispersive and complicated to understand if we outline recent CZ results by each discipline involved. Instead, we advocate a framework of “deep” science to help organize and comprehend research done in CZ science with a more synergistic perspective ( Fig. 3 ). Three foci are included in this perceived “deep” science framework: deep time , deep depth , and deep coupling ( Fig. 3 ). This “deep” science concept highlights the essence of integrating Earth surface processes at multiple spatial and temporal scales and signifies the unique contributions of CZ science to environmental and ecological research.

Deep Time: An Ever-Evolving Critical Zone System

Processes within the CZ take place across temporal scales that ranges from multimillion-year time frames of tectonics to rapid transformation of short-term events like C flux and water cycling ( Brantley et al., 2007 ). Recent CZ studies have suggested that the present landscape has been shaped through geologic time frames, and such geologic history (as recorded in the CZ) can provide scientific evidence and decision support to help project future CZ changes ( Banwart et al., 2013 ).

Fast Cyclic Processes and Slow Cumulative Changes

The CZ processes may be grouped into (i) fast cyclic processes (e.g., diurnal fluctuation of soil temperature, seasonal changes in soil moisture, and yearly changes in vegetation growth) and (ii) long-term cumulative changes (such as bedrock weathering, pedogenesis, and ecosystem succession). The shorter and longer time scale CZ processes are intertwined, with interactions, feedbacks, thresholds, and cumulative effects ( Lin, 2011 ). For example, it takes hundreds of thousands or more years for bedrock to be transformed into soil. These slow cumulative weathering and pedogenic processes lead to specific soil structures that control water movement in the soil profile and the periodic changes in soil moisture. On the other hand, each pulse of water moving through the soil profile causes varying degrees of physical translocation of materials, chemical reactions, and/or biological responses, thus imprinting certain marks of change in the soil profile. The cumulative effect of small changes over long periods of time can and do cause noticeable and permanent changes in soil evolution and soil structure ( Lin, 2011 ).

The special section “Soil Architecture and Functions” in Vadose Zone Journal in 2012 emphasized the need to link soil architecture, a result of longer time scale processes, to shorter time scale soil physical, chemical, and biological functions ( de Jonge et al., 2012 ). Studies in this special section explored major biophysical parameters and drivers for soil architecture and soil functions (e.g., Hamamoto et al., 2012 ; Subedi et al., 2012 ) and integrated emerging methods and techniques for assessing soil functions and visualizing soil architecture and its evolution in time and space (e.g., Markgraf et al., 2012 ; Sammartino et al., 2012 ).

Based on multidisciplinary studies of a 150-yr soil chronosequence at the Damma Glacier CZO in Switzerland, Bernasconi et al. (2011) found that the rapid evolution of microbial and plant communities strongly affected longer term weathering rate and soil formation. Biological variables indicated that the ecosystem in this glacier CZO evolved from sandy soils with a few plants to an ecosystem with almost complete vegetation cover in less than 70 yr, and to soils with a clear structure within 100 yr. The higher biomass and biodiversity in the older soils lead to more extensive biologic activities and faster pedogenesis. At the Koiliaris CZO in Greece, Moraetis et al. (2015) investigated the impacts from longer term natural processes and shorter term human activities on sediment provenance and soil formation. They found that longer term sediment transport from outcrops and strong eolian input from upslope, together with shorter term land use changes (e.g., man-made terraces), induced various soil development processes. At the Shale Hills CZO in Pennsylvania, Liu and Lin (2015) found that soil-terrain attributes formed over geological time scales played a key role in controlling the dynamics of fast subsurface preferential flow. Shi et al. (2015) suggested that detailed information on longer time landscape evolution and soil formation, such as soil type and soil thickness, were key to modeling day-to-day variations in soil moisture patterns at the Shale Hills CZO.

Threshold Changes and Gradual Changes

Evolution of the CZ entails a combination of threshold changes and gradual changes. Threshold pedogenic processes have been noted across contrasting soils and climates ( Vitousek and Chadwick, 2013 ). Thresholds may be reached either through gradual changes when cumulative effects exceed a certain level (i.e., an internal or intrinsic threshold, such as element depletion) or due to rapid exogenous or endogenous change, when the rate is faster than the adaptive capacity of the system (i.e., an external or extrinsic threshold, such as human disturbance) ( Lin, 2011 ). For example, based on a paddy soil chronosequence consisting of five profiles with cultivation history from 0 to 1000 yr, Chen et al. (2011) found that anthropogenic activities caused rapid changes in soil organic C (SOC), Ca, Na, and Mg contents within 50 yr, exhibiting external thresholds, whereas the changes in horizon differentiation of Fe oxide and clay illuviation during the 300- to 700-yr time period indicated internal thresholds for carbonate leaching.

The application of the threshold concept in CZ studies can propel our understanding of CZ processes and evolution. Some efforts have been pursued to investigate the causes of abrupt, rapid, and irreversible changes (i.e., thresholds) in CZ properties. At the Luquillo CZO in Puerto Rico, Porder et al. (2015) tested competing hypotheses (pedogenic thresholds, i.e., spatially sharp transitions in soil properties, vs. landscape homogeneity) on two parent materials (rock types). They found that, whereas the landscape underlain by volcaniclastics did not exhibit a regional knickpoint, strong knickpoints were observed in streams underlain by quartz diorite. Their results suggested that regional uplift, lithology, weathering, atmospheric inputs, and forest communities together controlled the spatial threshold distribution in soil cation availability (i.e., a significant difference above vs. below a knickpoint). At the Wüstebach CZO in Germany, Wiekenkamp et al. (2016) compared the frequency of occurrence of subsurface preferential flow across the catchment after different precipitation events. They identified the existence of a spatially variable threshold of precipitation on preferential flow initiation. Such activation thresholds for preferential flow response to precipitation were also observed by Dusek and Vogel (2014) , who modeled subsurface lateral flow above a shallow semipermeable soil–bedrock interface. In a related modeling effort, Nimmo (2016) developed a process-based model to predict the occurrence of subsurface preferential flow in macropores by considering the spatial distribution of soil matrix infiltrability as a threshold. In this model, an elementary matrix area was used to represent the local infiltrability of the soil matrix material between macropores. After each elementary matrix area absorbed water up to its matrix infiltrability (i.e., the threshold value), excess water flowed into a macropore and initiated preferential flow.

Linking Historical Evolution to Current Dynamics and Future Scenarios

Knowledge of CZ structure and its evolution in time and space is central to understanding CZ processes and predicting CZ response to future changes in climate and land use. Some efforts have been made to predict soil formation rate from bedrock porosity, matric potential, permeability, and mineralogy; to predict landform evolution from initial conditions and climatic, tectonic, and lithological forcing; and to predict CZ services from land use and management variations ( Banwart et al., 2013 ). For example, Todorovic et al. (2014) predicted SOC stocks at the 0- to 20-cm soil depth at the Fuchsenbigl CZO in Austria by using an improved and calibrated RothC-26.3 model. The model results indicated that a change in land use from forest to grassland and cropping would result in a clear decrease in the amount and quality of SOC. At the Catalina-Jemez CZO in Arizona and New Mexico, Zapata-Rios et al. (2015) applied the effective energy and mass transfer (EEMT) concept to predict water transit times (WTT). Significant correlations were identified between EEMT and WTT, which suggested that basic climatic data embodied in EEMT could help predict hydrological and hydrochemical responses. Hunt (2015) established a scaling theory of solute transport on percolation clusters to predict the thickness of weathering rinds as a function of time. Given the general consistency of the relationship between weathering rind thickness and permeability, this study suggested the possibility of establishing specific models of weathering rind development according to rock type, grain size, permeability, and ambient conditions.

To support research at the Shale Hills CZO, a group of modules for simulating coupled processes in the CZ have been developed within the framework of the Penn State Integrated Hydrologic Model (PIHM) ( Duffy et al., 2014 ). Flux-PIHM, Flux-PIHM-BGC, PIHM-SED, and Regolith-RT-PIHM have been or are being developed to model water and energy fluxes, C and N fluxes, sediment transport, and reactive transport, respectively, across a wide range of temporal scales from minutes to millions of years ( Shi et al., 2014 ; Duffy et al., 2014 ). Given the drawback of current landscape evolution models that could not take into account realistic groundwater and overland flow and channel–hillslope interactions, Zhang et al. (2016) integrated hydrologic processes with hillslope and channel sediment transport processes to establish a new hydrologic-morphodynamic model, called the 3-D LE-PIHM. Their study ( Zhang et al., 2016 ) tested the performance of LE-PIHM in modeling weathering and landscape evolution. Based on an ~2 × 10 4 –yr simulation, results from LE-PIHM indicated the importance of coupling groundwater flow in landscape evolution modeling. With the improvements in such an array of models, it should become possible to project the CZ under future scenarios of climate and land use.

Deep Depth: A Unified Critical Zone System

The concept of the CZ extends the traditional perception of soils (i.e., 1–2 m deep with a focus on the root zone, as emphasized in ecosystem and agronomic studies) to often much deeper weathered bedrock. This vertical integration of Earth surface components provides a more holistic view of terrestrial physical, chemical, and biological processes and offers a long-term understanding of CZ services to human society. Efforts are underway to understand the linkages between the rhizosphere and the deeper subsurface (lithosphere) and between aboveground dynamics and belowground processes (e.g., Naithani et al., 2013 ; Hahm et al., 2014 ; Stone et al., 2014 , 2015 ; Gaines et al., 2015 , 2016 ). In the meantime, it has been recognized that upscaling from field plot measurements to hillslope, catchment, and watershed scales is crucial to understanding CZ processes. In general, recent CZ studies have suggested viewing the surface, shallow, and deep subsurface as a unified system to more fully understand the processes, structures, and functions of the CZ across different spatial scales.

Linking Shallow Root Zone Soil to Deep Weathered Bedrock

As highlighted by Graham et al. (2010) and Brantley (2010) , chemical, physical, and biological processes combine to transform fresh bedrock (granite) surrounded by only a few pores or fractures into saprock and eventually weathered rock and then soils surrounding small, isolated rock fragments. Linking parent material to soil properties, soil microbial communities, and vegetation distributions is an active research topic in CZ science. For example, at the Luquillo CZO, Stone et al. (2015) found differences in the dominant bacterial community structure with depth for two contrasting parent materials and two forest types within the upper 1.4 m of soil profiles. That study indicated that in tropical forests, bacterial communities exhibit the capacity to perform N-cycle transformations in deeper parts of the soil profile. At the Southern Sierra CZO in California, Hahm et al. (2014) found that differences in forest cover could be explained by the varying geochemical composition of the underlying bedrock. That study indicated that, in addition to climatic regulation, bottom-up lithologic control could play a key role in the distribution and diversity of vegetation in granitic mountain ranges. At the Eel CZO in northern California, Salve et al. (2012) found that the weathered bedrock zone created a hydrologically active domain to conduct substantial rainfall penetrating to depth and recharge a groundwater table perched on the underlying fresh bedrock. During the summer dry season, the shallow weathered bedrock zone dried much more slowly than the soil layer, which would influence water delivery to depth and water availability to vegetation. In addition to soil water storage, the weathered bedrock zone was an essentially unmapped moisture reservoir at the study site.

The application and advancement in geophysical techniques have enhanced our ability to image and investigate the deep subsurface and promote CZ science in linking deep weathering processes to shallow subsurface and surface processes. Parsekian et al. (2015) reviewed geophysical methods for examining CZ form and function across multiple spatial scales, from centimeters to kilometers. Their study identified three advantages of geophysical measurements as a necessary complement to direct observations obtained by drilling or field sampling: (i) noninvasive imaging of the geometry of structural features in the space between direct measurements; (ii) repeated data acquisition in real time and with time in the field, and (iii) relatively larger spatial coverage, beyond what direct measurements can reveal, that can be compared with data sets from remote sensing techniques.

We have selected several studies to illustrate the strength of geophysical methods in advancing CZ science. St. Clair et al. (2015) applied seismic velocity and electrical resistivity surveys in three landscapes within the US CZO network (Boulder Creek, Calhoun, and Christina CZOs) and found that the pattern of bedrock fracture distribution as it neared the surface could not be easily explained by climate but probably depended on topographic stresses. Their study demonstrated the utility of geophysical methods to challenge traditional understanding of bedrock disaggregation, groundwater flow, chemical weathering, and the depth of the CZ ( Anderson, 2015 ). At the Southern Sierra CZO, Holbrook et al. (2014) used seismic refraction and resistivity imaging to detect variations in soil thickness and porosity across the catchment. Their results suggested that saprolite is a crucial reservoir for water storage, where the residence times of soil in the Southern Sierra are on the order of 10 5 years, indicating possible integrations of weathering over glacial–interglacial fluctuations. At the Shale Hills CZO, Guo et al. (2014) combined time-lapse ground-penetrating radar (GPR) surveys and artificial infiltration to map the pathways of subsurface lateral flow for the upper 1.4 m of soil profiles. They reported that enhanced radar data post-processing procedures provided a means to reconstruct the subsurface flow network and its dynamics with time. Zhang et al. (2014) compared the seasonal changes in GPR signals in two contrasting soils at the Shale Hills CZO. They found that repeated GPR surveys could identify the impacts of soil layering on water flow and water distribution in soils.

