PNNL

Abstract

Grassland soils store a substantial proportion of the global soil carbon (C) stock. The transformation of C in grassland soils with respect to phase (solid, dissolved, gas), chemical composition, and persistence strongly regulate the predicted terrestrial atmosphere C flux in global C biogeochemical cycling models. In addition, increasing atmospheric nitrogen (N) deposition alters C chemistry in grassland soils. However, there remains controversy about the importance of mineralogical versus biochemical stabilization of soil C, as well as uncertainty regarding how grassland soil C chemistry responds to elevated N. This study posited that soil organic matter (SOM) decomposition in grassland soil was constrained by both chemical recalcitrance and mineral associations resulting in a predictable pattern of C accumulation, by which grassland soils with high recalcitrant C content and/or short-range order minerals had high richness of organic compounds. Grassland soils from diverse locations were used in an 8-month aerobic incubation experiment to evaluate whether the chemical composition of SOM converged across sites over time, and how SOM persistence responded to N addition. Although the presence of O2 removed the thermodynamic constraint for microbial decomposition of SOM, the chemical composition of SOM shifted over the incubation such that labile compounds, such as carbohydrates, proteins, and lipids, were depleted at some sites, except those with higher abundance of ferrihydrite. Recalcitrant compounds, such as lignin, tannin, and condensed aromatics, were decomposed, and the composition of organic compounds that persisted after the 8-month incubation converged across unfertilized sites. However, N addition inhibited the cumulative C respired and the convergence of chemical composition across ecosystems. This study advanced our understanding of the dynamic biogeochemical cycling of C among diverse grassland ecosystems by demonstrating that both chemical recalcitrance and mineral-OM association simultaneously contribute to C persistence.