Under the context of global climate change， the influence mechanism of grassland degradation on soil carbon cycling remain poorly understood. In this study， we investigated 15 paired degraded and non-degraded grassland sites on the Qinghai-Tibet Plateau， integrated microcosm incubation， metagenomics and metabolomics to assess the impacts of degradation on key carbon cycling processes， organic carbon fractions， microbial functional genes， and metabolic intermediates. The results showed that grassland degradation significantly reduced labile carbon pools， with particulate organic carbon， dissolved organic carbon， and microbial biomass carbon decreasing by 48.21%， 30.87%， and 20.21%， respectively. Meanwhile， soil organic carbon mineralization increased by 56.71%， and carbon accumulation efficiency declined by 32.51%. Degradation also led to a decrease in the diversity of carbon cycling functional genes （-2.95%， P<0.05）， while the abundance of starch-degrading genes and carbohydrate-related metabolites increased significantly （by 9.39% and 49.6%， respectively）. These changes indicated that intensified microbial carbon limitation and a shift in metabolic strategy from synthesis to decoomposition. This study elucidates the key mechanisms underlying soil carbon cycling responses to grassland degradation from a multi-factor coupling perspective， revealing that environmental factors and microbial traits jointly regulated carbon cycling， with microbial functional genes and metabolic intermediates playing a dominat role.

Soil carbon cycling  /  Alpine grassland  /  Qinghai-Xizang Plateau  /  Grassland degradation  /  Functional genes  /  Microbial metabolism