All Is Not Lost! Contribution of Above-Ground Litter Mass Loss to the Formation of Soil Organic Matter.

Soil organic matter (SOM) is formed through the partial decomposition and transformation of plant litter. While studies have traditionally focused on the understanding of how climate, litter quality, and decomposer community composition affect the rate at which plant litter decomposes, we argue that litter mass loss rates are of little importance to the understanding of SOM formation and the long-term soil C and N balance. Rather, what really matters is the proportion of litter mass 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 litter enters 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 field decomposition experiment using 13C and 15N labelled Andropogon gerardii litter 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 SOM fractions, 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 (%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, due to their chemical recalcitrance. We observed the highest litter-C mineral stabilization efficiency during the early stages of decomposition when the most labile litter components are lost.