Linking Aboveground Systems to Belowground Systems

Increasing efforts have arisen in the CZ science community to link aboveground and belowground systems and to improve our understanding of how the surface and the deep subsurface are connected. These connections include: (i) water flux through the soil–plant–atmosphere continuum (SPAC); (ii) the influence of precipitation partitioning by canopy interception on subsurface preferential flow occurrence; (iii) interaction between vegetation distribution and soil moisture spatial pattern; and (iv) surface and subsurface flow networks at the hillslope and catchment scales ( Li et al., 2012 ).

The special section titled “Soil–Plant–Atmosphere Continuum” in Vadose Zone Journal in 2012 reported advances in the measurement and understanding of soil–water, soil–plant, and plant–atmosphere interfacial processes. A series of interdisciplinary studies were included in this special section that linked aboveground and belowground processes, including the new methods of quantifying and modeling soil–plant interactions from a single plant to the field scale (e.g., Zarebanadkouki et al., 2012 ; Assouline et al., 2012 ), the impact of canopy interception and root uptake on the variability of soil moisture in space and time (e.g., Moradi et al., 2012 ; Guswa, 2012 ), modeling water flux through the SPAC (e.g., Romano et al., 2012 ; Schröder et al., 2012 ), and the improved assessment of soil moisture and land–atmosphere interactions (e.g., Scanlon and Kustas, 2012 ). Although the synthesis of knowledge from various compartments of the SPAC into a holistic view remains challenging, these studies brought a new perspective that the whole CZ could function as a complex system.

The Shale Hills CZO, covered by temperate forest and underbrush, provides an ideal experimental site to study the interaction between aboveground ecohydrological and belowground hydropedological processes. Naithani et al. (2013) combined data sets of soil water content measurements (using time-domain reflectometry [TDR]), ground-based and satellite-sensed leaf area index, the spatial distribution of the dominant tree species, and high-resolution 0.5- by 0.5-m digital elevation model to evaluate the impacts of canopy growth on soil water content. They found that the spatial patterns of vegetation and soil moisture become increasingly homogenized and coupled from leaf onset to maturity but heterogeneous and uncoupled from leaf maturity to senescence. Based on multiple lines of evidence, including stable isotope natural abundance, sap flux, and soil moisture depletion patterns with depth, Gaines et al. (2015) inferred that root water uptake mainly occurred in the shallow (less than ~60 cm) soils throughout the Shale Hills CZO, even during the dry period of the growing season. Gaines et al. (2016) used a deuterium tracer, sap flux, and anatomical techniques to study tree water transport on a ridgetop at the Shale Hills CZO. They found that the soil-to-leaf driving force for water transport may control tracer velocity across tree species. Kraepiel et al. (2015) and Meek et al. (2016) conducted a multi-tracer study at the Shale Hills CZO to study the source and cycling of nutrients and metals among atmosphere, bedrock, soil, stream, groundwater, and vegetation. Their results indicated a strong interaction of slower weathering processes with rapid, biologically driven cycling between soils and vegetation.

Holistic View of the Critical Zone as a Unified System

In addition to the vertical integration of CZ processes from the rhizosphere down to the weathered bedrock and up to the aboveground canopy, the concept of deep depth also views the whole CZ as a unified system and extends field measurements to a larger spatial scale, such as catchment and watershed scales. The special section titled “HOBE: A Hydrological Observatory” in Vadose Zone Journal in 2011 reported findings from the Skjern catchment in Denmark and recognized the need to understand CZ processes at the catchment scale ( Jensen and Illangasekare, 2011 ). Studies in this special section reported on hydrological processes at different spatial scales, including regional estimates of precipitation from Doppler radar and its impact on the catchment-scale modeling of hydrological processes (e.g., He et al., 2011 ; Fu et al., 2011 ), measurement of evapotranspiration (ET) and greenhouse gas emissions and their dynamics and underlying controls at the catchment scale (e.g., Ringgaard et al., 2011 ; Herbst et al., 2011 ), and the impacts of climatic changes on hydrological processes at the regional scale (e.g., van Roosmalen et al., 2011 ).

High-density instrument arrays installed in CZOs can provide time series measurements that cover a whole catchment and help study CZ dynamics in a more comprehensive manner. For example, recent advances in sensor networks have created new opportunities to study soil water, soil temperature, soil matric potential, and other CZ variables with high temporal resolution and at the catchment scale. In the following, we use soil water content and soil matric potential as examples to illustrate the efforts of the CZ scientific community working toward a holistic view of CZ processes at the catchment and larger spatial scales.

Soil water content is a key variable in many processes within the CZ. The characterization of spatial soil water patterns and their temporal evolution is a cornerstone of our understanding of hydrological fluxes, flow pathways, and their impacts on biogeochemical dynamics. Using a 14-mo data set collected from 240 sensors across a 1.44-km 2 catchment at the Schäfertal CZO in Germany, Martini et al. (2015) characterized the spatial pattern of soil water content and inferred the hydrological processes that probably controlled the formation and dynamics of such a pattern. The results of that study indicated that the spatial variability of soil water content decreased with soil depth and that a variety of hydrological processes occurred at different times and at different topographic positions and soil horizons, thus the topsoil’s wetness might not necessarily reflect the hydrological processes occurring within the soil profile. This study pointed out the difficulty of upscaling soil water content from point measurement to the catchment scale. At the Rollesbroich CZO in Germany, Qu et al. (2014) investigated the spatiotemporal variability of soil water content patterns using a 2-yr time series measured at 41 locations across the 0.3-km 2 catchment. Temporally stable characteristics were found in the spatial variability of soil water content, which was correlated with the spatial variation in hydraulic properties. Webb et al. (2015) used a 3-yr data set collected from 97 sensors across 27 locations at the Southern Sierra CZO to investigate the wetting and drying dynamics of the shallow subsurface during snowmelt. Their study found a high variability of wetting and drying dynamics within the top 1 m of the soil in this mixed-conifer forest at the sub-hillslope and catchment scales, which could be influenced by subsurface lateral flows developed in the shallow soil or along the soil–bedrock interface after snowmelt. Given the cost of developing and maintaining a soil moisture sensor network, Schröter et al. (2015) used a terrain-based fuzzy c -means sampling and estimation approach at the Schäfertal CZO to identify representative measurement locations by stratifying the landscape into unique combinations of topographic attribute values. Their study demonstrated the effectiveness of the point-scale measurements in revealing the spatiotemporal pattern of soil moisture in a relatively small catchment.

While the spatiotemporal characteristics of soil moisture content have been extensively investigated, a limited number of studies have attempted to understand the spatiotemporal organization of the soil matric potential, especially at the catchment scale. Based on a 5.5-yr data set collected from 62 sites by standard nested tensiometers across the Shale Hills CZO, Yu et al. (2015) investigated the spatial variability and temporal stability of the soil matric potential at multiple depths (10–100 cm). They found that the spatiotemporal structure of the soil matric potential in the catchment was controlled by soil type, topography, season, and soil depth. Their results indicated that the downward parabolic trend in the spatial variability of the soil matric potential occurred when the spatial mean value of the soil matric potential decreased (i.e., became drier) across all soil depths.

In addition to the catchment or watershed scale, some studies have examined the spatiotemporal pattern of soil water content at the regional and even national scale. For example, based on the soil water content data obtained from the Soil Climate Analysis Network (SCAN), Wang and Franz (2015) assessed different climatic and soil controls on the soil water content spatial variability in Utah and the US Southeast. Their results suggested that instead of being controlled by climatic variables at the regional scale (~10 5 km 2 ), as traditionally believed, the spatial variability of soil water content in the study areas was strongly dependent on soil hydraulic properties. This regional-scale study also provided important implications for verifying remotely sensed soil moisture data (e.g., the Soil Moisture Active Passive mission [SMAP]) and initializing and parameterizing regional land surface and climate models. During 2009 to 2011, the US Climate Reference Network (USCRN) deployed Hydraprobe sensors to monitor soil water content across the United States. Bell et al. (2015) used the soil water content measurements from 114 USCRN sites at all observation depths to evaluate the 2012 drought. They detected an overall 11.07% decrease in soil water content from the average of the 2011 to 2013 summers. Their study demonstrated the utility of the USCRN for monitoring national soil water conditions, evaluating droughts, and tracking climate change with time. Coopersmith et al. (2016) quantified the random error of soil water content measurements from the USCRN. Based on the data from all of the ground sites (each site with three Hydraprobe sensors collocated), their results indicated that the average random error of Hydraprobe measurements was 0.012 m 3 /m 3 . The random error magnitudes of three repeated sensors at each ground site were related to precipitation patterns.

Deep Coupling: A Complex Critical Zone System

The integrated concept of the CZ reflects a recognition that the interactions and feedbacks among geologic, pedologic, hydrologic, chemical, biologic, atmospheric, and anthropogenic processes are coupled, especially those mediated by the flux of freshwater. Such close interplays among CZ processes require a synergistic approach from multidisciplinary and interdisciplinary perspectives to fully understand the co-evolution of CZ structures and functions.

Coupling Biogeochemical and Hydropedological Processes

A wide range of physical, chemical, and biological processes interact within the CZ. This diversity of interactions is central to understanding CZ evolution and function. Among various couplings, biogeochemical and hydropedological processes are ubiquitous across CZOs. The special section titled “Critical Zone Observatories” in Vadose Zone Journal in 2011 reported the initial results from the first several CZOs, which attempted to couple hydrology, biogeochemistry, and their interplay in the CZ ( Lin et al., 2011 ). For example, at the Boulder Creek CZO in Colorado, Dethier and Bove (2011) suggested that clay and Fe-oxide mineralogy could be used to distinguish alterations caused by weathering from that produced by hydrothermal activity. Chorover et al. (2011) developed a conceptual framework for quantifying the EEMT and found that the coupled C and water fluxes controlled the evolution of the CZ at the Catalina-Jemez CZO. At the Shale Hills CZO, Jin et al. (2011) showed that Mg concentration dynamics were controlled by the kinetics of clay mineral dissolution and soil exchange capacity, whereas changes in water isotopologues were strongly related to water residence times. They also found that rock weathering mechanisms and hydrochemical measurements could be used to infer hydrologic processes. Also at the Shale Hills CZO, Andrews et al. (2011) compared SOC storage and soil pore water dissolved organic C (DOC) across various landform units and reported that the higher SOC storage and soil pore water DOC concentration in swales were probably related to active subsurface hydrological processes in the swales because they were transport-driven hot spots of organic C in the studied catchment. These examples illustrate the importance of coupled biogeochemical and hydropedological processes in CZOs.

The special section titled “Frontiers of Hydropedology in Vadose Zone Research” in Vadose Zone Journal in 2013 investigated the two-way interactions between soil architecture and water movement ( Vogel et al., 2013 ). This special section highlighted hydropedology as a new scientific approach that emphasizes the central role of water in a variety of soil processes, including solute transport and various biogeochemical processes ( Vogel et al., 2013 ). For example, the spatiotemporal investigation of high-resolution sampling of stable isotopes (δO and δD) suggested the prevalence and mechanisms of preferential flow at the Shale Hills CZO ( Thomas et al., 2013 ). That study suggested the potential of using soil water isotopes to connect biogeochemical and hydropedological processes. Hardie et al. (2013) examined the dependence of subsurface flow patterns on the soil structure for soils of contrasting textures in Tasmania, Australia, and discussed the possible impacts of preferential flow on the eluviation of clay and chemical reduction in shallow soils. Arnold et al. (2013) examined fundamental hydropedological and ecohydrological relationships in natural Brigalow ecosystems of eastern Australia to support the rehabilitation of disturbed semiarid environments by promoting the development of native plants.

Dissolved organic matter (DOM) plays an important role in many biogeochemical and hydropedological processes that take place in the CZ. Variations in the quality or quantity of stream-water DOM can be used as a proxy of changes in land use or as guidance to catchment management. The special section titled “Dissolved Organic Matter in Soil” in Vadose Zone Journal in 2014 highlighted the role of DOM in C and nutrient cycling, pedogenesis, and microbial metabolism that link the CZ to aquatic systems ( Jansen et al., 2014 ). Studies in this special section improved the conceptual understanding of DOM dynamics in the CZ and aquatic systems by tracing changes in DOM concentration along its transport path from the soil to surface water (e.g., Roth et al., 2014 ; Vázquez-Ortega et al., 2014 ), explored the interactions of DOM with hydrology to build the linkage between the CZ and aquatic systems (e.g., Klotzbücher et al., 2014 ; Van Gaelen et al., 2014 ), and assessed the interrelations between DOM dynamics and human impacts and their possible changes in the future (e.g., De Troyer et al., 2014 ; Thangarajan et al., 2014 ). In addition to this special section, many DOM-related studies have been conducted across different CZOs. For example, Bol et al. (2015) used a 4-yr data set of weekly DOM features, pH, and Fe content sampled in the streams of the Wüstebach CZO to investigate the spatiotemporal variation of DOM. The positive correlation between DOC and Fe concentrations in the water sources studied indicated the transport of DOM via organo-mineral complexes. That study demonstrated the value of a long-term spatiotemporal data set covering DOM quality and quantity in both tributaries and main-stream water, as they apportion contributing sources and drivers of DOM dynamics in sub- and whole-catchment stream water. All these studies suggested the potential of DOM in coupling biogeochemical and hydropedological processes and promoting the frontier of CZ science.

