Soil microbial necromass shapes global carbon stocks in agricultural and natural ecosystems

Abstract. Soil carbon (C) plays an essential role in regulating global C cycle and climate. Microbial necromass is an important component of soil C, and yet the relative contribution of microbial necromass in shaping the global C stocks in agricultural and natural ecosystems worldwide remains virtually unknown. In this study, we compiled data on fungal and bacterial necromass along with soil organic carbon (SOC) from the 0–20 cm soil layer across 486 study sites (145 agricultural and 341 natural ecosystems) to evaluate the relative contribution of fungal necromass C (FNC) and bacterial necromass C (BNC) to SOC and the FNC/BNC ratio, after accounting for other biotic and abiotic factors. Our results indicated that, in both agricultural and natural ecosystems, the contribution of FNC to SOC significantly exceeded that of BNC, with FNC contributing approximately twice as much as BNC to SOC. However, the contributions of FNC and BNC to SOC were markedly higher in agricultural ecosystems than those in natural ecosystems, with a contrasting trend in the FNC/BNC ratio. Soil physicochemical properties (C/N and clay) were the most important predictors of the contributions of FNC and BNC to SOC in both ecosystems, while geographical factor (elevation) was the most important predictor of the FNC/BNC ratio. Our study enhances the current level of understanding regarding microbially mediated biogeochemical cycling and SOC dynamics, underscoring the critical role of microbial necromass in the global C cycle.