Soil carbon increase with straw returning: New evidence from soil extracellular enzymes

Straw returning is an effective practice to sequester soil carbon in agricultural soils. However, the response of soil extracellular enzyme activities (EEAs) to straw returning and their linkages to soil carbon pools remain poorly understood. Here, we present a global meta-analysis on the effect of straw returning on soil hydrolytic carbon-acquiring (C-acq) EEAs and their relationships with soil carbon pools in agricultural ecosystems. On average, straw returning increased C-acq EEAs by 23.2%, soil microbial biomass carbon (MBC) content by 29.2%, and soil organic carbon (SOC) storage by 21.1%, relative to no straw returning. The effect of straw returning on MBC content and SOC storage is best explained by their positive relationships with C-acq EEAs: when straw returning strongly stimulates C-acq EEAs, MBC content and SOC storage increase; conversely, when straw returning weakly stimulates C-acq EEAs, MBC content and SOC storage decline. Furthermore, soil pH and straw amount best predicted the straw returning effects on C-acq EEAs. Our results provide the first evidence that C-acq EEAs are linked to soil carbon increases under straw returning in agricultural ecosystems. Incorporating the soil microbial effects of straw returning revealed here on agricultural soils into Earth system models could improve predictions of soil carbon dynamics.