Coupling Multidisciplinary and Interdisciplinary Techniques

Due to the closely intertwined CZ processes occurring across a wide range of spatial and temporal scales, multidisciplinary and interdisciplinary techniques and expertise are required for comprehensive understanding of CZ dynamics. Remote sensing techniques have provided many new ways to monitor and estimate various CZ attributes across multiple scales, which can enhance CZ science. The special section titled “Remote Sensing for Vadose Zone Hydrology” in Vadose Zone Journal in 2013 demonstrated the improved temporal and spatial quantification of soil water content, ET, soil hydraulic parameters, soil salinity, and vegetation dynamics. The techniques use optical, microwave, gravitational, infrared, and other sensors ( Mohanty et al., 2013 ). A total of 14 applications of remote sensing techniques to study the CZ were included in this special section, aiming to (i) improve the estimation, scaling, and data assimilation of soil water content and its variability by microwave remote sensing (e.g., Han et al., 2013 ; Chaouch et al., 2013 ), (ii) enhance the estimation of ET and vadose zone properties by multispectral satellite data (e.g., Landsat, Moderate Resolution Imaging Spectroradiometer [MODIS] and Advanced Very High Resolution Radiometer [AVHRR]) and microwave remote sensing (e.g., Gowda et al., 2013 ; Shin et al., 2013 ; Teluguntla et al., 2013 ), and (iii) ground truth soil water content to calibrate and validate microwave remote sensing retrievals (e.g., Dorigo et al., 2013 ). Findings in this special section suggested the potential of remote sensing techniques in CZ research, including the investigation of land–atmosphere interactions, hydrology, water resource management, and hazard assessment ( Mohanty et al., 2013 ).

Besides the studies included in this special section, Harpold et al. (2015) reviewed the application of lidar technology in obtaining precise three-dimensional information on the Earth’s surface characteristics and described the unique features of lidar data sets in advancing CZ research. For example, terrestrial laser scanning was used to reveal the important influence of alluvial and colluvial processes on vegetation and soil dynamics on semiarid hillslopes ( Harman et al., 2014 ). Without the high resolution and precision of lidar information on surface topography and canopy volume, it is very difficult to quantitatively investigate the co-evolution of microtopography and vegetation ( Harman et al., 2014 ). Lidar-derived canopy structure was used to test a CZ development model based on eco-pedo-geomorphic feedbacks and to identify the crucial role that ecological processes (e.g., vegetation density) have played in landscape evolution ( Pelletier and Perron, 2012 ). These exemplar CZ studies elucidated the potential of lidar to simultaneously quantify topographic, vegetative, hydrological, and other CZ processes ( Harpold et al., 2015 ). Harpold et al. (2015) also identified three areas where lidar technology can further the frontier of CZ science: (i) quantifying landscape adjustments through time, (ii) parameterizing and verifying CZ models, and (iii) improving the understanding of coupled CZ processes across multiple scales.

In addition to using single technologies to monitor CZ processes across multiple scales (e.g., lidar), increasing efforts are being made to integrate different techniques to study CZ dynamics. For example, at the Koiliaris CZO, Tsiknia et al. (2014) applied molecular biotechnology, soil physical, chemical, and biochemical analyses, and geostatistical mapping to investigate the underlying factors that shape the spatial distribution of dominant soil microorganisms in the watershed. At the Luquillo CZO, Stone et al. (2014) integrated soil chemical analyses, phospholipid fatty acid analyses, and enzyme assays to determine how extracellular enzyme activity changes as a function of soil depth. Moreover, some studies have compared the consistency of different technologies in measuring the same CZ variable. For example, at the Reynolds CZO in Idaho, Flerchinger and Seyfried (2014) compared several methods for quantifying ET during an 8-yr period, including eddy covariance systems with and without adjusting the turbulent fluxes to force energy balance closure, soil water storage loss measured by TDR and neutron probes, and simulation by the Simultaneous Heat and Water (SHAW) model. Their results suggested that, compared with other approaches, ET estimates from eddy covariance significantly underestimated seasonal ET unless the turbulent fluxes were adjusted to force the closure of the energy balance. In a 0.1-km 2 forest catchment, Lv et al. (2014) compared soil water content measured using a cosmic-ray neutron probe (CRNP) and an in situ time-domain transmissometry (TDT) sensor network from a total of 108 sensors. Additionally, the near-surface soil water content was simulated using a HYDRUS-1D numerical model. Then the simulated and TDT-measured soil water content were used to construct the depth-weighted mean areal soil water content for comparison with the CRNP measurements. During a period of 6 mo, the CRNP estimates exhibited a dry bias under relatively wet conditions at the beginning of the snow-free period. Using a combination of soil water content measurements and near-surface simulations, the CRNP output was recalibrated to capture the wetter conditions. This study indicated possible discrepant soil moisture content from different devices and calibration methods, which calls for attention to the needed standardization in methods, equipment, protocols, databases, and models to maximize the potential of a global network of CZOs. The studies mentioned above indicated the importance of advancing and integrating multidisciplinary and interdisciplinary techniques in achieving a more comprehensive understanding of the CZ. In addition to the combination of established observation methods, the interdisciplinary aspect also requires developing new concepts and methods that can study the CZ as a whole system to integrate results from multidisciplinary observation methods and to provide systematic understanding of CZ processes.

Coupling Services with Management

Although widespread recognition that CZ processes have important societal relevance, depletion of natural resources from the CZ is accelerating, and in many cases the CZ is managed without regard for the limits of supporting ecosystems. The intensified pressure on the CZ calls for sustainable management of the CZ ( Robinson et al., 2013 ). To facilitate the application of CZ science to sustainability practices, a new concept of CZ services was proposed by Banwart et al. (2012) and Field et al. (2015) . Banwart et al. (2012) conceptualized mass and energy flows that arise from CZ processes that supply goods and services for human benefit. Field et al. (2015) presented a CZ perspective to complement and enrich the current perspectives on ecosystem services in terms of context, constraints, and currency. Understanding ecosystem services within the context of CZ services requires enhanced efforts to address a grand challenge in integrating bio- and geosciences. The concept of CZ services integrates biological services with soil generation, landscape evolution, and water cycling, emphasizing geological processes and nonrenewable resources beyond the scale of a human life span, which extends the biologically focused perspectives of ecosystem services ( Field et al., 2015 ).

In essence, CZ services extend the context for ecosystem services in three important aspects: (i) explicitly addressing how the physical structure of the terrestrial surface (e.g., parent material, topography, and orography) provides a broader spatial and temporal template that determines the co-evolution of physical, chemical, and biological processes in ecosystems; (ii) emphasizing the rate-limiting processes of ecosystem services that are fundamentally constrained by CZ processes, such as soil formation, nutrient supply, hydrologic partitioning, and streamflow generation; and (iii) integrating CZ processes into an evaluation currency by quantifying the amount of EEMT (i.e., a measure of the energy flux available to do physical, chemical, and biological work within the CZ) ( Field et al., 2015 ). This proposed concept of CZ services builds a linkage between ecosystem services and the constraints thereon associated with CZ processes and thus provides a framework to enable more effective long-term management and valuation of ecosystem services in response to a changing climate and increasing human disturbances ( Banwart et al., 2012 ; Field et al., 2015 ).

Given the vital influence of the CZ to life and humanity, CZ management can no longer be based on a single function but instead should be coupled with multiple services that the CZ offers ( Robinson et al., 2013 ; Baveye et al., 2016 ). In particular, with the recognition that mankind has entered the new Anthropocene era in which human activities substantially impact Planet Earth, research outcomes from CZ science should provide decision support to evaluate different adaptive strategies for mitigating the impacts of climatic change and human disturbance so as to sustain CZ services ( Banwart et al., 2013 ; Montanarella and Panagos, 2015 ; Baveye et al., 2016 ). Montanarella and Panagos (2015) demonstrated how the CZ can be used as a new paradigm to contribute to environmental policy decisions, including climate change, water management, biodiversity protection, air quality, water quality, waste management, and agricultural policies. This study suggested that when policymakers make decisions on land use, water management, and agricultural practices, they should consider soil threats and soil functions and the trade-off between increasing food production vs. organic C loss or pollutant transformation. For example, at the Intensively Managed Landscapes CZO inn Illinois, Iowa, and Minnesota, Papanicolaou et al. (2015) compared the spatial variability of soil saturated hydraulic conductivity (a key soil property for CZ service that influences the amount of runoff, erosion, and stream-water quality) at three sites with different agricultural management practices. They found that different land use and management practices (i.e., conventional tillage, no-till, and Conservation Reserve Program) triggered statistically significant differences in the soil saturated hydraulic conductivity. Their results can provide decision support to policymakers to consider the balance between agricultural activity and CZ services. Future efforts are required to translate new CZ science knowledge into policy-relevant data and information to support policy and management decisions related to CZ sustainability.

Challenges and Opportunities in Critical Zone Science

Global network of observatories and cross-observatory collaborations.

The past few years have seen a global network of CZOs beginning to take shape, offering a potentially fertile ground to extend the breadth and depth of interdisciplinary CZ science ( Fig. 1 ). Up to this point, however, most CZ studies have focused on the outcomes from a single CZO, with a limited number of studies comparing several different CZOs. No study has been done to identify common catchment behaviors across CZOs, albeit that each CZO has its unique features. Given the growing interest in CZOs worldwide, a real challenge and opportunity for the CZ scientific community is to synergize outcomes from multiple CZOs and to synthesize data or knowledge via the network of CZOs to reveal the underlying controls on CZ structure, evolution, processes, and services.

This synergistic effort calls for establishing CZO standards to maximize the potential of a global array of CZOs. As the results reported by Flerchinger and Seyfried (2014) and Lv et al. (2014) showed, different measuring devices and calibration methods would probably lead to discrepant values of the same variables measured, which will hamper comparability across CZOs. Thus, more efforts are needed to standardize the protocols of CZO investigations in terms of observation variables and processes, measurement equipment and methods, database organization and analysis, and model development and intercomparison. In addition to unique features and site-specific processes in individual CZOs, common infrastructure and baseline measurements of a CZO may include: (i) real-time monitoring of energy, water, solutes, and sediment fluxes across CZ components and boundaries via eddy covariance systems, sensor networks, and other standardized observation equipment; (ii) isotopes and other tracers of water, particles, and chemicals to couple hydrological and biogeochemical processes; (iii) geophysical investigations of the three-dimensional architecture of the CZ and the characterization of CZ storages and changes across different temporal scales; (iv) documentation of the presence and role of the biota community in the CZ, especially the microbial community in the deep subsurface; and (v) open database access with adequate metadata and a standardized format to facilitate interdisciplinary data exchange and meta-analysis across CZOs (e.g., Niu et al., 2014 ). Development of cross-CZO collaborations, in particular, will contribute to such needed standards to meet the full potential of a global CZO network through joint efforts in various areas, such as sample collection and analysis, monitoring setup and instrumentation, and model development and comparison, among others.

Efforts thus far in developing a global network of CZOs have been mainly pursued in the United States, Europe (especially Germany, the United Kingdom, and France), China, and Australia. However, CZOs in other countries are relatively rare, in particular in Africa and South America ( Fig. 1 ). Given the goal of the global network of CZOs to develop predictive knowledge across terrestrial ecosystems, more efforts are required to reach out to other parts of the world to extend the spatial coverage of the global network of CZOs, on which comparative studies could be conducted across CZOs along environmental gradients or under different anthropogenic impacts to advance our understanding of the evolution, structure, and functions of the CZ.

Library of Critical Zone Models and Databases

Recent advances in Earth science have greatly enhanced our ability to develop Earth system models to represent a wide variety of processes that are coupled under different climatic conditions. Critical Zone science also creates new modeling opportunities to integrate Earth surface processes. Hence, similar efforts are needed to develop comprehensive CZ models or a library of models and databases ( Vereecken et al., 2016 ). Two modeling needs have been recognized that may lead to substantial advancements in CZ science:

Models need to be developed and tested across CZOs with a focus on common CZ features. The recent findings in CZ science provide rich knowledge to develop a unified theoretical framework to understand CZ evolution and function, on which common models could be built to quantify basic CZ processes. These common models need to be tested across a variety of site conditions and should be more transferrable to conditions outside of those in which they were developed or in which extensive monitoring data are available and thus more capable of solving challenges associated with heterogeneity and regional differences. Extrapolating the common models to unmonitored sites and to access global CZ processes and services is essential to advance CZ science.

A common library of CZ models and databases should be established. Instead of one supermodel for all CZ processes, it may be more practical to create a library of models to include links between pedogenesis and landscape evolution, connections between aboveground and belowground, coupling of geophysics and geochemistry, integration of hydrology and biogeochemistry, and combination of anthropogenic and natural processes. A library of comprehensive CZ databases is also crucial, and better tools are needed for querying and sharing data sets with adequate metadata. These databases can then be linked to models and used for model development, calibration, and testing, as well as for hypothesis-driven investigations. To promote efficient data sharing and integration, more efforts are required to develop interdisciplinary data exchange platforms and to enhance the inter-operability of CZ databases across CZOs.

