Decadal persistence of grassland soil organic matter derived from litter and pyrogenic inputs

Abstract

The stabilization of carbon (C) and nitrogen (N) from organic inputs in soil organic matter constitutes a critical process in ecosystem biogeochemistry, yet the underlying mechanisms are not yet fully understood. Several frameworks have been proposed to explain particulate- and mineral-associated organic matter persistence, but a lack of long-term data has stymied their reconciliation. Here we present the results of an in-field incubation in a grassland in Kansas, USA, that followed 13C- and 15N-labelled plant litter and pyrogenic organic matter through the decomposition process and into soil organic matter fractions over the course of a decade. At the end of the experiment, 7.0% and 24.2% of the initial litter C and N, respectively, remained in the soil, while 60.8% and 54.4% of the initial pyrogenic organic matter C and N, respectively, remained. Litter-derived mineral-associated organic matter formed within the first year of litter decomposition, and 10-year sampling revealed that it had persisted relatively unchanged, in terms of both litter-derived C stocks and C:N ratio. These results provide further evidence that mineral-associated organic matter is stabilized via the sorption of soluble inputs and suggest that stabilization and persistence can occur largely independent of particulate organic matter dynamics. Plant litter-derived mineral-associated organic matter that formed in the first year and pyrogenic organic inputs both persist on a decadal scale in grassland soil via distinct mechanisms, according to a soil organic matter decomposition experiment.