Introduction:
Managing soils in agricultural systems to sequester carbon (C) in soil
organic matter (SOM) may be a powerful approach to offset anthropogenic
C emissions (Lal, 2004). Soils are the largest terrestrial C pool, and
experimental manipulations like changing vegetation type, increasing
organic inputs, or altering management practices demonstrate the
potential for significant and rapid SOM accumulation (Minasny et al.,
2017; Paustian et al., 2016). However, there is a high degree of
uncertainty in understanding, predicting, and optimizing soil C
accumulation (Sulman et al., 2018). Much of this uncertainty arises
because plant roots and soil microbes, the active drivers of soil
biogeochemical cycling, both build and deplete SOM through
simultaneously occurring processes. As such, our ability to optimize
soil C sequestration relies on improving our understanding of how roots
and microbes drive the transfer of new litter C inputs into SOM.
As per the current understanding of SOM formation, litter inputs are
decomposed into simpler compounds that can be physically protected from
microbial decomposers by occlusion in soil aggregates or sorption to
mineral surfaces (Lehmann & Kleber, 2015). As such, SOM is often
delineated into three main pools (Fig. 1a): undecomposed or
partially-decomposed particulate organic matter (here, light POM),
aggregate-occluded SOM (here, heavy POM), and mineral associated organic
matter (MAOM) (Lavallee et al., 2020). Light POM accumulation depends
upon the balance between litter inputs to soil and litter decomposition,
and can accumulate with no apparent upper limit but is also vulnerable
to factors like warming that enhance decomposition rates (Benbi et al.,
2014; Cotrufo et al., 2019). Heavy POM is operationally separated from
light POM by density fractionation and is linked with stable soil
aggregates (Lavallee et al., 2020). Accumulation in this pool may
saturate and is vulnerable to factors like soil disturbance and land use
change (Bronick & Lal, 2005). MAOM is generally considered to be the
most persistent or protected form of SOM (Cotrufo et al., 2013; Liang et
al., 2017). However, optimizing MAOM accumulation may only be practical
in soils like those in degraded agricultural ecosystems that have lost
nearly 50% of their C since ploughing the prairie (Stockmann et al.,
2015) as MAOM accumulation appears to saturate (Cotrufo et al., 2019;
but see Georgiou et al., 2022). To manage ecosystems for soil C
sequestration, it is critical to understand what drives the transfer of
new litter inputs between these SOM pools to enhance our predictive
understanding of how much soil C can accumulate and how persistent this
soil C may be in a changing climate.