The ongoing funding-source-mandated open-data/open-model paradigm shift that is underway has improved the availability and accessibility of CZ data. The US CZO network has applied the Theory–Model–Data fusion framework to integrate CZ data across CZOs ( http://criticalzone.org/national/data/ ). Currently, there are 287 CZ data sets publically available (as of July 2016). Online search and visualization tools have also been developed to help users to access CZ data. The Hydrologic Information System (HIS; http://his.cuahsi.org/ ) and HydroShare ( https://www.hydroshare.org/ ) of the US National Science Foundation supported Consortium of Universities for the Advancement of Hydrologic Sciences (CUAHSI) are other examples of sharing data, models, and codes. On these two platforms, data publishers can organize, store, and make data available to others, and data users can search, retrieve, visualize, analyze, and run models on data from these data servers. Ongoing efforts by the CUASHI will further improve these platforms to become a standard system for storing, managing, organizing, indexing, documenting, and sharing data. Although online data sharing has great potential to improve CZ data accessibility, high-level integration of cyberinfrastructure is still needed to promote the information exchange among different data sharing systems. Because individual researchers can publish data on online platforms, data homogeneity (both data quality and data format) becomes an important issue for data sharing, which again calls for establishing data standards. Additionally, more efforts are needed to encourage individuals to share data through online platforms. Contributions to data publishing should be more recognized, such as assigning a DOI number to online published data sets, models, and codes, making them more widely citable and searchable.

Processes taking place within the Earth’s CZ are essential for sustaining life and humanity. Critical Zone science has been recognized as one of the most compelling research fields in the 21st century. The past few years have seen an increasing number of CZOs around the world that are being connected into a global network. This focus-topic update summarizes recently published work to highlight major results and new concepts related to CZ science, especially those generated from recognized CZOs. A framework of “deep” science—deep time, deep depth, and deep coupling—is used to synthesize recent CZ investigations across a broad range of spatial and temporal scales.

Deep time emphasizes the recognition of the intertwined short-term and long-term CZ processes and the linkage from past CZ evolution to current CZ functions to future CZ scenarios.

Deep depth provides a holistic view of the entire three-dimensional CZ as a unified system and suggests the need to integrate the surface and deep subsurface and to upscale field-plot measurements to hillslope-, catchment-, and watershed-scale understanding.

Deep coupling indicates the complex interplay of CZ processes (especially across biotic and abiotic processes) and suggests the need to link CZ services to CZ management.

The knowledge gaps and major opportunities suggested to advance the frontiers of CZ science are (i) a global network of CZOs and cross-CZO collaborations, and (ii) a library of CZ models and databases. This update should facilitate continuous discussions and stimulate further advancement of CZ science around the world.

The authors are supported in part by the US National Science Foundation Hydrologic Sciences Program (Grant EAR-1416881, PI: H. Lin) and the Critical Zone Observatory Program (EAR-0725019, PI: C. Duffy; and EAR-1239285, EAR-1331726, PI: S. Brantley). We thank Vadose Zone Journal Editor Harry Vereecken for this opportunity to update the community on the latest studies in Critical Zone research and observatories. We thank Jirka Šimůnek, Dave Stonestrom, Harry Vereecken, and another anonymous reviewer for their comments that have helped improve the quality of this paper.

Data & Figures

A global network of Critical Zone Observatories (CZOs) and CZO-like sites. Red dots indicate the US national CZOs ( http://criticalzone.org/national/ ), including Boulder Creek CZO, Calhoun CZO, Catalina/Jemez CZO, Christina CZO, Eel CZO, IML CZO, Luquillo CZO, Reynolds Creek CZO, Shale Hills CZO, and Southern Sierra CZO; orange dots indicate the four Terrestrial Environmental Observatories (TERENO) CZOs in Germany ( http://teodoor.icg.kfa-juelich.de/overview-en ); and dark red dots indicate the four CZOs established by Soil Transformations in European Catchments (SoilTrEC) in Switzerland, Austria, Greece, and Czech Republic ( http://www.soiltrec.eu/index.html ; http://esdac.jrc.ec.europa.eu/projects/critical-zone-observatories ). Blue dots indicate the other CZO-like sites summarized by Banwart et al. (2013) and registered in SoilTrEC. Five Chinese CZOs co-funded by a Sino-UK joint program and the Australian CZOs are being developed and are not included in this figure. This map of the global network of CZOs may not be complete.

A total of 133 Critical Zone (CZ) related studies have been published in Vadose Zone Journal and another 91 in other scientific journals from 2011 to 2015 based on the ISI Web of Knowledge core database (as of 22 May 2016). These 133 papers include 106 published in seven special sections (colored bars) plus 27 others (dashed bars). Grid histograms indicate the CZ studies published in other scientific journals besides Vadose Zone Journal . “Critical Zone” or “catchment” were used in the title field in searching for CZ-related studies published in Vadose Zone Journal , while only “Critical Zone” was used in the title field to search for CZ related studies in other journals. Each paper from the search results was carefully inspected, and studies irrelevant to CZ science were excluded. Out of these CZ-related papers, 60 from Vadose Zone Journal and 41 from other journals are cited in this update. The CZ-related special sections (SS) published in Vadose Zone Journal and summarized in this update include the following: Critical Zone Observatories (CZO) in 2011, HOBE : A Hydrological Observatory (HOBE) in 2011, Soil Architecture and Functions (SAF) in 2012, Soil–Plant–Atmosphere Continuum (SPAC) in 2012, Frontiers of Hydropedology in Vadose Zone Research (HP) in 2013, Remote Sensing for Vadose Zone Hydrology (RSVZH) in 2013, and Dissolved Organic Matter in Soil (DOM) in 2014.

The framework of “deep” science proposed to synthesize recent research results in Critical Zone (CZ) science. This “deep” science concept highlights the integration of Earth surface processes at multiple spatial and temporal scales and sets CZ science apart from other environmental research. Deep time emphasizes the linkage from past CZ evolution to current CZ functions and future CZ scenarios. Deep depth provides a holistic view of the entire CZ as a unified three-dimensional system. Deep coupling indicates the complex interplay of CZ processes and suggests the need to link CZ service to CZ management. Two major opportunities and knowledge gaps are suggested to advance the frontiers of CZ science: (i) a global network of CZ observatories and cross-observatory collaborations; and (ii) a library of CZ models and databases.

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ORIGINAL RESEARCH article

Ecological and human health risk of heavy metals in nubui river: a case of rural remote communities, ghana.

• 1 University of South Africa, Pretoria, South Africa
• 2 Chair in Nanoscience and Nanotechnology, College of Graduate Studies, University of South Africa, Pretoria, South Africa
• 3 Nanosciences African Network, iThemba Laboratory, Cape Town, South Africa
• 4 University of Kinshasa, Kinshasa, Kinshasa, Democratic Republic of Congo
• 5 Institute for Nanotechnology and Water Sustainability, College of Science, Engineering and Technology, University of South Africa, Johannesburg, Free State, South Africa

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The Nubui river is a primary source of water for drinking purpose and other domestic activities in the rural communities dotted along its riparian zone, with agriculture being the major activity occurring in this important ecotone. The river has become a potential sink for agrochemical residue, including heavy metals and has apparent aesthetic water quality issues, with associated health consequences. This study, therefore, assessed the health risks of heavy metals within the rural populations in the catchment areas, who have limited sources of improved water supply. The concentration of Iron (Fe), Lead (Pb), Cadmium (Cd), Mercury (Hg) and Zinc (Zn) was assessed on cumulatively 275 water samples, using a Perkin Elmer PINAAcle 900T atomic absorption spectrophotometer for eleven months. A cross sectional survey was conducted among 338 community members, following field observations on utilisation types, aesthetic appeal and perceived quality of water from the Nubui river. To determine the potential human and ecological risks of heavy metals, the hazard quotient, chronic daily Intake, contamination factor and health pollution indices of heavy metals were computed. STATA version 16 was used to analyse the survey results. Descriptive statistics of average concentrations of heavy metals in surface water at all sampling stations showed the pattern Hg ˂ Pb ˂ Cd ˂ Zn ˂ Fe, with relatively low concentrations, between 0.001mg/L-0.004 mg/L for Hg, 0.0011 mg/L -0.0019 mg/L for Pb, 0.0461 mg/L -0.0739 mg/L for Zn and 0.2409 mg/L -0.377 mg/L for Fe. The findings, however, showed relatively high Cadmium levels between 0.0215 mg/L -0.0383 mg/L in two out of five sampling stations in comparison to the World Health Organisation (WHO) drinking water guideline values in some months. Hazard quotient values indicate that, the population is safe from the noncarcinogenic health risks of heavy metals exposure through oral routes. Contamination factor and Heavy Metal pollution indices for Cadmium exceed recommended guideline values of 1 and 100 respectively. Meanwhile 73.1% community members evidently preferred the Nubui River for various domestic activities with 86.1% of these utilising it for drinking purposes. This occurrence, results in an exposure to associated health risks.

Keywords: Water Quality, heavy metals, health risk, Nubui River, Ghana

Received: 08 Mar 2024; Accepted: 13 May 2024.

Copyright: © 2024 Norvivor, Azizi, Fuku, Atibu, Idris, Sibali, Maaza and Kamika. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY) . The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

* Correspondence: Ilunga Kamika, University of South Africa, Pretoria, 0003, South Africa

Disclaimer: All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.

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Forging New Purpose: Updated Edwards Building supports MSU’s innovative steel manufacturing research

Contact: James Carskadon

STARKVILLE, Miss.—A newly renovated facility on the Mississippi State campus is forging new capabilities and partnerships in steel manufacturing and materials research.

The Edwards Building, located behind the Industrial Education Building at the eastern edge of the parking lot, has been optimized to meet the research needs of MSU’s Michael W. Hall School of Mechanical Engineering and the Center for Advanced Vehicular Systems. The updated facility is helping MSU researchers improve steel production methods as the industry works toward the next generation of steel alloys.

Originally built in the 1960s for nuclear engineering research, it later transitioned for the study of steel. The space now is a place to design and prototype novel steels, bridging the gap between research labs producing a few ounces of steel and industrial steel production facilities.

The Edwards Building operates like a small-scale steel mill, giving researchers the ability to create steel plates up to 50 pounds that can be analyzed for different quality measures. New equipment was added as part of the renovation, including a metal formability tester, and an equipment layout redesign will maximize efficiency.

“From melting and casting to rolling and forging downstream processes, these facilities enable MSU researchers to advance steel development in areas relevant to both industrial and government sectors,” said CAVS Research Engineer Dawn Van Iderstine. “Precise impurity control through vacuum induction melting provides an ideal environment for alloy design.  Subsequent thermomechanical processing can be carried out through controlled hot and cold rolling or by forging schedules, making CAVS research facilities relevant to a variety of industries.”

Ross Smith, assistant professor of practice in the Hall School of Mechanical Engineering, said the updated facility and state-of-the-art equipment will serve as a nucleus for interdisciplinary collaboration and empower MSU researchers to push the boundaries of metallurgy and materials engineering.

“This initiative not only enhances MSU’s research prowess but also provides students with unparalleled opportunities for hands-on learning and innovation, strengthening the school’s academic excellence,” Smith said. “Moreover, by fostering collaboration between academia and industry stakeholders, the renovation project serves as a catalyst for regional economic development and industry partnership, positioning MSU as a leader in materials science and engineering.”

Haitham El Kadiri, Hall School of Mechanical Engineering Director and PACCAR Endowed Chair, said MSU research is helping to bridge the gap between new materials developed in a lab and industrial production.

“Closing the valley of death for materials microstructure development at the production scale through integrated computational materials engineering (ICME) techniques continues to be a daunting challenge in materials science and engineering research,” El Kadiri said. “The manufacturing capabilities at the Edwards Building have been designed to generate and study microstructure evolution mechanisms that operate at intermediate size and times scales suitable to create and validate high-fidelity ICME models.”

The upgrades come at a time for growth in the regional steel industry. In 2022, Steel Dynamics Inc. announced plans to invest $2.5 billion into an expansion of its facility in nearby Lowndes County, with the company currently building an aluminum mill. With the success of Steel Dynamics’ current operation and plans for growth, CAVS Executive Director Clay Walden said the center is making strategic investments to be a resource for the steel manufacturing community, helping to grow regional economic development.

“While we are doing work in next-generation steel alloys, a lot of our more immediate activity has been around ways to better support production and strategies for the alloys that are already in existence,” Walden said. “If you look at all of the different elements in quenching, rolling and heating, it is very expensive to experiment with those different parameters at a mill. These facilities allow us to do smaller-scale experiments here and identify potential production improvements.”

Representatives from CAVS and the Hall School of Mechanical Engineering closely examined the hot and cold working processes and identified efficiencies that significantly reduced the amount of employee movement and space needed for the process in the Edwards Building. Several cosmetic improvements were made, including the creation of a conference room.

For more on CAVS’ steel research capabilities, visit www.cavs.msstate.edu/research/steel.php .

Mississippi State University is taking care of what matters. Learn more at www.msstate.edu .

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SoilTrEC: a global initiative on critical zone research and integration

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Soil is a complex natural resource that is considered non-renewable in policy frameworks, and it plays a key role in maintaining a variety of ecosystem services (ES) and life-sustaining material cycles within the Earth's Critical Zone (CZ). However, currently, the ability of soil to deliver these services is being drastically reduced in many locations, and global loss of soil ecosystem services is estimated to increase each year as a result of many different threats, such as erosion and soil carbon loss. The European Union Thematic Strategy for Soil Protection alerts policy makers of the need to protect soil and proposes measures to mitigate soil degradation. In this context, the European Commission-funded research project on Soil Transformations in European Catchments (SoilTrEC) aims to quantify the processes that deliver soil ecosystem services in the Earth's Critical Zone and to quantify the impacts of environmental change on key soil functions. This is achieved by integrating the research results into decision-support tools and applying methods of economic valuation to soil ecosystem services. In this paper, we provide an overview of the SoilTrEC project, its organization, partnerships and implementation.

