The health and diversity of natural ecosystems—and human civilization—depend on our coordinated responses
to global changes that threaten earth’s long-term habitability. Soils, the thin veneer on the global
land surface that supports terrestrial life, are an integral component of anthropogenic climate change
mitigation strategies (Paustian et al., 2016; Loisel et al., 2019). Soils are a necessary part of the
solution for human-induced climate change because they represent one of the largest terrestrial carbon
(C) reservoirs, storing twice as much C as the earth’s atmosphere and vegetation combined (up to 2500 Pg
C; IPCC, 2013; Friedlingstein et al., 2020). Terrestrial C pools are a powerful C sink, with the
potential to offset up to 30% of anthropogenic C emissions, where some of the sequestered C persists in
soil over millennial time scales (Friedlingstein et al., 2020). Because of the relative sizes of the
different C reservoirs, even slight changes in the amount of C stored in soil can represent significant
changes in the global atmospheric concentration of carbon dioxide (CO2) and the earth’s
How do we unlock soil’s potential for combating climate change? An important component of a comprehensive
response is to store more C in soils, particularly in soil pools that cycle C at slower rates compared
to the other reservoirs (ex., atmosphere, biomass, and on near surface soil layers) (Schmidt et al.,
2011). The amount of carbon stored in soil (soil organic C or SOC) is a balance between inputs and
outputs of carbon (Berhe, 2019a; Lavallee and Cotrufo, 2020). SOC storage in a given area (plot,
catchment, region, or another spatially constrained system) has been likened to a bank account, where
the “balance” is the bulk SOC stock or inventory (Fig. 1). Bank “deposits” are contributed by vegetation
litter, root exudates, living soil biota, deposition of eroded C, and remains of formerly living
organisms. The depletion of the balance in the soil carbon bank account is driven by microbial
decomposition of organic C inputs to CO2 and dissolved and particulate transport of C through
leaching and/or erosion.
Soil organic carbon (SOC) is a dynamic and complex admixture. Here, three contrasting ecosystems reveal
differing SOC richness and dynamics: (A) agricultural, (B) grassland/shrubland, and (C) forested.
Conventional agriculture (A) often leads to lower carbon stocks, and overall, less carbon input to the
soil carbon pool. Grasslands (B) can harbor plants with deeper and more extensive root systems, medium
to high amounts of SOC stock, and greater carbon inputs to the SOC pool. Forests (C) can have the
deepest rooting system, a high amount of soil C stock, greatest density of mineral-associated C, and
high rate of input of C to soils. Overall, organo-mineral association(s) and SOC pool is a function of
the “balance” of C inputs and outputs in the soil organic carbon “bank account.”
The SOC that exists in soil can be subdivided into “slow-cycling” and “fast-cycling” pools akin to
checking and savings accounts (Lavallee and Cotrufo, 2020), respectively. Slow-cycling C is either
mineral-associated C that is found physically protected in soil aggregates or chemically bound to the
surfaces of reactive soil minerals; both mechanisms restrict decomposition and associated losses of SOC,
allowing it to persist in soil for decadal to millennial time scales (Schmidt et al., 2011; Hemingway et
al., 2019). In contrast, fast-cycling C is more readily degradable and prone to physical transport in
shorter time scales (Schmidt et al., 2011; Hemingway et al., 2019). Fast C cycling, which is akin to
funds in a checking account, is critical for maintenance of life in soil, because decomposition is the
main mechanism that recycles nutrients needed by organisms that call the soil home (Janzen, 2006). Even
small, but sustained, deposits into the soil C savings account over time allow for long-term buildup of
C in the slow-cycling pool with significant potential for climate change mitigation.
