Mineral‐Associated Organic Carbon Dominates Soil Organic Carbon Sequestration Under Increasing Grazing Intensity

ABSTRACT Soil organic carbon (SOC) sequestration is vital for maintaining the function of grassland ecosystems under grazing conditions, yet the relationship between different carbon fractions and carbon sequestration remains unclear. This study investigated SOC dynamics under a 5‐year controlled grazing gradient (ranging from grazing exclusion to extremely heavy grazing) in the Qilian Mountains, China. It aimed to elucidate the variations in SOC and its fractions and further discuss the mechanisms of SOC sequestration under grazing. The results showed that with increasing grazing intensity, SOC and all carbon fractions exhibited a significant declining trend, except for dissolved organic carbon (DOC). The decline in SOC was directly influenced by aboveground biomass and available phosphorus. Furthermore, organic carbon dynamics were regulated by the mineral protection mechanism represented by particulate organic carbon (POC) and mineral‐associated organic carbon (MAOC), whereas other carbon fractions did not exhibit significant effects. However, Partial Least Squares Structural Equation Modeling (PLS‐SEM) revealed distinct regulatory pathways for the two; specifically, the reduction in POC induced by changes in vegetation characteristics was a significant cause of the SOC decline. In contrast, MAOC, serving as the primary reservoir for carbon sequestration, was mainly regulated by soil factors. Overall, the insensitivity of MAOC to grazing pressure rendered it the most stable carbon fraction. Meanwhile, the contribution of MAOC to long‐term carbon preservation exceeded that of POC, highlighting the critical significance of stable organic carbon for the long‐term retention of SOC. These findings suggest that maintaining grazing at moderate levels can preserve the labile organic carbon inputs required for nutrient cycling, while the stable carbon fraction (MAOC) ensures the long‐term carbon sink function of alpine meadows.