Above-Ground Litter-Derived C and N Stabilization in SOM: The Key Role of Dissolved Organic Matter Leaching and Litter Fragmentation

Soil organic matter (SOM) is formed through the partial decomposition and transformation of plant organic matter (OM) inputs. While much is known about how climate, litter chemistry, and decomposer community composition affect the rate at which plant inputs are decomposed, we argue that mass loss rates are of little importance to long-term net soil C and N balance. Rather, what really matters is the proportion of plant OM inputs that is eventually incorporated into SOM and further stabilized by spatial inaccessibility or through interactions with minerals versus the amount which is mineralized. Above ground plant inputs enter the soil in the form of dissolved organic matter (DOM) and litter fragments, yet we have very little data on those two fluxes and how DOM and litter fragments contribute to SOM formation and stabilization. Through an integrated set of laboratory and modelling work and a long-term field decomposition experiment using 13C and 15N labelled plant OM we are elucidating the controls of DOM versus CO2 production during decomposition and the fate and stabilization of DOM and litter fragments in soil, and soil fragments, to a 20 cm depth. Our work shows that during the early stages of decomposition, a sizable fraction of litter C is lost to the soil in the form of DOM, and that this flux is largely controlled by the initial litter chemistry (i.e., %N and lignocellulose index). This DOM appears to be fast transformed by microbes and to stabilize on mineral particles. Later, most of the litter enters the soil in the form of litter fragments, recovered as light fraction in the soil, where they accumulate, likely due to their chemical recalcitrance. In agreement with the Microbial Efficiency Mineral Stabilization framework we observed the highest litter-C mineral stabilization efficiency during the early stages of decomposition when the most labile litter components are lost.