Increasing urgency for addressing the global climate emergency demands that we reduce the release of
greenhouse gasses from burning of fossil fuels, while finding appropriate alternatives to draw down some
atmospheric carbon through soil carbon sequestration and other means. As we seek these solutions, it is
important to remember that decomposition of organic matter (i.e., withdrawal of some of the balance from
the soil carbon checking account) is a critical ecosystem process because decay of organic residue
provides essential nutrients for plants and microbes in soil (Janzen, 2006). For this reason, we cannot
expect zero withdrawals from the soil carbon bank and must figure out how we can continue to “invest” in
soil C to maximize its input and retention in the soil, thus preventing fast release of C as greenhouse
gasses to the atmosphere. Maintenance of soil health through “smart” management practices has been
proven to simultaneously achieve SOC sequestration and provision of clean air, water, and a functional
habitat (Billings et al., 2021; Kopittke et al., 2022). Here, we explore prevailing issues with
conventional soil management, vulnerability of SOC to loss in a changing world, and strategies to
alleviate climate-change impacts on soil resources. In this framework, we identify strategies for soil C
sequestration and ways to prevent “overspending” in an uncertain future marked by changing climate and
increased demands to ensure food and nutritional security of the growing human population.
Carbon Losses Due to Conventional Soil Use and Degradation
An increasing human population and onset of the industrial age led to an increased demand for food,
energy, and water resources, and overall intensification of the agricultural sector. With intensive
agricultural practices came large-scale degradation of the global soil resource that included increased
rates of soil erosion (i.e., loss from working lands) that outpaced new soil production by 1–2 order(s)
of magnitude, largely resulting from deforestation to clear land for agriculture, conventional tillage
practices, and overgrazing (Lal, 2004; Montgomery, 2007). Conventional land management practices cause
physical disturbance of soils and have historically promoted enhanced agricultural yields, to the
detriment of SOC content, topsoil thickness, and overall soil health and structural stability (Phillips
et al., 1980; Reganold et al., 1987; Amundson et al., 2015). The systematic exploitation and
modification of undisturbed soils has led to the resulting agricultural soils being dubbed
“domesticated,” lacking hallmark resilience of their wild predecessors (Amundson et al., 2015). Soil
domestication for agriculture also presents broader, associated ecosystem issues, such as diminished
biodiversity from engineered crop community monocultures, introduction of chemical pesticides to hydro-
and pedospheres, and the delivery of vast quantities of esp. nitrogen and phosphorus fertilizers to
coastal margins. Conservation tillage and organic farming have been proposed as alternative approaches
that enhance soil health and to limit unsustainable soil “mining” and associated SOC overspending
(Montgomery, 2007). Estimates maintain that tillage management, when paired with cropping systems, can
sequester 0.03–0.11 Pg C yr-1 (Follett, 2001). Despite these promising advances, human
civilization and associated changes in land use and land cover led to the loss of 120 Pg C in the upper
~2 m of soils since humans adopted agriculture, with the fastest rate of loss occurring in the past 200
years (Sanderman et al., 2018).
Land Use/Land-Use Change (LULUC) practices such as conventional agriculture, deforestation, and wetland
conversion contribute 10%–14% of overall anthropogenic greenhouse gas emissions (Paustian et al., 2016).
The SOC pools impacted by LULUC have the potential to release massive amounts of C to the atmosphere,
making the preservation of these environments critical to protect soil C from loss both by reducing
future releases of C from soil to the atmosphere (avoided fluxes) and promoting drawdown of C that is
already in the atmosphere (sequestration of atmospheric CO2). Deforestation was historically
practiced to clear land for agriculture, but also continues to occur due to urban development, logging,
and an increase in wildfire frequency and intensity. These activities can destabilize SOC, releasing
slow-cycling C stored even in deeper soil layers (Drake et al., 2019). This also lowers ecosystem
functions that SOC can provide, such as water retention and nutrient cycling (Veldkamp et al., 2020).