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Acknowledgements

This work is supported by the European Commission 7th Framework Programme as a Large Integrating Project (SoilTrEC, www.soiltrec.eu , Grant Agreement No. 244118).

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Menon, M., Rousseva, S., Nikolaidis, N.P. et al. SoilTrEC: a global initiative on critical zone research and integration. Environ Sci Pollut Res 21 , 3191–3195 (2014). https://doi.org/10.1007/s11356-013-2346-x

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To help our clients understand responses to COVID-19, McKinsey launched a research effort to gather insights from physicians into how the pandemic is affecting their ability to provide care, their financial situation, and their level of stress, as well as what kind of support would interest them. Nationwide surveys were conducted online in 2020 from April 27–May 5 (538 respondents), July 22–27 (150 respondents), and September 22–27 (303 respondents), as well as from March 25–April 5, 2021 (379 respondents).

The participants were US physicians in a variety of practice types and sizes, and a range of employment types. The specialties included general practice and family practice; cardiology; orthopedics, sports medicine and musculoskeletal; dermatology; general surgery; obstetrics and gynecology; oncology; ophthalmology; otorhinolaryngology and ENT; pediatrics; plastic surgery; physical medicine and rehabilitation; psychiatry and behavioral health; emergency medicine; and urology. These surveys built on a prior one of 1,008 primary-care, cardiology, and orthopedic-surgery physicians in April 2019.

To provide timely insights on the reported behaviors, concerns, and desired support of adult consumers (18 years and older) in response to COVID-19, McKinsey launched consumer surveys in 2020 (March 16–17, March 27–29, April 11–13, April 25–27, May 15–18, June 4–8, July 11–14, September 5–7, October 22–26, and November 20–December 6) and 2021 (January 4–11, February 8–12, March 15–22, April 24–May 2, June 4–13, and August 13–23). These surveys represent the stated perspectives of consumers and are not meant to indicate or predict their actual future behavior. (In these surveys, we asked consumers about “Coronavirus/COVID-19,” given the general public’s colloquial use of coronavirus to refer to COVID-19.)

Many digital start-ups and tech and retail giants are rising to the occasion, but our most recent (2021) McKinsey Physician Survey indicates that physicians may prefer a return to pre-COVID-19 norms. In this article, we explore the trends creating disconnects between consumers and physicians and share ideas on how providers could offer digital services that work not only for them but also for patients. Bottom line: a seamless IRL/URL offering could retain patients while delivering high-quality care. Everybody benefits.

The rise of telehealth

These materials reflect general insight based on currently available information, which has not been independently verified and is inherently uncertain. Future results may differ materially from any statements of expectation, forecasts, or projections. These materials are not a guarantee of results and cannot be relied upon. These materials do not constitute legal, medical, policy, or other regulated advice and do not contain all the information needed to determine a future course of action.

At the onset of the COVID-19 pandemic, both physicians and patients embraced telehealth: in April 2020, the number of virtual visits was a stunning 78 times higher than it had been two months earlier, accounting for nearly one-third of outpatient visits. In May 2021, 88 percent of consumers said that they had used telehealth services at some point since the COVID-19 pandemic began. Physicians also felt dramatically more comfortable with virtual care. Eighty-three percent of those surveyed in the 2021 McKinsey Physician Survey offered virtual services, compared with only 13 percent in 2019. 1 See sidebar on methodology; McKinsey Physician Surveys conducted nationally in five waves between May 2019 and April 2021; May 1, 2019, n = 1,008; May 5, 2020, n = 500; July 2, 2020, n = 150; September 27, 2020, n = 500; April 5, 2021, n = 379.

However, as of mid-2021, consumers’ embrace of telehealth appeared to have dimmed a bit  from its early COVID-19 peak: utilization was down to 38 times pre-COVID-19 levels. Also, more physicians were offering telehealth but recommending in-person care when possible in 2021, which could suggest that physicians are gravitating away from URL and would prefer a return to IRL care delivery (Exhibit 1).

Three trends from the late-stage pandemic

As COVID-19 continues, three emerging trends could set the stage for the next few years.

The number of virtual-first players keeps growing, and physicians struggle to keep up

The growth (and valuations) of virtual-first care providers suggest that demand by patients is persistent and growing. Teladoc increased the number of its visits by 156 percent in 2020, and its revenues jumped by 107 percent year over year. Amwell increased its supply of providers by 950 percent in 2020. 2 “Teladoc Health reports fourth-quarter and full-year 2020 results,” Teledoc Health, February 24, 2021; “Amwell announces results for the fourth quarter and full year 2020,” Amwell, March 24, 2021. By contrast, only 45 percent of physicians have been able to invest in telehealth during the pandemic, and only 16 percent have invested in other digital tools. Just 41 percent believe that they have the technology to deliver telehealth seamlessly. 3 McKinsey Physician Survey, April 5, 2021.

Some workflows, for example, require physicians to log into disparate systems that do not integrate seamlessly with an electronic health record (EHR). Audiovisual failures during virtual appointments continue to occur. To make these models work, providers may need to determine how to design operational workflows to make IRL/URL care as seamless as possible for both providers and patients. The workflows and care team models may need to vary, depending on the physician’s specialty and the amount of time they plan to devote to URL versus IRL care.

Patient–physician relationships are shifting

In McKinsey’s April 2021 Physician Survey, 58 percent of the respondents reported that they had lost patients to other physicians or to other health systems since the start of the COVID-19 pandemic. Corroborating those findings, our August 2021 survey of consumers showed that of those who had a primary-care physician (PCP), 15 percent had switched in the past year. Thirty-five percent of all consumers reported seeing a new healthcare provider who was not their regular PCP or specialist in the past year. Among consumers who had switched PCPs, 35 percent cited one or more reasons related to the patient experience—the desire for a PCP who better understood their needs (15 percent of respondents), a better experience (10 percent), or more convenient appointments (6 percent). Just half (50 percent) of consumers with a PCP say they are very satisfied. What’s more, Medicare regulations now give patients more ownership over their health data, and that could make it easier for them to switch physicians. 4 “Policies and technology for interoperability and burden reduction,” Centers for Medicare & Medicaid Services, December 9, 2021.

Physicians and patients see telehealth differently

Our surveys show that doctors and patients have starkly different opinions about telehealth and broader digital engagement (Exhibit 2). Take convenience: while two-thirds of physicians and 60 percent of patients said they agreed that virtual health is more convenient than in-person care for patients, only 36 percent of physicians find it more convenient for themselves.

This perception may be leading physicians to rethink telehealth. Most said they expect to return to a primarily in-person delivery model over the next year. Sixty-two percent said they recommend in-person over virtual care to patients. Physicians also expect telehealth to account for one-third less of their visits a year from now than it does today.

These physicians may be underestimating patient demand. Forty percent of patients in May 2021 said they believe they will continue to use telehealth in the pandemic’s aftermath. 5 McKinsey Consumer Health Insights Survey , May 7, 2021.

In November 2021, 55 percent of patients said they were more satisfied with telehealth/virtual care visits than with in-person appointments. 6 McKinsey Consumer Health Insights Survey , November 19, 2021. Thirty-five percent of consumers are currently using other digital services, such as ordering prescriptions online and home delivery. Of these, 42 percent started using these services during the pandemic and plan to keep using them, and an additional 15 percent are interested in starting digital services. 7 McKinsey Consumer Health Insights Survey , June 24, 2021.

Convenience is not the only concern. Physicians also worry about reimbursement. At the height of the COVID-19 pandemic in the United States, the Centers for Medicare & Medicaid Services (CMS) and several other payers switched to at-parity (equal) reimbursement for virtual and in-person visits. More than half of physician respondents said that if virtual rates were 15 percent lower than in-person rates, they would be less likely to offer telehealth. Telehealth takes investment: traditional providers may need time to transition their capital and operating expenses to deliver virtual care at a cost lower than that of IRL.

Four critical actions for providers to consider

Providers may want to define their IRL/URL care strategy to identify the appropriate places for various types of care—balancing clinical appropriateness with the preferences of physicians and patients.

Determine the most clinically appropriate setting

Clinical appropriateness may be the most crucial variable for deciding how and where to increase the utilization of telehealth. Almost half of physicians said they regard telehealth as appropriate for treatment of ongoing chronic conditions, and 38 percent said they believe it is appropriate when patients have an acute change in health—increases of 26 and 17 percentage points, respectively, since May 2019.

However, physicians remain conservative in their view of telehealth’s effectiveness compared with in-person care. Their opinions vary by visit type (Exhibit 3). Health systems may consider asking their frontline clinical-care delivery teams to determine the clinically appropriate setting for each type of care, taking into account whether physicians are confident that they can deliver equally high-quality care for both IRL and URL appointments.

Assess patient wants and needs in relevant markets and segments

Patient demand for telehealth remains high, but expectations appear to vary by age and income group, payer status, and type of care. Our survey shows that younger people (under the age of 55 ), people in higher income brackets (annual household income of $100,000 or more), and people with individual or employer-sponsored group insurance are more likely to use telehealth (Exhibit 4). Patient demand also is higher for virtual mental and behavioral health. Sixty-two percent of mental-health patients completed their most recent appointments virtually, but only 20 percent of patients logged in to see their primary-care provider, gynecologist, or pediatrician.

To meet market demand effectively, it may be crucial to base care delivery models on a deep understanding of the market, with a range of both IRL and URL options to meet the needs of multiple patient segments.

Partner with physicians to define a new operating model

Many physicians are turning away from the virtual operating model: 62 percent recommended in-person care in April 2021, up five percentage points since September 2020. As physicians evaluate their processes for 2022, 46 percent said they prefer to offer, at most, a couple of hours of virtual care each day. Twenty-nine percent would like to offer none at all—up ten percentage points from September 2020. Just 11 percent would dedicate one full day a week to telehealth, and almost none would want to offer virtual care full time (Exhibit 5).

To adapt to these views, care providers can try to meet the needs and the expectations of physicians. They could offer highly virtualized schedules to physicians who prefer telehealth, while allowing other physicians to remain in-person only. Matching the preferences of physicians may create the best experience both for them and for patients. Greater flexibility and greater control over decisions about when and how much virtual care to offer may also help address chronic physician burnout issues (Exhibit 6). Digital-first solutions (for example, online scheduling, digital registration, and virtual communications with providers) could also increase the reach of in-person-only care providers to the 60 percent of consumers interested in using these digital solutions after the pandemic abates.

Communicate clearly to patients and others

Physicians consistently emerge as the most trusted source of clinical information by patients: 90 percent consider providers  trustworthy for healthcare-related issues. 8 McKinsey Consumer Survey, May 2020. Providers could play a pivotal role in counseling patients on the importance of continuity of care, as well as what can be done safely and effectively by IRL and URL, respectively. The goal is to help patients receive the care that they need in a timely manner and in the most clinically appropriate setting.

Potential benefits to providers

The strategic, purposeful design of a hybrid IRL/URL healthcare delivery model that respects the preferences of patients and physicians and offers virtual care when it is appropriate clinically may allow healthcare providers to participate in the near term, retain clinical talent, offer better value-based care, and differentiate themselves strategically for the future.

Telehealth and broader digital engagement tools have enjoyed persistent patient demand throughout the pandemic. That demand may persist well after it. Investment in digital health companies has grown rapidly—reaching $21.6 billion in 2020, a 103 percent year-over-year increase—which also suggests that this approach to medicine has staying power. 9 Q4 and annual 2020 digital health (healthcare IT) funding and M&A report , Executive Summary, Digital Health Funding and M&A, Mercom Capital Group.

That level of demand offers the potential for growth when physicians can meet it. If only new entrants fully meet consumer demand, traditional providers who do not offer URL options may risk losing market share over time as a result of patients’ initial visit and downstream care decisions. What’s more, as healthcare reimbursement continues to move toward value, virtual-delivery options could become a strategic differentiator that helps providers better manage costs. 10 Brian W. Powers, MD, et al., “Association between primary care payment model and telemedicine use for Medicare Advantage enrollees during the COVID-19 pandemic,” JAMA Network , July 16, 2021.

In all likelihood, one of the critical steps in the process will be engaging physicians in the design of new virtual-care models—for example, determining clinical appropriateness, how and where physicians prefer to deliver care, and the workflows that will maximize their productivity. This has the added benefit of potentially also addressing the problem of physician burnout by offering a range of options for how and where clinicians practice.

Most important, virtual care can offer an opportunity to improve outcomes for patients meaningfully by delivering timely care to those who might otherwise delay it or who live in areas with provider shortages. In addition, patients’ most trusted advisers on care decisions are physicians, so virtual care gives them a meaningful opportunity to help patients access the care they need in a way that both parties may find convenient and appropriate. 11 “Public & physician trust in the U.S. healthcare system,” ABIM Foundation, surveys conducted on December 29, 2020 and February 5, 2021.

Physicians are evaluating a variety of factors for delivering care to patients during and, eventually, after the COVID-19 pandemic. The strategic, purposeful design of a hybrid IRL/URL healthcare delivery model offers a triple unlock: improving the value of healthcare while better meeting consumer demand and improving physicians’ engagement. The full unlock is not easy—it requires deep engagement and cooperation between administrators, clinicians, and frontline staff, as well as focused investment. But it will yield dividends for patients and providers alike in the long run.

