Grassland degradation is a major driver affecting soil structure stability and carbon cycling processes； however， its underlying pathways and regulatory mechanisms under humid climatic conditions remain insufficiently quantified. In this study， a typical montane meadow in Nanshan Pasture， Hunan Province， China， was selected to investigate the characteristics of soil aggregate structure and organic carbon distribution across a degradation gradient， including non-degraded （ND）， lightly degraded （LD）， moderately degraded （MD）， and heavily degraded （HD） grasslands within the 0-30 cm soil layer. The results showed that， with increasing degradation intensity， the proportion of >5 mm macroaggregates decreased by approximately 47%， whereas the proportion of <0.25 mm microaggregates increased by approximately 168%. Meanwhile， the mean weight diameter （MWD） and geometric mean diameter （GMD） of both mechanically stable and water-stable aggregates declined by approximately 46% and 53%， respectively， accompanied by a significant increase in fractal dimension， indicating a transition of soil structure from a stable state to a fragmented state. The breakdown of macroaggregates released organic carbon， part of which was subsequently adsorbed and stabilized by microaggregates， resulting in a shift in carbon distribution from a “macroaggregate-dominated” pattern to a “microaggregate-dominated” pattern. Grassland degradation also led to a decline in total soil organic carbon and induced substantial structural reorganization of soil carbon pools， highlighting the pivotal role of soil aggregates in carbon protection and redistribution. These findings elucidate the structural mechanisms underlying carbon pool dynamics driven by grassland degradation in humid regions and provide a theoretical basis for ecological restoration and carbon sequestration management of degraded grasslands.

Grassland degradation  /  Soil aggregates  /  Soil organic carbon  /  Humid region