Similarly, histosols (wetland soils, including peatlands with no underlying permafrost) can play a
critical role because they make up only 1% of soils globally, yet contain a larger proportion of SOC
(179 Pg C, or ~12% of SOC in the upper 100 cm globally: Brady and Weil, 2017). This SOC accumulation can
be attributed to a lower rate of decomposition of SOC due to waterlogging and resultant limitation in
availability of free oxygen for the heterotrophic soil microorganisms that can otherwise effectively
decompose organic matter. Histosols have historically been targets for drainage and conversion to
high-yielding agricultural lands (Holden et al., 2004). Draining of histosols, due to atmospheric
warming and/or anthropogenic practices, can lead to rapid decomposition of SOC release to the atmosphere
(Couwenberg et al., 2011). Overall, the soil system stores large amounts of carbon, but it has continued
to experience rapid degradation due to human actions. However, adoption of climate-smart land management
practices has a clear potential to reduce the atmospheric CO2 burden and increase the amount
of carbon stored in the soil carbon bank, with multiple benefits for improving ecosystem health and
Vulnerability of SOC to Loss with Uncertain Future
Climate is a primary factor driving the rate of decomposition of SOC (Brady and Weil, 2017). Global
climate change can accelerate SOC losses due to increasing global atmospheric temperature, altered
precipitation patterns, and other changes (Bellamy et al., 2005; Walker et al., 2018). Warming often
increases the rate of microbial decomposition of SOC and subsequent CO2 efflux to the
atmosphere (Lloyd and Taylor, 1994; Lehmeier et al., 2013; Min et al., 2019). The effects of increasing
temperature on SOC losses vary with molecular complexity of SOC and environmental conditions (e.g.,
water limitation, aggregation, mineral association) (Davidson and Janssens, 2006). Complex SOC, with
high activation energy, is more sensitive to temperature than simple SOC (Lehmeier et al., 2013; Lefèvre
et al., 2014). The temperature sensitivity of protected, slow-cycling C has been less studied (Karhu et
al., 2019), which necessitates future studies that explore the relationship between slow-cycling C and
its sensitivity to environmental changes. Contrary to the positive relationship between temperature and
SOC decomposition rate, increases in water availability can increase (Kaiser et al., 2015; Min et al.,
2020) or decrease SOC decomposition (Freeman et al., 2001), depending on the systems of interest.
Precipitation can also indirectly affect SOC storage by inducing soil erosion, changes in pore
connectivity, and altering ecosystem structure (Pimentel et al., 1995; Smith et al., 2017; Wu et al.,
2018). In eroding landscapes, lateral distribution of topsoil C and its deposition in lower-lying
landform positions (Berhe et al., 2018) causes mixing of the relatively fast-cycling C with slow-cycling
C in deep soil layers.
The response of carbon stored in soil to climate change and other perturbations varies depending on the
nature of the soils and the type of change to the system (Berhe, 2019b). Here, we highlight how SOC will
respond to climate change using three important areas of concern and uncertainty (e.g., gelisols,
paleosols, and deep soil).
Gelisols are soils of very cold climate conditions and store ~1000 Pg C in the upper 3 m of active and
underlying layers of permafrost soils (Tarnocai et al., 2009; Hugelius et al., 2014). Gelisols have
accumulated C because of climate-driven slow decomposition rates (Ping et al., 2015; Turetsky et al.,
2020). Warming in the northern hemisphere is predicted to release 12.2–112.6 Pg C by 2100, according to
Representative Concentration Pathway 4.5 and 8.5 warming scenarios (IPCC, 2013). This huge uncertainty
in the projected C release in the northern hemisphere is partly due to considerable variability in
hydrology, soil conditions, and vegetation (McGuire et al., 2009; Schuur and Abbott, 2011; Ping et al.,
2015). The rapid destabilization of polar and high-altitude environments, often referred to as the most
sensitive barometers of climate change, serves as a benchmark for understanding anthropogenic
modifications to the global climate system.
Paleosols are soils that developed in different environmental conditions when topsoil was transported
downhill and buried by alluvial, colluvial, aeolian deposition, volcanic eruption, or human activities
over centuries to millennia (Marin-Spiotta et al., 2014; Chaopricha and Marin-Spiotta, 2014). This
process promotes SOC-mineral association(s) (Rumpel and Kögel-Knabner, 2011) that build up soil C stock
in the slow-cycling soil C savings account (Schmidt et al., 2011). Recent estimates suggest that
paleosol C is a significant global C reservoir (Lehmkuhl et al., 2016), but it is spatially variable
depending on landscape and climate history, thus making it difficult to estimate the total storage. The
effect of any environmental change on buried SOC is complex and poorly understood because paleosols are
not considered for the global C stock inventory and models. The possibility of the vast storage of SOC
raises questions on how the previously buried SOC will interact in the presence of water, modern soil
surface microbes, and addition of new fresh SOC, and finally if they will become a sink or a source of
greenhouse gasses in the presence of all the optimal conditions for decomposition.
