Complex climatic, CO2, and grazing controls on the net primary productivity and carbon stocks in grasslands of the Tibetan Plateau

The Tibetan Plateau (TP) grasslands, the world’s largest alpine ecosystem and highest altitude pastoral region, are particularly susceptible to climate change and grazing activities. These alpine grassland ecosystems are experiencing rapid warming, altered precipitation patterns, and changing grazing pressure. However, the relative contributions of climatic factors, CO₂ enrichment, and grazing, and their interactions, to carbon dynamics in these ecosystems remain poorly quantified, which limits our understanding of how these factors affect carbon cycling and feedbacks of alpine ecosystems. Using the process-based biogeochemical model Biome-BGCMuSo integrated with a dynamic grazing module, we conducted multi-scenario simulations to isolate the individual and interactive effects of environmental and anthropogenic factors on carbon dynamics of the TP grasslands over the past 40 years. The results demonstrated that precipitation was the dominant factor in 67.9 % of TP grasslands. Precipitation promoted vegetation growth, increased net primary productivity (NPP) (positive contribution of 54.6 %), enhanced soil carbon inputs (56.6 %), and ultimately drove increases in total carbon (TOTC) (53.1 %). On average, warming alone reduced NPP (negative contribution of −4.6 %) and TOTC (−2.7 %), highlighting its negative impact on carbon stocks. However, when warming occurred with increased precipitation, the combined positive effect on carbon dynamics was found to be more pronounced than that of precipitation alone. This emphasizes the significant interactive effect of temperature and precipitation, where their combined influence enhances carbon sequestration beyond a simple additive response. The effects of CO₂ enrichment, and its interaction with climate change, were both positive and significant, resulting in increased carbon sequestration. Although changes in grazing intensity had a detrimental effect on NPP and TOTC, their impact was comparatively limited. Under the future warming and wetting trend on the TP, alpine grasslands have the potential to sequester more carbon as increasing precipitation, and the interactive effects of warming and wetting, continues to enhance plant growth.