The effects of grazing exclusion on soil organic carbon sequestration in Chinese grasslands: nonlinear responses, threshold effects, and driving mechanisms

Grazing exclusion, as a primary management measure for restoring degraded grasslands in China, is widely regarded as an effective approach to enhancing soil organic carbon (SOC) storage. However, it remains unclear whether SOC accumulation shows a temporal threshold effect following grazing exclusion. Based on 519 paired observations from grassland ecosystems across China, this study employed a meta-analysis to investigate the temporal dynamics of SOC accumulation after grazing exclusion and to identify the key influencing factors. The results showed that grazing exclusion significantly promoted SOC accumulation, with an average increase of approximately 25%. Specifically, SOC increased by 61.12% in typical and by 31.65% in desert grassland, whereas changes in meadow grassland were not significant. With increasing years of grazing exclusion, the SOC accumulation rate initially increased and then decreased, reaching a peak at 16 years of exclusion. The study further revealed significant nonlinear relationships between SOC accumulation rate and initial SOC content, mean annual temperature (MAT), mean annual precipitation (MAP), and elevation, with clear threshold turning points. Boosted regression tree (BRT) analysis indicated that site-specific soil properties, including initial SOC, total nitrogen, and total phosphorus, were the dominant drivers of SOC changes following grazing exclusion, followed by climatic factors (MAT and MAP). However, their relative importance varied among different soil depths and grassland types. Overall, the positive effect of grazing exclusion on grassland SOC is jointly regulated by a dynamic trade-off between nutrient limitation and carbon saturation effects. By identifying the nonlinear relationships and threshold turning points in SOC accumulation and its key influencing factors following grazing exclusion, this study provides important scientific evidence for optimizing grazing exclusion duration and developing site-specific grassland restoration and carbon sequestration management strategies.