Jenny Cordina is a partner in McKinsey’s Detroit office,  Jennifer Fowkes is a partner in the Washington, DC, office,  Rupal Malani, MD , is a partner in the Cleveland office, and  Laura Medford-Davis, MD , is an associate partner in the Houston office.

The article was edited by Elizabeth Newman, an executive editor in the Chicago office.

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A, Adjusted odds ratios and 95% CIs for the outcome of any in-hospital PT or OT were derived from logistic regression models. B, Adjusted rate ratios and 95% CIs for the outcome of rate of in-hospital PT or OT were derived from Poisson regression models. All models were constructed separately for each exposure and adjusted for the same set of covariates (age, sex, pre–intensive care unit count of disabilities in activities of daily living, use of mechanical ventilation, and count of organ dysfunction as a surrogate for severity of illness).

Estimates were derived from the multivariable logistic regression model adjusting for covariates of age, sex, pre–intensive care unit count of disabilities in activities of daily living, use of mechanical ventilation, and count of organ dysfunction as a surrogate for severity of illness.

Estimates were derived from the multivariable Poisson regression model adjusting for covariates of age, sex, pre–intensive care unit count of disabilities in activities of daily living, use of mechanical ventilation, and count of organ dysfunction as a surrogate for severity of illness, as described in the methods.

eFigure. Assembly of the Analytic Sample

eTable 1. ICD-9 and ICD-10 Codes Used to Identify Mechanical Ventilation and Organ Dysfunction

eTable 2. Sensitivity Analysis for the Outcome of Any In-Hospital Physical and/or Occupational Therapy Excluding ICU Hospitalizations From NHATS Participants Who Were Admitted From a Nursing Home or Had a Stay in a Nursing Home of ≥100 Days Between Pre-ICU NHATS Interview and Index ICU Hospitalization

eTable 3. Sensitivity Analysis for the Outcome of Rate of In-Hospital Physical and/or Occupational Therapy Excluding ICU Hospitalizations From NHATS Participants Who Were Admitted From a Nursing Home or Had a Stay in a Nursing Home of ≥100 Days Between Pre-ICU NHATS Interview and Index ICU Hospitalization

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Jain S , Murphy TE , Falvey JR, et al. Social Determinants of Health and Delivery of Rehabilitation to Older Adults During ICU Hospitalization. JAMA Netw Open. 2024;7(5):e2410713. doi:10.1001/jamanetworkopen.2024.10713

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Social Determinants of Health and Delivery of Rehabilitation to Older Adults During ICU Hospitalization

• 1 Section of Pulmonary, Critical Care, and Sleep Medicine, Department of Internal Medicine, Yale School of Medicine, New Haven, Connecticut
• 2 Department of Public Health Sciences, Pennsylvania State University, State College
• 3 Department of Physical Therapy and Rehabilitation Science, University of Maryland School of Medicine, Baltimore
• 4 Program on Aging, Yale School of Medicine, New Haven, Connecticut
• 5 Department of Sociology, Yale University, New Haven, Connecticut
• 6 Center for Outcomes Research and Evaluation, Yale New Haven Hospital, New Haven, Connecticut

Question   Are social determinants associated with differential delivery of skilled rehabilitation services to older adults during hospitalization with a critical illness?

Findings   In this cohort study of 1618 hospitalizations in older adults, after accounting for prehospitalization disability and acute illness characteristics, dual Medicare and Medicaid eligibility and rural residence were associated with a lower likelihood of delivery of any rehabilitation, whereas limited English proficiency was associated with reduced amount of rehabilitation services delivered during a critical illness hospitalization.

Meaning   These findings suggest social determinants of health should be taken into consideration in efforts to enhance equitable delivery of skilled rehabilitation to older adults who are critically ill.

Importance   Older adults with socioeconomic disadvantage develop a greater burden of disability after critical illness than those without socioeconomic disadvantage. The delivery of in-hospital rehabilitation that can mitigate functional decline may be influenced by social determinants of health (SDOH). Whether rehabilitation delivery differs by SDOH during critical illness hospitalization is not known.

Objective   To evaluate whether SDOH are associated with the delivery of skilled rehabilitation during critical illness hospitalization among older adults.

Design, Setting, and Participants   This cohort study used data from the National Health and Aging Trends Study linked with Medicare claims (2011-2018). Participants included older adults hospitalized with a stay in the intensive care unit (ICU). Data were analyzed from August 2022 to September 2023.

Exposures   Dual eligibility for Medicare and Medicaid, education, income, limited English proficiency (LEP), and rural residence.

Main Outcome and Measures   The primary outcome was delivery of physical therapy (PT) and/or occupational therapy (OT) during ICU hospitalization, characterized as any in-hospital PT or OT and rate of in-hospital PT or OT, calculated as total number of units divided by length of stay.

Results   In the sample of 1618 ICU hospitalizations (median [IQR] patient age, 81.0 [75.0-86.0] years; 842 [52.0%] female), 371 hospitalizations (22.9%) were among patients with dual Medicare and Medicaid eligibility, 523 hospitalizations (32.6%) were among patients with less than high school education, 320 hospitalizations (19.8%) were for patients with rural residence, and 56 hospitalizations (3.5%) were among patients with LEP. A total of 1076 hospitalized patients (68.5%) received any PT or OT, with a mean rate of 0.94 (95% CI, 0.86-1.02) units/d. After adjustment for age, sex, prehospitalization disability, mechanical ventilation, and organ dysfunction, factors associated with lower odds of receipt of PT or OT included dual Medicare and Medicaid eligibility (adjusted odds ratio, 0.70 [95% CI, 0.50-0.97]) and rural residence (adjusted odds ratio, 0.65 [95% CI, 0.48-0.87]). LEP was associated with a lower rate of PT or OT (adjusted rate ratio, 0.55 [95% CI, 0.32-0.94]).

Conclusions and Relevance   These findings highlight the need to consider SDOH in efforts to promote rehabilitation delivery during ICU hospitalization and to investigate factors underlying inequities in this practice.

Surviving a critical illness, an increasingly common occurrence among older adults, 1 - 3 is frequently accompanied by new or worsening disability. 4 , 5 Skilled rehabilitation with physical therapy (PT) and occupational therapy (OT) during hospitalization facilitates mobilization of patients recovering from critical illness and is known to prevent functional decline 6 - 8 and help to identify post-acute care needs. 9 Therefore, inequitable delivery of rehabilitation services to older adults with social or economic disadvantage may contribute to downstream disparities in disability. 10 Whether social determinants of health (SDOH) are associated with differences in delivery of skilled rehabilitation during a critical illness hospitalization is unknown.

Prior studies have described wide variation in clinician-reported delivery of rehabilitation services to patients who are critically ill. 11 - 13 Cross-sectional studies of hospitals participating in trial networks or quality improvement collaboratives have yielded similar results. 14 - 16 However, these studies did not investigate associations between SDOH and delivery of in-hospital rehabilitation. Many patient- and hospital-level factors known to be associated with prescription of skilled rehabilitation to patients who are critically ill can be affected by SDOH. 11 , 17 - 19 For example, limited English proficiency (LEP) could be associated with lower rehabilitation delivery due to perceived barriers to engagement with therapy services or differences in management of sedation and delirium by clinicians. 20 Furthermore, variation in resources and practices at hospitals caring for a higher proportion of patients with socioeconomic disadvantage could lead to reduced delivery of skilled rehabilitation services, as has been observed for other care processes. 21 , 22 A common challenge in evaluation of health care disparities is the absence of granular information on SDOH, such as LEP, income, and education, and preexisting health status, that could influence in-hospital treatment needs, such as prehospitalization disability in the case of rehabilitation services. We leveraged a nationally representative longitudinal study of aging with detailed information on SDOH and prehospitalization geriatric risk factors, linked with administrative claims, to investigate whether SDOH are associated with differences in the delivery of skilled rehabilitation to older adults during hospitalization in an intensive care units (ICU).

The protocol for the National Health and Aging Trends Study (NHATS) was approved by the Johns Hopkins University institutional review board, and our cohort study using these data was approved by the Yale University institutional review board. All participants provided informed consent. We followed the Strengthening the Reporting of Observational Studies in Epidemiology ( STROBE ) reporting guideline.

Data were drawn from the National Health and Aging Trends Study (NHATS), a longitudinal, nationally representative survey of community-dwelling Medicare beneficiaries ages 65 years and older living in the contiguous United States. 23 The initial sample was drawn from the Medicare enrollment database on September 30, 2010. 24 The survey collected information on sociodemographics, including race and ethnicity, insurance, education, income, English proficiency, rural residence, and clinical characteristics, through annual in-person interviews starting in 2011. If a participant was unavailable for interview, a proxy knowledgeable about their health was interviewed. Race and ethnicity were categorized as Hispanic, non-Hispanic Black, non-Hispanic White, and other (including participants reporting race as American Indian, Alaska Native, Asian, Native Hawaiian, Pacific Islander, or other race). Race and ethnicity were included in descriptive analyses because they are key SDOH.

ICU admissions were identified through linked inpatient claims files for Medicare fee-for-service and Medicare Advantage participants using critical care revenue codes indicating admission to general, specialty, or coronary care units but excluding psychiatric and intermediate care units. 25 Information on mechanical ventilation and organ dysfunction was obtained using International Classification of Diseases, Ninth Revision, Clinical Modification ( ICD-9-CM ) and International Statistical Classification of Diseases, Tenth Revision, Clinical Modification ( ICD-10-CM ) diagnosis and procedure codes (eTable 1 in Supplement 1 ). 26 , 27 ICU length of stay was determined based on days with a critical care revenue code.

Our primary outcomes were delivery of any PT or OT, as determined by revenue center codes 042.X and 043.X respectively, and amount of PT or OT, determined as number of units of evaluation or treatment delivered during ICU hospitalization. In general, PT and OT are billed in 15-minute increments; therefore, 1 billed unit represents 15 minutes of intervention by a therapist. These units were modeled as units per day to account for differences in hospital length of stay. Since services between 8 and 22 minutes are aggregated as a single unit, the observed rate of therapy is a rounded assessment of the actual delivered amount of therapy.

Given its previously reported associations with increased risk for functional decline following critical illness and reduced delivery of in-hospital rehabilitation to older adults, our primary exposure was dual eligibility for Medicare and Medicaid. 10 , 28 We additionally explored associations of other SDOH that have been linked to rehabilitation delivery in other populations or settings 29 - 31 and were available in NHATS or Medicare claims data. SDOH assessed in this study included income, education, LEP, and rurality. Dual-eligibility for Medicare and Medicaid was assessed using the dual Medicare-Medicaid status indicator in the Medicare Master Beneficiary Summary File at any time during the year preceding the ICU hospitalization. Information on other SDOH was derived from the participant’s NHATS survey immediately preceding ICU hospitalization. Income and assets were assessed using a composite of income from Social Security; Department of Veterans Affairs; pension; retirement plans; funds, stocks, and bonds; and checking and savings accounts and operationalized as quartiles in our sample. 23 The only missing data were on household income; missing data were imputed using values provided by NHATS. 32 Education was characterized as less than high school vs more. LEP was operationalized as a response of not well or not at all, as opposed to well or very well, to questions about how well respondents understand or speak English. 23 , 33 Residence was classified as rural (nonmetropolitan) vs urban (metropolitan), as assigned by NHATS based on the Office of Management and Budget classification of county of residence. We considered but did not evaluate the exposure of race and ethnicity because of small proportions of participants who identified as a race other than Black or White in our sample.

We selected covariates that could be potential confounders in rehabilitation delivery based on prior research and clinical relevance. We included age categorized into intervals based on proportions in our sample (65-74, 75-79, 80-84, 85-89, and ≥90 years), sex, count of disabilities in the NHATS interview preceding ICU hospitalization (defined as need for help or inability to perform activities of daily living, including 4 self-care activities [eating, bathing, using the toilet, and dressing] and 3 mobility activities [getting outside, getting around inside one’s home, and getting out of bed]), 10 , 34 use of mechanical ventilation (eTable 1 in Supplement 1 ), and severity of illness (determined as count of organ dysfunction). 27

Assembly of the analytic sample is presented in the eFigure in Supplement 1 . We identified 2832 NHATS participants from 2011 to 2018 who had a hospitalization with an ICU admission for at least 1 day. Participants could contribute multiple observations; however, we restricted our sample to 2299 first ICU hospitalizations in the interval between consecutive annual NHATS interviews to allow updating model covariates. After excluding hospitalization from 681 participants who were not community-dwelling at the pre-ICU NHATS interview, our sample included 1618 ICU hospitalizations.

