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.