The overwhelming majority of soil C studies have focused on shallow soil depths, with little attention
paid to the amount of C stored in or the vulnerability of C in deep soil layers. Soils can develop to
>10 m depth, and deep soils (below 30 cm) can store up to 74% of the total profile C with radiocarbon
ages of 5,000–20,000 years old (Moreland et al., 2021). It is estimated that 28 Pg C is stored in soils
with deep weathered bedrock, suggesting that deep soil C is a large C reservoir that may be potentially
vulnerable to a changing climate (Moreland et al., 2021). Some soils are already showing evidence of
warming by 2 °C, since 1961, which has been observed at up to 3 m depths (Zhang et al., 2016). Although
decomposition rates are slower in deeper soils than in surface soils, recent studies have shown that
deep SOC is more vulnerable to loss than previously thought (Rumpel and Kögel-Knabner, 2011; Hicks Pries
et al., 2017; Min et al., 2020). Experimental warming to a depth of 1 m found that warming increased
annual soil respiration by ~35% and estimated that with a 4 °C increase, deep soils have the potential
to release 3.1 Pg C yr-1, equivalent to 30% of fossil fuel emissions (Hicks Pries et al.,
2017; Friedlingstein et al., 2020).
In the following section, we focus on “working lands,” where the global soil degradation problem can be
effectively addressed (in a cost- and time-efficient manner) through a suite of natural climate change
Soils as Natural Climate Change Solutions
Intergovernmental Panel on Climate Change (IPCC) assessment reports and the Paris Agreement have
highlighted the importance of immediate action to prevent catastrophic changes to the earth system.
Inclusion of soils in local to global climate change mitigation strategies is a proven and
cost-effective strategy. Natural climate solutions can provide 37% of cost-effective CO2
mitigation necessary for a >66% chance of holding warming below 2 °C by 2030 (Griscom et al., 2017).
The “4 per 1000” effort has proposed soil as a natural climate change solution and endeavors to increase
SOC storage by 0.4% annually (Rumpel et al., 2020), thereby offsetting one third of global fossil fuel
emissions. Here, we provide a review of the available solutions to increase the amount of C stored in
the soil C savings account through a variety of land stewardship practices, including use of amendments
such as compost, biochar, waste, and management interventions such as reforestation, inclusion of deep
root perennials, and cover crops.
Restoring degraded lands and avoiding further land conversion (e.g., afforestation) can also help
mitigate climate change (Fig. 2; Table 1). Afforestation of degraded sites in the United States is
estimated to potentially sequester 2.43 Pg C yr-1 in the upper 30 cm of soil over 30 years
(Cook-Patton et al., 2020). Although afforestation efforts can increase SOC storage on decadal time
scales, the effects are largely site-specific. For example, depending on the prevailing climate of an
area, restoring grasslands might be a better option for C sequestration than
afforestation/reforestation, and converting grasslands to forest may yield less net SOC storage than
converting cropland to forest (Li et al., 2012; Bárcena et al., 2014). Soil restoration, specifically
for wetlands, has the potential to return these environments to a net C sink (Table 1; Waddington et
al., 2010) and represents a cost-efficient mitigation strategy—projected to cost ~US$20 per Mg of
sequestered C (Humpenöder et al., 2020).
Various management strategies in forested, agriculture/grassland, and wetland ecosystems exhibit
differing propensities to take up CO2
. Overall, these strategies represent a way to expand
terrestrial ecosystem uptake of carbon (Friedlingstein et al., 2020; Paustian et al., 2016; Griscom et
Regenerative agriculture (RA) also holds a substantial role in attaining negative carbon emissions from
rangeland and agricultural soils (Fig. 2; Table 1). RA is a set of locally adapted land practices that
minimize soil disturbance (e.g., no-till, minimum tillage, cover cropping) and losses (e.g., erosion,
degradation), while self-sustaining its ecosystem services (e.g., productivity, biodiversity;
Gonzalez-Sanchez et al., 2015) using agroecology-based theory and management (e.g., compost application,
crop, and grazing rotation, etc.). Hence, RA promotes C sequestration and soil health while
simultaneously reducing net SOC losses by providing a direct layer of protection from disturbance.
