Response of carbon flux of alpine ecosystems to hydrological and meteorological factors on the Qinghai-Tibet Plateau

Alpine meadows, alpine wetlands, and alpine desert steppes are the three typical vegetation types on the Qinghai-Tibet Plateau. The complex terrain and harsh climatic conditions across this region lead to considerable diversification in the vegetation growth environment, resulting in substantial spatial heterogeneity in ecosystem carbon flux and its controlling mechanisms. Using eddy covariance data collected from March to August 2019, this study examined the responses of carbon and water fluxes in different ecosystems on the Tibetan Plateau to typical hydrometeorological factors, focusing on Net Ecosystem CO₂ Exchange (NEE) and Evapotranspiration (ET). The results indicate that: 1) The Longbao alpine wetland primarily acted as a carbon sink from May to August, while serving as a carbon source from March to April. In the Maqin alpine meadow, it functioned as a carbon sink during June and July but acted as a carbon source in March, April, May, and August. The Tuotuohe alpine desert strppe was predominantly a net carbon sink from March to August. Overall, after the entire growing season (March to August), the Longbao alpine wetlands, Maqin alpine meadow, and Tuotuohe alpine desert steppe all showed net carbon sink properties, with net CO₂ uptakes of 236.12 g/m², 291.45 g/m², and 290.28 g/m², respectively. 2) The importance of meteorological factors to NEE varies with scale and ecosystem type, with global radiation (Rg) being the most critical factor influencing NEE variation. Volumetric soil water content (Soil_VWC) and soil temperature (Soil_T) had a positive effect on NEE at Maqin alpine meadow and Tuotuohe alpine desert steppe, while higher values of these variables showed a negative contribution. Furthermore, the sensitivity of NEE to Soil_T at Longbao alpine wetland and Tuotuohe alpine desert steppe was greater than its sensitivity to air temperature (Tair). 3) The effect of Gross Primary Productivity (GPP) on NEE in alpine desert steppes is significantly greater than in alpine meadows. Both Ecosystem Respiration (Reco) and NEE were substantially limited by GPP, with 84% of GPP in alpine wetlands contributing to Reco and 16% to NEE; 92% of GPP in alpine meadows contributing to Reco and 8% to NEE; and 40% of GPP in high-altitude desert grasslands contributing to Reco and 60% to NEE. 4) The strong correlation between NEE and evapotranspiration suggests that water availability is the primary factor controlling changes in the carbon and water budgets of alpine ecosystems.