Plant community changes determine the vegetation and soil δ13C and δ15N enrichment in degraded alpine grassland

δ¹³C and δ¹⁵N are extensively used to understand the biogeochemical mechanisms that moderate ecosystem carbon (C) and nitrogen (N) processes. Little is known about the responses of δ¹³C and δ¹⁵N 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 δ¹³C and δ¹⁵N, soil δ¹³C and δ¹⁵N, 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 δ¹³C and δ¹⁵N, and soil δ¹³C and δ¹⁵N, 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 δ¹³C, and soil δ¹³C, and accounted for the largest proportion of the variance for both vegetation δ¹³C and soil δ¹³C (17.25 and 23.65%, respectively). The vegetation δ¹⁵N and soil δ¹⁵N 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 C₄ 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.