We describe demographic and clinical characteristics of our sample using means and SDs or medians and IQRs for continuous variables and counts and weighted percentages for categorical variables, as appropriate. For each person-year of NHATS data, we used specific analytic weights that adjust for differential probabilities of selection and nonresponse within each strata (region) and cluster (zip code within county); this allows generalization to the 2011 Medicare population. 35 , 36 For income, NHATS provided 5 imputed data sets that were used only in the models testing this exposure. 32 Among other exposures considered in the models, only education had any missing data (0.7%). Hence, our models were based on complete case data. We separately fit multivariable logistic regression models for the binary outcome of delivery of any in-hospital PT or OT on each of the 5 exposures with adjustment for covariates. For exposures with a significant association with this outcome, we calculated risk differences. For the rate of PT or OT delivered per day of hospital stay, we fit multivariable Poisson regression models on each of the exposures with adjustment for the same covariates. We calculated least square means of the outcomes significantly associated with this outcome. Because the provision of PT or OT could be prioritized to patients presumed to be discharging to a facility, we conducted sensitivity analyses excluding participants admitted from a nursing home or with a stay 100 days or more between pre-ICU NHATS interview and index ICU hospitalization. We used SAS software version 9.4 (SAS Institute) for descriptive analyses and SAS-callable SUDAAN software version 11 (RTI International) for all models. To account for the small number of participants who contributed multiple hospitalizations, we used generalized estimating equations with an exchangeable covariance structure based on its minimization of quasilikelihood under the independence model criterion. In all analyses, significance was defined as a 2-tailed P  < .05. Data were analyzed from August 2022 to September 2023.

Our sample included 1618 ICU hospitalizations across 569 hospitals ( Table ). Patients had a median (IQR) age of 81.0 (75.0-86.0) years, and 842 (52.0%) were female. The sample included 371 patients (22.9%) with dual Medicare and Medicaid eligibility, 523 patients (32.6%) with less than high school education, 320 patients (19.8%) with rural residence, and 56 patients (3.5%) with LEP. Median (IQR) income was $22 000 ($12 000-$41 000).

A total of 1076 patients (68.5%) received any PT or OT during ICU hospitalizations. We observed decreased receipt of any PT or OT for hospitalizations among patients with Medicare and Medicaid dual eligibility (228 hospitalizations [62.7%] vs 848 hospitalizations among patients without dual eligibility [69.9%]), rural residence (185 hospitalizations [60.1%] vs 891 hospitalizations among patients from urban areas [70.3%]), and below-median income (520 hospitalizations [65.5%] vs 586 hospitalizations among patients with above-median income [70.8%]). A mean of 0.94 (95% CI, 0.86-1.02) units/d was delivered. Patients with LEP received a lower rate of PT or OT (0.79 [95% CI, 0.76- 0.82] units/d vs 0.95 [95% CI, 0.90-0.99] units/d for those without LEP), as did patients with income above the median (0.84 [95% CI, 0.73-0.96] units/d vs 1.02 [95% CI, 0.91-1.03] units/d for those below median income).

Figure 1 presents the results of our multivariable models. Dual Medicare and Medicaid eligibility (adjusted odds ratio [aOR], 0.70 [95% CI, 0.50-0.97]) and rural residence (aOR, 0.65 [95% CI, 0.48-0.87]) were associated with lower odds of delivery of any PT or OT ( Figure 1 A). For risk differences, the percentage of ICU hospitalizations in which any PT or OT was delivered was 7.8% lower for dual-eligible older adults than for non-dual-eligible older adults and 9.5% lower for residents of rural vs urban areas ( Figure 2 ). For the rate of total therapy, LEP was associated with lower rates of in-hospital PT or OT compared with not having LEP (adjusted rate ratio [aRR], 0.55 [95% CI, 0.32-0.94]) ( Figure 1 B). On the absolute scale, a mean rate of 0.7 (95% CI, 0.4-1.2) units/d of PT or OT was delivered to participants with LEP, compared with 1.3 (95% CI, 1.1-1.5) units/d to those proficient in English ( Figure 3 ). Over a 5-day hospital stay, this would translate into 24 to 66 fewer minutes of therapy for patients with LEP compared with those proficient in English. Having an income between $12 000 and $22 000 was also significantly associated with a lower rate of therapy delivered (aRR, 0.71 [95% CI, 0.52-0.95]) compared with the highest quartile of income; the other income categories did not demonstrate significant associations ( Figure 1 B).

In sensitivity analyses excluding hospitalization for participants who were admitted from a nursing home or had a nursing home stay of at least 100 days between their pre-ICU NHATS interview and the index ICU hospitalization, the magnitude and direction of association between the exposures and both outcomes were similar, albeit with wider CIs (eTable 2 and eTable 3 in Supplement 1 ).

In this nationally representative cohort study of older adults, we found that select SDOH were associated with reduced delivery of skilled rehabilitation services during hospitalization for critical illness. After accounting for prehospitalization disability and severity of acute illness, older adults who were dually eligible for Medicare and Medicaid and those who resided in rural areas had 30% to 35% lower odds of receiving any PT or OT during an ICU hospitalization than patients who were not dually eligible or who were proficient in English. Patients with LEP received lower amounts of therapy than patients proficient in English. Given the well-documented value of in-hospital rehabilitative PT and OT in preventing functional decline and identifying care needs at discharge following critical illness, 6 - 9 our findings of reduced delivery of these services to older adults by dual eligibility status, rural residence, and LEP warrant consideration of targeted efforts to mitigate inequities.

The prevalence of in-hospital rehabilitation delivery in our study was comparable with estimates from contemporary studies of acutely hospitalized patients. 28 , 37 , 38 Among patients who are critically ill, clinician surveys 11 , 12 , 39 and cohort studies of hospitals participating in trials or quality reporting initiatives 14 , 16 have reported wide variability in the use of PT and OT. Despite this known variability, to our knowledge, prior work has not evaluated the role of SDOH in the delivery of skilled rehabilitation services among Medicare beneficiaries. The availability of patient-level information on SDOH in NHATS, beyond those usually available in administrative data, allowed us to evaluate this important question.

Our findings of reduced delivery of skilled rehabilitation to older adults who are dually eligible for Medicare and Medicaid, live in rural areas, or have LEP may have explanations rooted in structural- and individual-level factors underlying in-hospital care delivery. First, factors related to resources and organization of rehabilitation services at hospitals deserve consideration. Staffing by physical therapists and nurses is associated with rehabilitation among patients who are critically ill. 11 , 18 Hospitals serving more dually eligible patients and those located in rural areas are generally more underresourced and may not be adequately staffed by professionals essential to delivering rehabilitation. 40 Similar to our findings, dual Medicare and Medicaid eligibility was associated with a lower likelihood of skilled rehabilitation among patients with acute stroke. 28 Second, percolation of evidence-based strategies, such as the ABCDEF bundle (assess, prevent, and manage pain; both spontaneous awakening and spontaneous breathing trials; choice of analgesia and sedation; assess, prevent, and manage delirium; early mobility and exercise; and family engagement and empowerment), 6 , 41 to promote rehabilitation may be lower at these hospitals. Study collaboratives promoting adoption of the bundle have typically engaged urban, academic hospitals 15 ; whether and how this guidance is translated into practice in rural hospitals should be explored. Third, the association between LEP and reduced PT and OT suggests that interpersonal communication barriers or implicit biases can underlie suboptimal delivery of rehabilitation, as described for other care processes. 20 , 42 Consistent with our observations, in a 2020 study at a safety-net hospital in Texas, speaking a language other than English or Spanish was associated with fewer minutes of therapy delivered to older adults hospitalized with prolonged acute illness. 29 Among patients who are critically ill, the reduced amount of rehabilitation services delivered to those with LEP could be due to differential prescription of rehabilitation services by physicians because of perceived barriers to engagement or differences in management of sedation and delirium that might influence a patient’s ability to participate in rehabilitation. 17 , 19 , 20

Our findings have important implications. First, immobility, a common occurrence during ICU hospitalization, is an important and modifiable risk factor for post-ICU disability. 6 - 8 While mobilizing hospitalized patients who are acutely ill can be achieved by personnel other than rehabilitation therapists, the complex feasibility and safety considerations for older adults who are critically ill usually warrant an interdisciplinary approach. 13 , 43 , 44 Evaluation by physical or occupational therapists is a part of recommended ICU mobilization protocols 6 , 7 , 45 and associated with its delivery in observational studies. 13 , 14 Therefore, while not equivalent to it, the lower delivery of PT or OT services by SDOH in our study likely represents underlying differences in ICU mobilization. Second, evaluation by physical and occupational therapists, usually on the wards, is important to identify postacute rehabilitation needs before hospital discharge. Reduced delivery of any PT or OT during hospitalization suggests that postacute rehabilitation needs are assessed less frequently among older adults with Medicaid and those in rural areas. In-hospital skilled rehabilitation is associated not only with higher mobility at discharge 9 , 46 , 47 but also improved long-term function among adults recovering from critical illness. 48 Therefore, reduced delivery of any rehabilitation during ICU hospitalization may represent a missed opportunity to improve long-term functional outcomes after critical illness and inequities in this practice can be a mechanism underlying disparities in post-ICU disability. 10

Our study has several strengths. First, we used a nationally representative sample of older adults with ICU hospitalization. Second, we included granular assessment of prehospitalization disability that could influence in-hospital rehabilitation needs. Third, we had information on SDOH using instruments specifically tailored for older adults, 23 Fourth, we used rigorous methods to determine delivery of PT and OT using claims data. 28 , 49 , 50

Our findings should be interpreted in the context of a few limitations. First, we could not distinguish the contribution of hospital-level effects or evaluate hospital factors, such as supply of physical therapists, due to the limited number of observations per hospital and hospital-level information in our claims-linked NHATS data. This should be investigated in future work to understand the role of structural factors in driving differences in rehabilitation delivery. Second, we could not distinguish the delivery of PT and OT in the ICU from that delivered elsewhere in the hospital; future studies evaluating this could guide efforts to mitigate differences. Third, we could not evaluate nurse-driven mobility. While this practice exists, an interdisciplinary approach, including at least an evaluation by rehabilitation therapists, remains usual practice in most US ICUs, 13 , 44 suggesting that in-hospital skilled rehabilitation, although not a surrogate for ICU mobilization, is likely associated with differences in this practice. Fourth, we did not have information on severity of illness scores, such as the SOFA score and Acute Physiology and Chronic Health Evaluation scale score, which are known to be associated with rehabilitation in the ICU. 51 Nevertheless we used a validated claims-based organ dysfunction algorithm to account for severity of illness. 27 Fifth, because of the small number of participants with self-reported race and ethnicity other than non-Hispanic Black or non-Hispanic White in our sample, we could not evaluate it as an exposure; future work should investigate this. Furthermore, because LEP and rurality were evaluated as exploratory exposures in our study, the observed associations should be further investigated in future studies.

In this nationally representative cohort study of older adults, dual eligibility for Medicare and Medicaid and rural residency were associated with lower likelihood and LEP was associated with reduced rate of delivery of skilled rehabilitation therapy during hospitalization with critical illness. Our findings highlight the need to consider these SDOH in efforts to enhance equitable delivery of skilled rehabilitation services during hospitalization. Future research is needed to distinguish individual- vs structural-level factors underlying differences in in-hospital rehabilitation delivery by SDOH.

Accepted for Publication: March 9, 2024.

Published: May 10, 2024. doi:10.1001/jamanetworkopen.2024.10713

Open Access: This is an open access article distributed under the terms of the CC-BY License . © 2024 Jain S et al. JAMA Network Open .

Corresponding Author: Snigdha Jain, MD, MHS, Section of Pulmonary, Critical Care, and Sleep Medicine, Department of Internal Medicine, Yale School of Medicine, 15 York St, New Haven, CT 06510 ( [email protected] ).

Author Contributions: Drs Jain and Ferrante had full access to all of the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis.

Concept and design: Jain, Murphy, Zang, Ferrante.

Acquisition, analysis, or interpretation of data: All authors.

Drafting of the manuscript: Jain, Murphy.

Critical review of the manuscript for important intellectual content: All authors.

Statistical analysis: Jain, Murphy.

Obtained funding: Jain.

Administrative, technical, or material support: Jain, Leo-Summers, O'Leary, Zang.

Supervision: Jain, Falvey, Ferrante.

Conflict of Interest Disclosures: Dr Krumholz reported receiving grants from American Heart Association, Agency for Healthcare Research and Quality, National Institutes of Health (NIH), Johnson & Johnson, Janssen, Centers for Disease Control and Prevention, and Pfizer (paid to institution); personal fees from Massachusetts Medical Society Completed, Identifeye, F-Prime, UpToDate, Ensight, and Element Science; and serving as cofounder of ENSIGHT-AI, Refactor Health, and Hugo Health outside the submitted work. No other disclosures were reported.

Funding/Support: Drs Jain, Ferrante, Zang, and Gill received support from the Claude D. Pepper Older Americans Independence Center at Yale School of Medicine (award No. P30AG021342). Dr Jain was supported by the Parker B. Francis Family Foundation Fellowship Award, Yale Physician-Scientist Development Award (award No. KL2 TR001862) from the National Center for Advancing Translational Science of the NIH, and National Institute On Aging (NIA) of the NIH (award No. R03AG078942). Dr Ferrante is supported by the NIA (award No. K76 AG057023. Falvey is supported by the University of Maryland Claude D. Pepper Center and the NIA (award No. K76AG074926). Dr Zang is supported by the NIA (award No. R21AG074238-01).

Role of the Funder/Sponsor: The funders had no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication.

Data Sharing Statement: See Supplement 2 .

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Changing Partisan Coalitions in a Politically Divided Nation

5. party identification among religious groups and religiously unaffiliated voters, table of contents.

