CONTEXT Straw return is widely used to sustain yields, mitigate climate change, and increase soil fertility. However, regional differences in climate, soil, and management lead to divergent magnitudes of soil organic carbon (SOC) responses, while the underlying mechanisms remain unclear. OBJECTIVE This study integrates meta-analysis and structural equation modelling to assess topsoil (0-20 cm) SOC responses to straw return across mainland China. METHODS K-Means clustering and random forest models were further used to simulate optimal straw carbon input strategies and SOC sequestration potential under future climate scenarios. RESULTS AND CONCLUSIONS In arid Northwest China, increasing precipitation raised SOC retention, whereas excessive rainfall in the South and Northeast suppressed C accrual. Alkaline soils (pH > 8) sequestered more SOC than acidic ones due to slower straw decomposition and Ca2+ stabilization. Larger straw inputs and longer durations generally increased SOC, though with diminishing marginal benefits. Excessive nitrogen fertilization reduced SOC gains by accelerating acidification and decomposition. Carbon sequestration was region-specific. The highest cumulative increase occurred in the Northeast (1.4 t ha(-1)) under moderate inputs (3.6 t ha(-1) yr(-1) for 11 years), followed by the South, where high inputs and long durations (4.7 t ha(-1) yr(-1) for 19 years) yielded 0.83 t ha(-1). In contrast, gains were lower in the North and Northwest (similar to 0.40 t ha(-1)) despite extended durations or optimized practices. SOC sequestration in the South would increase by 10% through amelioration of soil acidity.