Straw Return Drives the Differential Responses of Soil Organic Carbon Fractions Regulated by Initial Soil Carbon Pool Status

Straw return is a key agronomic practice for enhancing soil organic carbon (SOC) storage in agricultural ecosystems. However, the mechanisms governing its allocation between labile and stable carbon fractions, as well as the environmental thresholds regulating this process, remain inadequately understood. This knowledge gap constrains the accurate assessment and optimization of its regional carbon sequestration potential. To address this, the present study synthesizes data from a global meta‐analysis of 102 published studies and three located long‐term field experiments, including 303 paired observations for SOC, 145 for DOC, 134 for MBC, 102 for POC and 96 for MAOC, to systematically quantify the effects of straw return on SOC fractions and identify their key controlling factors. Results show that straw return increased SOC by an average of 9.8%, with distinct responses observed among different carbon fractions. Among the labile fractions, POC exhibited the highest increase (27.0%), while MAOC showed a more modest gain of 9.5%. This pattern reflects a dynamic characterized by rapid turnover of labile components versus slow accrual of stable carbon. Path analysis confirmed that MAOC was the primary contributor to the net increase in SOC following straw return, with POC being secondary. Random forest analysis further indicated that experimental duration and straw return amount were the major drivers of SOC, mean annual temperature and mean annual precipitation were the key predictors of POC and experimental duration was the dominant factor controlling MAOC. Straw application rate significantly influenced DOC, whereas MBC was primarily regulated by inherent soil properties. The study also identified a threshold effect of initial SOC content, with a critical value of approximately 8.93 g/kg. When the initial soil SOC is below this threshold, straw return is more conducive to the accumulation of POC and is accompanied by an increase in MAOC, demonstrating high carbon sequestration potential. In contrast, in carbon‐rich soils approaching saturation, the carbon sequestration effect of straw return was significantly attenuated. In summary, the carbon sequestration efficacy of straw return is manifested not only in the increase of total SOC but, more critically, in the coordinated accumulation of labile and stable fractions. This process is constrained by the interactive effects of climate, initial soil carbon status and management practices. Future cropland carbon management should integrate the compositional dynamics of SOC fractions and the threshold behaviours of key drivers to develop regionally tailored straw return strategies, thereby maximizing soil carbon sequestration potential.