Soil organic matter (SOM) is the largest store of carbon (C) in terrestrial ecosystems, representing 2/3 of the total terrestrial C pool. The rates at which soils store or lose C could either mitigate or exacerbate atmospheric CO2 concentrations. New frameworks have recently been proposed regarding the mechanisms controlling the formation and persistence of SOM, which point to the role of both plant litter chemistry and soil C saturation deficit as major controls. However, these mechanisms have not been tested with a comprehensive approach. This study is the first to comprehensively analyze these newly proposed SOM formation and stabilization mechanisms. We incubated isotopically (13C and 15N) labeled litters with differing chemistries and tracked the fate of the litter-derived C and N as they formed SOM fractions characterized by different mechanisms of stabilization (i.e., inherent litter chemical recalcitrance or mineral-association) in two soils with contrasting degrees of C saturation. We hypothesized that labile litter components (i.e., hot water-extractable fraction) would be preferentially incorporated in microbial biomass and ultimately contribute more to mineral-associated SOM, in soils with a higher C saturation deficit. Whereas recalcitrant litter components (i.e., acid-unhydrolyzable fraction) would be used less efficiently by microbes and contribute more to CO2 respiration.
Results/Conclusions
After one year of incubation, independent of soil type, litter types with the highest proportion of hot-water extractable components were preferentially decomposed compared to litter types with the highest proportion of acid-unhydrolyzable components (35% and 54% mass remaining, respectively). However, the most recalcitrant litters produced more litter-derived CO2 , (332 mg C compared to 217 mg C) for both soil types, indicating a lower mass loss as dissolved organic carbon, and/or less efficient microbial biomass incorporation. Additionally, litter types with a greater proportion of labile litter components contributed 30% more to the mineral-associated SOM (MAOM) fraction than litter types with more recalcitrant components, supporting our hypothesis. However, this difference was only apparent in the soil type with a lower soil C saturation deficit, which was contrary to our hypothesis. So far, our results confirm the role of litter chemistry in determining pathways and efficiencies of new SOM formation, but less so that of mineral C saturation deficits.