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 (<em>C-acq</em>) EEAs and their relationships with soil carbon pools in agricultural ecosystems. On average, straw returning increased <em>C-acq</em> 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 <em>C-acq</em> EEAs: when straw returning strongly stimulates <em>C-acq</em> EEAs, MBC content and SOC storage increase; conversely, when straw returning weakly stimulates <em>C-acq</em> EEAs, MBC content and SOC storage decline. Furthermore, soil pH and straw amount best predicted the straw returning effects on <em>C-acq</em> EEAs. Our results provide the first evidence that <em>C-acq</em> 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.

秸秆还田是农业土壤固碳的有效途径。然而，土壤胞外酶活性（extracellular enzyme activities, EEAs）对秸秆还田的响应及其与土壤碳库的关联仍未得到充分解析。本研究针对农业生态系统中秸秆还田对土壤水解性碳获取型（C-acq）胞外酶活性的影响，及其与土壤碳库的关联开展了一项全球元分析（meta-analysis）。平均而言，相较于不秸秆还田处理，秸秆还田可使碳获取型胞外酶活性提升23.2%，土壤微生物量碳（microbial biomass carbon, MBC）含量提升29.2%，土壤有机碳（soil organic carbon, SOC）储量提升21.1%。秸秆还田对土壤微生物量碳含量与土壤有机碳储量的影响，可通过二者与碳获取型胞外酶活性的正向关联得到最佳阐释：当秸秆还田强烈刺激碳获取型胞外酶活性时，土壤微生物量碳含量与土壤有机碳储量随之升高；反之，当秸秆还田对碳获取型胞外酶活性的刺激较弱时，土壤微生物量碳含量与土壤有机碳储量则会下降。此外，土壤pH值与秸秆还田用量可最佳预测秸秆还田对碳获取型胞外酶活性的调控效应。本研究首次证实，在农业生态系统中，秸秆还田条件下碳获取型胞外酶活性与土壤碳储量提升存在显著关联。将本研究揭示的秸秆还田对农业土壤的微生物效应纳入地球系统模型（Earth system models），可优化土壤碳动态的预测精度。