Increasing Contribution of Microbial Residue Carbon to Soil Organic Carbon Accumulation in Degraded Grasslands

Grassland degradation driven by overgrazing, invasive species, and climate change profoundly affects the dynamics and stability of soil organic carbon (SOC), yet the molecular mechanisms underlying these changes remain unclear. This study investigated the molecular composition and origins of SOC across different degradation stages—native grassland (NG), weed-dominated grassland (WG), and invasive grassland (IG) dominated by Pedicularis kansuensis—in the Bayinbuluke alpine region of China. Soil samples from three depth intervals (0–10 cm, 10–20 cm, and 20–30 cm) were analyzed using gas chromatography-mass spectrometry (GC-MS) to quantify biomarkers, including free lipids, ester-bound lipids, lignin phenols, and amino sugars. Principal component analysis (PCA) was applied to assess the overall variation in SOC composition. Compared to NG, plant-derived long-chain lipids and lignin phenols in WG and IG surface soils increased by 44–90% and 68–76% (p < 0.05), respectively, while cutin content increased by 96% and 150%. However, suberin content in IG decreased by 58% across all layers (p < 0.05). Microbial-derived carbon (MRC), including bacterial and fungal residues, increased significantly in the surface layer of degraded soils (IG > WG > NG), with MRC contributions to SOC also rising markedly in the subsurface layers (10–30 cm). PCA revealed a distinct separation of SOC components along the PC1 axis, highlighting the substantial impact of degradation on SOC composition and sources. These findings emphasize the role of vegetation shifts in SOC transformation and provide insights for grassland management and restoration strategies.