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can provide, such as water retention and its sensitivity to environmental changes. This process promotes SOC-mineral asso-
nutrient cycling (Veldkamp et al., 2020). Contrary to the positive relationship between ciation(s) (Rumpel and Kögel-Knabner,
Similarly, histosols (wetland soils, including temperature and SOC decomposition rate, 2011) that build up soil C stock in the slow-
peatlands with no underlying permafrost) increases in water availability can increase cycling soil C savings account (Schmidt et
can play a critical role because they make up (Kaiser et al., 2015; Min et al., 2020) or al., 2011). Recent estimates suggest that
only 1% of soils globally, yet contain a larger decrease SOC decomposition (Freeman et paleosol C is a significant global C reser-
proportion of SOC (179 Pg C, or ~12% of al., 2001), depending on the systems of voir (Lehmkuhl et al., 2016), but it is spa-
SOC in the upper 100 cm globally: Brady interest. Precipitation can also indirectly tially variable depending on landscape and
and Weil, 2017). This SOC accumulation affect SOC storage by inducing soil erosion, climate history, thus making it difficult to
can be attributed to a lower rate of decom- changes in pore connectivity, and altering estimate the total storage. The effect of any
position of SOC due to waterlogging and ecosystem structure (Pimentel et al., 1995; environmental change on buried SOC is
resultant limitation in availability of free Smith et al., 2017; Wu et al., 2018). In erod- complex and poorly understood because
oxygen for the heterotrophic soil microor- ing landscapes, lateral distribution of top- paleosols are not considered for the global C
ganisms that can otherwise effectively soil C and its deposition in lower-lying land- stock inventory and models. The possibility
decompose organic matter. Histosols have form positions (Berhe et al., 2018) causes of the vast storage of SOC raises questions
historically been targets for drainage and mixing of the relatively fast-cycling C on how the previously buried SOC will
conversion to high-yielding agricultural with slow-cycling C in deep soil layers. interact in the presence of water, modern
lands (Holden et al., 2004). Draining of The response of carbon stored in soil to soil surface microbes, and addition of new
histosols, due to atmospheric warming climate change and other perturbations var- fresh SOC, and finally if they will become a
and/or anthropogenic practices, can lead ies depending on the nature of the soils and sink or a source of greenhouse gasses in
to rapid decomposition of SOC release to the type of change to the system (Berhe, the presence of all the optimal conditions
the atmosphere (Couwenberg et al., 2011). 2019b). Here, we highlight how SOC will for decomposition.
Overall, the soil system stores large respond to climate change using three
amounts of carbon, but it has continued to important areas of concern and uncertainty Deep Soil
experience rapid degradation due to human (e.g., gelisols, paleosols, and deep soil). The overwhelming majority of soil C
actions. However, adoption of climate- studies have focused on shallow soil depths,
smart land management practices has a clear Gelisols with little attention paid to the amount of C
potential to reduce the atmospheric CO Gelisols are soils of very cold climate stored in or the vulnerability of C in deep
2
burden and increase the amount of carbon conditions and store ~1000 Pg C in the upper soil layers. Soils can develop to >10 m
stored in the soil carbon bank, with multiple 3 m of active and underlying layers of per- depth, and deep soils (below 30 cm) can
benefits for improving ecosystem health and mafrost soils (Tarnocai et al., 2009; Hugelius store up to 74% of the total profile C with
human welfare. et al., 2014). Gelisols have accumulated C radiocarbon ages of 5,000–20,000 years old
because of climate-driven slow decomposi- (Moreland et al., 2021). It is estimated that
VULNERABILITY OF SOC TO LOSS tion rates (Ping et al., 2015; Turetsky et al., 28 Pg C is stored in soils with deep weath-
WITH UNCERTAIN FUTURE 2020). Warming in the northern hemisphere ered bedrock, suggesting that deep soil C
Climate is a primary factor driving the is predicted to release 12.2–112.6 Pg C by is a large C reservoir that may be poten-
rate of decomposition of SOC (Brady and 2100, according to Representative Con- tially vulnerable to a changing climate
Weil, 2017). Global climate change can centration Pathway 4.5 and 8.5 warming sce- (Moreland et al., 2021). Some soils are already
accelerate SOC losses due to increasing narios (IPCC, 2013). This huge uncertainty showing evidence of warming by 2 °C, since
global atmospheric temperature, altered in the projected C release in the northern 1961, which has been observed at up to 3 m
precipitation patterns, and other changes hemisphere is partly due to considerable depths (Zhang et al., 2016). Although decom-
(Bellamy et al., 2005; Walker et al., 2018). variability in hydrology, soil conditions, and position rates are slower in deeper soils than
Warming often increases the rate of micro- vegetation (McGuire et al., 2009; Schuur and in surface soils, recent studies have shown
bial decomposition of SOC and subsequent Abbott, 2011; Ping et al., 2015). The rapid that deep SOC is more vulnerable to loss
CO efflux to the atmosphere (Lloyd and destabilization of polar and high-altitude than previously thought (Rumpel and Kögel-
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Taylor, 1994; Lehmeier et al., 2013; Min et environments, often referred to as the most Knabner, 2011; Hicks Pries et al., 2017; Min
al., 2019). The effects of increasing temper- sensitive barometers of climate change, et al., 2020). Experimental warming to a
ature on SOC losses vary with molecular serves as a benchmark for understanding depth of 1 m found that warming increased
complexity of SOC and environmental con- anthropogenic modifications to the global annual soil respiration by ~35% and estimated
ditions (e.g., water limitation, aggregation, climate system. that with a 4 °C increase, deep soils have the
mineral association) (Davidson and Janssens, potential to release 3.1 Pg C yr , equivalent
–1
2006). Complex SOC, with high activation Paleosols to 30% of fossil fuel emissions (Hicks Pries
energy, is more sensitive to temperature Paleosols are soils that developed in dif- et al., 2017; Friedlingstein et al., 2020).
than simple SOC (Lehmeier et al., 2013; ferent environmental conditions when top- In the following section, we focus on
Lefèvre et al., 2014). The temperature sensi- soil was transported downhill and buried by “working lands,” where the global soil
tivity of protected, slow-cycling C has been alluvial, colluvial, aeolian deposition, vol- degradation problem can be effectively
less studied (Karhu et al., 2019), which canic eruption, or human activities over addressed (in a cost- and time-efficient
necessitates future studies that explore the centuries to millennia (Marin-Spiotta et al., manner) through a suite of natural climate
relationship between slow-cycling C and 2014; Chaopricha and Marin-Spiotta, 2014). change solutions.

6 GSA TODAY | May 2022