Ultimately, avoiding land conversion and disturbance a priori is the most effective strategy to maintain
SOC storage, as restoration of degraded lands accrues SOC slowly (Guo and Gifford, 2002). Both active
and preventative restoration practices are vital in providing ecosystem service co-benefits such as
water filtration and storage.
Land managers have added organic amendments to their soils since the early periods of agriculture. The
addition of C-rich amendments can improve soil health via enhancing nutrients and water storage, plant
productivity, microbial diversity, and soil structure (Woolf et al., 2010; Farooqi et al., 2018; Amelung
et al., 2020). Studies have now documented significant, positive impacts of organic amendments that
include a 2.3 Mg C ha-1 yr-1 increase in SOC stock in corn fields after six years
of biochar amendments (Blanco-Canqui et al., 2020), and a projected SOC sequestration potential of 1.2
Mg C ha-1 yr-1 in croplands after application of manure, sewage sludge, or straw
(Smith, 2004). In parts of the world that have large amounts of excess biomass (ex., agricultural
residue, manure, forest clippings, etc.), these amendments are viable options for climate change
mitigation (Fig. 2; Table 1), while at the same time replenishing C and nutrient stocks to increase the
ecosystem’s overall health and resilience (Koide et al., 2015).
Recent advances in plant-based strategies have also provided new insights to address net SOC loss. These
strategies rely on the ability of plants to self-regulate and self-optimize resource uptake and
allocation, and thus are considered cost-effective and sustainable with limited environmental
footprints. Plant roots are known to be a main source of SOC (Rasse et al., 2005), and root-derived SOC
is preferentially retained by minerals (Bird et al., 2008). Therefore, the introduction of roots into
deep soils can enhance slow-cycling C formation (Kell, 2011; Paustian et al., 2016). However, root
exudates enhance soil microbial activity and reduce SOC stock via priming (Fontaine et al., 2007;
Keiluweit et al., 2015). For this reason, plant roots are considered as a double-edged sword for SOC
formation (Dijkstra et al., 2021). Still, there is evidence that deeply rooting vegetation (esp.
perennial grasses) can sequester C into the deep soil (Slessarev et al., 2020). Extensive root systems
introduce C to the subsoil, enhancing SOC-mineral associations, aggregate protection, and reduced access
to SOC by soil microbes. In this manner, rhizosphere engineering benefits overall soil health and
resource use efficiency (Dessaux et al., 2016). With proper implementation, plant-based strategies can
synergize with existing strategies (e.g., conservation agriculture) to promote more SOC in the long-term
savings account (Fig. 2).
Soils have supported life and stored C throughout geological history. However, human civilization has
spurred drastic land use changes through agriculture and other activities. Additionally, profound
alteration to the global climate system has resulted from widespread fossil fuel utilization and
resulting greenhouse gas emissions. As we apply sophisticated models and propose novel technologies for
understanding and addressing anthropogenic climate change, a piece of the solution is found in the soil.
Natural climate change solutions involving soil health are not only cost effective, but also
non-negotiable, because they are key for securing the food, fuel, and fiber necessary for an
ever-increasing human population. Earth scientists, land managers, and policy makers must collaborate to
continue “spending” SOC while “investing” in SOC to increase its retention in the soil and maximize its
ability to support life. It’s a win-win climate solution that’s right beneath our feet. Let’s keep it
This article was supported by the National Science Foundation (EAR 1623812) and University of California
Merced and Falasco Endowed Chair to AAB; and University of California Merced Chancellor’s Postdoctoral
Fellowship and the National Research Foundation of Korea (MSIT, NRF-2018R1A5A7025409) to KM. Icons used
in Figure 1 made by Freepik and Flat Icons, from www.flaticon.com.
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