Straw return reshapes soil organic carbon stability: A global meta-analysis

Straw return is widely adopted in croplands to promote soil organic carbon (SOC) sequestration and mitigate climate change. However, its replenishing effects on particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) are still not well-understood. Here, we conducted a meta-analysis to evaluate the responses of POC and MAOC to straw return, whilst the variations of other SOC pools were also examined to provide a comprehensive assessment of SOC dynamics [i.e., dissolved organic carbon (DOC), easily oxidizable organic carbon (EOC), microbial biomass carbon (MBC), and microbial necromass carbon (MNC)]. Results indicated that straw return significantly enhanced SOC pools compared to straw removal, with the most pronounced increases in POC (35.73%), EOC (41.17%), and MNC (35.59%). Straw return significantly decreased the MAOC/SOC ratio and increased EOC content, suggesting a decline in SOC stability. Linear regression analyses indicated that POC and MAOC accumulation decreased with increasing initial soil total nitrogen (TN) and mean annual precipitation (MAP). Shapley additive explanation (SHAP) analyses further revealed that initial soil TN, sand, and silt contents were key factors regulating POC responses to straw return. Specifically, straw return tended to suppress POC accumulation under conditions of high initial soil TN or sand contents, with corresponding thresholds of 1.57 g kg^-1 and 24%, respectively. Meanwhile, initial soil pH, straw return duration, and potential evapotranspiration (PET) were the primary drivers of MAOC accumulation. Straw return preferentially promoted MAOC accumulation when soil pH exceeded 8.01 or straw return duration was longer than six years. Further structural equation modeling revealed that MAOC played a dominant role in SOC sequestration compared to POC under straw return conditions, whereas straw-induced POC increases were strongly dependent on environmental conditions. Collectively, this study highlights that straw return preferentially increases the POC pool, which may undermine its contribution to SOC sequestration. Therefore, integrating additional agricultural practices may represent a promising approach to enhance the efficacy of straw return for SOC sequestration, warranting further investigation in the future.