• What this report tells us – and what it doesn’t
• Partisans and partisan leaners in the U.S. electorate
• Party identification and ideology
• Education and partisanship
• Education, race and partisanship
• Partisanship by race and gender
• Partisanship across educational and gender groups by race and ethnicity
• Gender and partisanship
• Parents are more Republican than voters without children
• Partisanship among men and women within age groups
• Race, age and partisanship
• The partisanship of generational cohorts
• Religion, race and ethnicity, and partisanship
• Party identification among atheists, agnostics and ‘nothing in particular’
• Partisanship and religious service attendance
• Partisanship by income groups
• The relationship between income and partisanship differs by education
• Union members remain more Democratic than Republican
• Homeowners are more Republican than renters
• Partisanship of military veterans
• Demographic differences in partisanship by community type
• Race and ethnicity
• Age and the U.S. electorate
• Education by race and ethnicity
• Religious affiliation
• Ideological composition of voters
• Acknowledgments
• Overview of survey methodologies
• The 2023 American Trends Panel profile survey methodology
• Measuring party identification across survey modes
• Adjusting telephone survey trends
• Appendix B: Religious category definitions
• Appendix C: Age cohort definitions

The relationship between partisanship and voters’ religious affiliation continues to be strong – especially when it comes to whether they belong to any organized religion at all.

The gap between voters who identify with an organized religion and those who do not has grown much wider in recent years.

Protestants mostly align with the Republican Party. Protestants remain the largest single religious group in the United States. As they have for most of the past 15 years, a majority of Protestant registered voters (59%) associate with the GOP, though as recently as 2009 they were split nearly equally between the two parties.

Partisan identity among Catholics had been closely divided, but the GOP now has a modest advantage among Catholics. About half of Catholic voters identify as Republicans or lean toward the Republican Party, compared with 44% who identify as Democrats or lean Democratic.

Members of the Church of Jesus Christ of Latter-day Saints remain overwhelmingly Republican. Three-quarters of voters in this group, widely known as Mormons, identify as Republicans or lean Republican. Only about a quarter (23%) associate with the Democratic Party.

Jewish voters continue to mostly align with the Democrats. About seven-in-ten Jewish voters (69%) associate with the Democratic Party, while 29% affiliate with the Republican Party. The share of Jewish voters who align with the Democrats has increased 8 percentage points since 2020.

Muslims associate with Democrats over Republicans by a wide margin. Currently, 66% of Muslim voters say they are Democrats or independents who lean Democratic, compared with 32% who are Republicans or lean Republican. (Data for Muslim voters is not available for earlier years because of small sample sizes.)

Democrats maintain a wide advantage among religiously unaffiliated voters. Religious “nones” have become more Democratic over the past few decades as their size in the U.S. population overall and in the electorate has grown significantly. While 70% of religiously unaffiliated voters align with the Democratic Party, just 27% identify as Republicans or lean Republican.

Related: Religious “nones” in America: Who they are and what they believe

Over the past few decades, White evangelical Protestant voters have moved increasingly toward the GOP.

• Today, 85% of White evangelical voters identify with or lean toward the GOP; just 14% align with the Democrats.

• Over the past three decades, there has been a 20 percentage point rise in the share of White evangelicals who associate with the GOP – and a 20-point decline in the share identifying as or leaning Democratic. 

Over the past 15 years, the GOP also has made gains among White nonevangelical and White Catholic voters.

About six-in-ten White nonevangelicals (58%) and White Catholics (61%) align with the GOP.    Voters in both groups were equally divided between the two parties in 2009.

Partisanship among Hispanic voters varies widely among Catholics and Protestants.

• 60% of Hispanic Catholic voters identify as Democrats or lean Democratic, but that share has declined over the past 15 years.
• Hispanic Protestant voters are evenly divided: 49% associate with the Republican Party, while 45% identify as Democrats or lean Democratic.

A large majority of Black Protestants identify with the Democrats (84%), but that share is down 9 points from where it was 15 years ago (93%).

Atheists and agnostics, who make up relatively small shares of all religiously unaffiliated voters, are heavily Democratic.

Among those who identify their religion as “nothing in particular” – and who comprise a majority of all religious “nones” – Democrats hold a smaller advantage in party identification.

• More than eight-in-ten atheists (84%) align with the Democratic Party, as do 78% of agnostics.
• 62% of voters who describe themselves as “nothing in particular” identify as Democrats or lean Democratic, while 34% align with the GOP.

Voters who regularly attend religious services are more likely to identify with or lean toward the Republican Party than voters who attend less regularly.

In 2023, 62% of registered voters who attended religious services once a month or more aligned with Republicans, compared with 41% of those who attend services less often.

This pattern has been evident for many years. However, the share of voters who identify as Republicans or lean Republican has edged up in recent years.

For White, Hispanic and Asian voters, regular attendance at religious services is linked to an increase in association with the Republican Party.

However, this is not the case among Black voters.

Only about one-in-ten Black voters who are regular attenders (13%) and a similar share (11%) of those who attend less often identify as Republicans or Republican leaners.

Higher GOP association among regular attenders of religious services is seen across most denominations.

For example, among Catholic voters who attend services monthly or more often, 61% identify as Republicans or lean toward the Republican Party.

Among less frequent attenders, 47% align with the GOP.

Black Protestants are an exception to this pattern: Black Protestant voters who attend religious services monthly or more often are no more likely to associate with the Republican Party than less frequent attenders.

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Biden hikes tariffs on Chinese EVs, solar cells, steel, aluminum — and snipes at Trump

President Joe Biden slapped major new tariffs on Chinese electric vehicles, advanced batteries, steel, aluminum and other goods Tuesday as he embraced a strategy that’s increasing friction between the world’s two largest economies.

President Joe Biden sits down to sign a document in the Rose Garden of the White House in Washington, Tuesday, May 14, 2024, imposing major new tariffs on electric vehicles, semiconductors, solar equipment and medical supplies imported from China. (AP Photo/Susan Walsh)

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President Joe Biden speaks in the Rose Garden of the White House in Washington, Tuesday, May 14, 2024, announcing plans to impose major new tariffs on electric vehicles, semiconductors, solar equipment and medical supplies imported from China. (AP Photo/Susan Walsh)

FILE - A worker assembles an SUV at a car plant of Li Auto, a major Chinese EV maker, in Changzhou in eastern China’s Jiangsu province on March 27, 2024. The Biden administration is announcing plans to slap new tariffs on Chinese electric vehicles, advanced batteries, solar cells, steel, aluminum and medical equipment. (Chinatopix Via AP, File)

WASHINGTON (AP) — President Joe Biden slapped major new tariffs on Chinese electric vehicles, advanced batteries, solar cells, steel, aluminum and medical equipment on Tuesday, taking potshots at Donald Trump along the way as he embraced a strategy that’s increasing friction between the world’s two largest economies.

The Democratic president said that Chinese government subsidies ensure the nation’s companies don’t have to turn a profit, giving them an unfair advantage in global trade.

“American workers can outwork and outcompete anyone as long as the competition is fair,” Biden said in the White House Rose Garden. “But for too long, it hasn’t been fair. For years, the Chinese government has poured state money into Chinese companies ... it’s not competition, it’s cheating.”

The tariffs come in the middle of a heated campaign between Biden and Trump, his Republican predecessor, to show who’s tougher on China. In a nod to the presidential campaign, Biden recognized lawmakers from Michigan in his remarks and spoke about workers in Pennsylvania and Wisconsin, all battleground states in November’s election.

Asked to respond to Trump’s comments that China was eating America’s lunch, Biden said of his rival, “He’s been feeding them a long time.” The Democrat said Trump had failed to crack down on Chinese trade abuses as he had pledged he would do during his presidency.

Karoline Leavitt, the Trump campaign’s press secretary, called the new tariffs a “weak and futile attempt” to distract from Biden’s own support for EVs in the United States, which Trump says will lead to layoffs at auto factories.

The Chinese government was quick to push back against the tariffs, saying they “will seriously affect the atmosphere of bilateral cooperation.” The foreign ministry used the word “bullying.”

The tariffs are unlikely to have a broad inflationary impact in the short term because of how they’re structured, some not to take effect until 2026, but there could be price increases in the meantime for EV batteries, solar and some other specific items.

Biden administration officials said they think the tariffs won’t escalate tensions with China, yet they expect China will explore ways to respond to the new taxes on its products. It’s uncertain what the long-term impact on prices could be if the tariffs contribute to a wider trade dispute.

The tariffs are to be phased in over the next three years, with those that take effect in 2024 covering EVs, solar cells, syringes, needles, steel and aluminum and more. There are currently very few EVs from China in the U.S., but officials worry low-priced models made possible by Chinese government subsidies could soon start flooding the U.S. market.

Chinese firms can sell EVs for as little as $12,000 . China’s solar cell plants and steel and aluminum mills have enough capacity to meet much of the world’s demand, with Chinese officials arguing their production keeps prices low and would aid a transition to the green economy.

China’s commerce ministry said in a statement that the tariffs were “typical political manipulation” as it expressed its “strong dissatisfaction” and pledged to “take resolute measures to defend its rights and interests.”

Under the findings of a four-year review on trade with China, the tax rate on imported Chinese EVs will rise to 102.5% this year, up from total levels of 27.5%. The review was undertaken under Section 301 of the Trade Act of 1974, which allows the government to retaliate against trade practices deemed unfair or in violation of global standards.

Under the 301 guidelines, the tariff rate is to double to 50% on solar cell imports this year. Tariffs on certain Chinese steel and aluminum products will climb to 25% this year. Computer chip tariffs will double to 50% by 2025.

For lithium-ion EV batteries, tariffs will rise from 7.5% to 25% this year. But for non-EV batteries of the same type, the tariff increase will be implemented in 2026. There are also higher tariffs on ship-to-shore cranes, critical minerals and medical products.

The new tariffs, at least initially, are largely symbolic since they will apply to only about $18 billion in imports. A new analysis by Oxford Economics estimates the tariffs will have a barely noticeable impact on inflation by pushing up inflation by just 0.01%.

The Chinese EV maker BYD has explored the possibility of opening factories in Mexico for the Mexican market, possibly creating a way to ship goods into the United States. U.S. Trade Representative Katherine Tai said she was talking with industry and workers about the possibility and to “stay tuned.”

The auto industry is still trying to assess the impact of the tariffs. But at present, it appears they could be assessed on only two Chinese-made vehicles, the Polestar 2 luxury EV and potentially Volvo’s S90 luxury gas-electric hybrid midsize sedan.

“We’re still reviewing the tariffs to understand exactly what’s affected and how,” said Russell Datz, spokesman for Volvo, a Swedish brand now under China’s Geely group. A message was left seeking comment from Polestar, which also falls under Geely.

The Chinese foreign ministry spokesperson, Wang Wenbin, said the U.S. is trampling on the principles of a market economy and international economic and trade rules.

“It’s a naked act of bullying,” Wang said.

The Chinese economy has been slowed by the collapse of the country’s real estate market and past coronavirus pandemic lockdowns, prompting Chinese President Xi Jinping to try to jumpstart growth by ramping up production of EVs and other products, making more than the Chinese market can absorb.

This strategy further exacerbates tensions with a U.S. government that claims it’s determined to strengthen its own manufacturing to compete with China, yet avoid a larger conflict.

“China’s factory-led recovery and weak consumption growth, which are translating into excess capacity and an aggressive search for foreign markets, in tandem with the looming U.S. election season add up to a perfect recipe for escalating U.S. trade fractions with China,’’ said Eswar Prasad, professor of trade policy at Cornell University.

The Europeans are worried, too. The EU launched an investigation last fall into Chinese subsidies and could impose an import tax on Chinese EVs.

After Xi’s visit to France last week, European Commission President Ursula von der Leyen warned that government-subsidized Chinese EVs and steel “are flooding the European market” and said, “The world cannot absorb China’s surplus production.’’

Biden’s administration views China, with its subsidies of manufacturing, as trying to globally control the EV and clean-energy sectors, whereas the administration says its own industrial support is geared toward ensuring domestic supplies to help meet U.S. demand.

“We do not seek to have global domination of manufacturing in these sectors, but we believe because these are strategic industries and for the sake of resilience of our supply chains, that we want to make sure that we have healthy and active firms,” Treasury Secretary Janet Yellen said.

The tensions go far beyond a trade dispute to deeper questions about who leads the world economy as a seemingly indispensable nation. China’s policies could make the world more dependent on its factories, possibly giving it greater leverage in geopolitics. At the same time, the United States says it’s seeking for countries to operate by the same standards so competition can be fair.

China maintains the tariffs are in violation of the global trade rules the United States originally helped establish through the World Trade Organization. It accuses the U.S. of continuing to politicize trade issues and on Friday said the new tariffs compound the problems caused by tariffs the Trump administration previously put on Chinese goods, which Biden has kept.

Those issues are at the heart of November’s presidential election, with a bitterly divided electorate seemingly united by the idea of getting tough with China. Biden and Trump have overlapping but different strategies.

Biden sees targeted tariffs as needed to defend key industries and workers, while Trump has threatened broad 10% tariffs against all imports from rivals and allies alike.

Biden has staked his presidential legacy on the U.S. pulling ahead of China with its own government investments in factories to make EVs, computer chips and other advanced technologies.

Trump tells his supporters America is falling further behind China by not betting on oil to keep powering the economy, despite its climate change risks. The ex-president may believe tariffs can change Chinese behavior, but he believes the U.S. will be reliant on China for EV components and solar cells.

“Joe Biden’s economic plan is to make China rich and America poor,” he said at a rally this month in Wisconsin.

AP autos writer Tom Krisher contributed to this report from Detroit.