Grazing accelerates soil carbon loss by enhancing the degradability of soil water-extractable organic matter and microbial turnover in fragile grasslands

• Grazing altered soil water-extractable OM composition to be more degradable. • Grazing increased microbial biomass carbon and its ratio to SOM. • Grazing reduced soil organic matter (SOM) by enhancing microbial decomposition. • Grazing intensity strengthened the microbial turnover and decomposition of SOM. The decrease in soil organic matter (SOM) induced by grazing may be primarily due to reduced plant litter inputs. However, the grazing effects on SOM molecular compositions that control its decomposition by microbes are not well understood. Herein we investigated the molecular composition of soil water-extractable organic matter (WEOM, representing the soluble part of SOM), and the associated microbial metabolisms in grassland soils on the Tibetan Plateau. The paired fields (i.e., grazed versus non-grazed) had been established for ten years at eight sites, covering ecologically fragile grasslands on the plateau. The results demonstrated that grazing substantially changed the soil WEOM compositions by enhancing the number and relative abundance of labile C molecules, including lipids and proteins. Subsequently, grazing increased WEOM degradability index by 26.3 % (0.48 vs. 0.38), and enhanced predicted microbial metabolisms degrading labile compounds (i.e., lipid, amino acid, and carbohydrate). Consistently, grazing increased microbial biomass C (MBC) (1.3 folds higher, 298.4 vs. 232.3 mg kg −1 ) (all P < 0.001), and its proportion to the SOC pool (1.5 folds higher, 0.020 vs. 0.013), indicating that grazing strengthened the microbial pathway of SOM transformation processes. These observed increases of soil WEOM degradability and microbial decomposing capacity coincided with the grazing-induced reduction of SOM by 25.9 % ( P < 0.001), and the effects were positively correlated with the grazing intensity across grassland types. We conclude that grazing can accelerate soil C loss by enhancing WEOM degradability and directly facilitating the metabolism of soil microbes degrading labile organic compounds. This study provides a novel insight into the soil C cycling under human disturbances in fragile grasslands.