Moderate grazing modulates depth-dependent soil organic carbon stock responses to climatic variability in an alpine meadow on the Qinghai–Xizang Plateau

Background Grassland soils on the Qinghai–Xizang Plateau (QXP) represent an important terrestrial carbon reservoir and are affected by grazing, nitrogen (N) deposition, and climate variability. However, the interactive effects of grazing and N addition on soil organic carbon (SOC) stocks across soil depths remain poorly understood. We conducted a three-year factorial field experiment in an alpine meadow on the QXP to examine the effects of three stocking rates and four N-addition rates on plant biomass, plant functional composition, and SOC stocks at 0–30 cm depth. Climatic variables were obtained from an on-site weather station and were treated as observational interannual covariates rather than experimental factors. Results During the experimental period, a stocking rate of 8 sheep ha –1 was associated with higher SOC stocks at selected soil depths and sampling years. N addition mitigated some negative effects of grazing on plant aboveground biomass (AGB), but had negligible net effects on SOC stocks. Grazing weakened negative associations between interannual climatic covariates and SOC stocks. The contributions of grazing, climatic covariates, plant functional composition, and soil properties to SOC stocks varied with soil depth. Plant functional composition, particularly the AGB ratio of traditional herbage (TH) to functional native herbage (FNH), made an important contribution to SOC stocks in the upper soil layers, whereas the contributions of climatic covariates and soil properties increased with depth. Conclusions Stocking rates at ~ 8 sheep ha –1 were compatible with higher SOC stocks under the conditions of this three-year experiment, but long-term and multi-site studies are needed to evaluate their broader applicability.