Characterizing relationships between leaf litter decomposability and soil organic carbon stability: Does rapid decomposition lead to stable soil organic carbon?
Two observations have led soil organic carbon (SOC) researchers to hypothesize that, over time, labile leaf litter leads to greater amounts of stabilized SOC formation compared to recalcitrant leaf litter. First, labile substrates maximize microbial growth and, second, microbial biomass is a primary precursor for the formation of mineral-organic associations which can protect SOC for decades or centuries. Therefore, we predicted that soils dominated by plants with labile litter should store more SOC in protected forms than soils dominated by plants with recalcitrant litter. In central hardwood forests, all trees associate with either arbuscular mycorrhizal (AM) or ectomycorrhizal (ECM) fungi. Relative to ECM-associated trees, AM-associated trees tend to have rapidly decomposing leaf litter. Thus, we expected the amount of protected SOC to increase with the dominance of AM-associated trees. Using a spatially explicit approach, we sampled soil from a forest mega-plot in South-Central Indiana where more than 28,000 trees had been identified, measured, and geo-referenced. We then used density fractionation to separate SOC into “protected” (>1.65 g/cm3) and “unprotected” (<1.65 g/cm3) pools.
Overall, total SOC stocks were positively, but marginally, associated with ECM dominance in shallow soils (0-5 cm) and unrelated to AM or ECM dominance in deeper soils (5-15 cm). These differences were largely driven by a strong positive relationship with unprotected SOC. Compared to AM-dominated soils, ECM-dominated soils had 145% more unprotected SOC in shallow soils and 34% more unprotected SOC in deeper soils. Because the unprotected pool isolated from density fractionation is generally considered to be plant-derived, these results suggest that the selective preservation of recalcitrant plant litter is a primary mechanism of SOC storage in ECM-dominated soils. In contrast, AM dominance was positively associated with protected SOC, but only in deeper soils where AM-dominated soils stored 64% more SOC than ECM -dominated soils. Collectively, our results suggest that although AM-dominated soils store less total C in surface soils, a greater fraction of this C is stored in protected forms and may therefore be less susceptible to environmental changes.