Abstract: 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.

Key words: Grassland degradation, Soil aggregates, Soil organic carbon, Humid region

摘要： 草地退化是影响土壤结构稳定性与碳循环过程的重要驱动因素，但湿润气候条件下其作用路径及调控机制仍缺乏定量解析。本研究以湖南南山牧场典型山地草甸为对象，基于退化梯度（ND，LD，MD，HD），系统分析了0~30 cm土层土壤团聚体结构及有机碳分布特征。结果表明，随着退化程度加剧，>5 mm大团聚体含量下降约47%，<0.25 mm微团聚体含量增加约168%，机械稳定性和水稳性团聚体的平均重量直径（MWD）和几何平均直径（GMD）分别降低约46%，53%，分形维数显著升高，表明土壤结构由稳定态向破碎态转变。大团聚体破碎释放碳，其中一部分被微团聚体吸附封存，导致碳在团聚体中的分布由“大团聚体主导”向“微团聚体主导”转变。草地退化同时引起土壤有机碳总量下降与碳结构性重组，显示土壤团聚体在碳保护和分配中的关键作用。本研究揭示了湿润区草地退化驱动的碳库变化及其结构性机制，为退化草地生态修复与碳汇管理提供了理论依据。

关键词: 草地退化, 土壤团聚体, 土壤有机碳, 湿润区