Plant Community Changes Determine The Vegetation And Soil Delta C-13 And Delta N-15 Enrichment In Degraded Alpine Grassland

delta C-13 and delta N-15 are extensively used to understand the biogeochemical mechanisms that moderate ecosystem carbon (C) and nitrogen (N) processes. Little is known about the responses of delta C-13 and delta N-15 to alpine grassland degradation on the Qinghai-Tibetan Plateau (QTP), which prevents a full understanding of degradation-induced changes in C and N cycling there. We investigated the vegetation delta C-13 and delta N-15, soil delta C-13 and delta N-15, soil properties, and plant community composition of alpine grassland on the QTP that were in different states of degradation. Our results show that the vegetation delta C-13 and delta N-15, and soil delta C-13 and delta N-15, increased with the severity of degradation, whereas soil organic carbon (SOC) and total N content decreased as degradation became more severe. The aboveground biomass percentage of forbs was positively correlated with the soil C/N ratio, vegetation delta C-13, and soil delta C-13, and accounted for the largest proportion of the variance for both vegetation delta C-13 and soil delta C-13 (17.25 and 23.65%, respectively). The vegetation delta N-15 and soil delta N-15 were negatively correlated with the soil C/N ratio, which explained the largest proportion of the variance (18.01 and 25.81%, respectively). Our results suggest that C cycling is strongly moderated by plant community composition, because forbs species and C-4 species, were more prevalent in degraded alpine grassland. Meanwhile, N cycling is indirectly regulated by changes in community composition via its effect on the soil C/N as the degradation became more severe for alpine grassland on the QTP.