Linking soil organic carbon mineralization to microbial interactions during alpine grassland degradation

Grassland degradation induces substantial losses of soil organic carbon (SOC), which are closely associated with the activities and interactions of various soil microorganisms. However, the response of microorganisms and their interactions to degradation and their relationships with SOC dynamics remain unclear. Here, we established four distinct grassland degradation transects on the Qinghai-Xizang Plateau to investigate the effects of grassland degradation on soil microorganisms (including bacteria, fungi, and protists) and reveal the relationships between microbial network, microbial metabolic activity, and SOC mineralization dynamics. Our findings revealed that grassland degradation increased SOC cumulative mineralization rates by 4.5-47 % and individual microbial metabolic capacity by 10 % to threefold, evidenced by higher cumulative CO₂ release per unit SOC and enzyme activity per unit microbial biomass. Grassland degradation increased microbial network complexity, with fungal and cross-kingdom network complexity increasing substantially while bacterial and protistan network complexity decreased. This increase in fungal network complexity was likely driven by community assembly processes and intensified niche overlap among fungi. The enhanced fungal and cross-kingdom interactions may indicate more efficient microbial metabolic networks, thereby accelerating SOC decomposition and loss. Using enhanced microbial network complexity and microbial metabolic capacity as diagnostic indicators can help assess the risk of SOC loss and degradation severity in grassland ecosystems, ultimately improving our ability to predict and manage soil carbon dynamics.