Consistent responses of ecosystem CO2 exchange to grassland degradation in alpine meadow of the Qinghai-Tibetan Plateau

The effect of grassland degeneration on the emissions and sinks of carbon dioxide (CO2) received extensive attention because of the increase of degraded area and the degeneration level in alpine meadow. To quantify its effect, we investigated net ecosystem exchange (NEE), gross primary productivity (GPP), ecosystem respiration (Reco), plant respiration (Rplant) and heterotrophic respiration (Rh), as well as environmental variability (soil water content (SWC), soil total carbon (STC), soil total nitrogen content (STN) and aboveground biomass (AGB)) from different degraded grasslands and non-degraded (ND) grassland in alpine meadow, Qinghai-Tibetan Plateau in growing season. The results indicated that compared with ND grassland, land degradation significantly decreased net CO2 uptake (-NEE), GPP, Reco, Rplant and Rh by 60.61%, 63.22%, 67.53% 78.82% and 43.56%, respectively in extremely degraded (ED) grassland. These consistent responses suggested that the ecosystem CO2 fluxes were very sensitive to grassland degradation. Degradation also decreased Rplant/Reco and Rplant/GPP by 16.8% and 8.1%, respectively, while increased Rh/Reco, Rh/Rplant and Rh/GPP by 16.8%, 41.4% and 3.8%, respectively, suggested that grassland degradation could eventually shifted Reco from autotrophic dominated to heterotrophic dominated. Four structural equation models (SEM) indicated that decline in NEE, GPP and Reco directly related to AGB and STC and decline in Rplant directly related to AGB, and indirectly related to SWC and STN. Our study highlighted that the consistent responses of CO2 fluxes to grassland degradation could alter the ecosystem's carbon balance, and further would be influence carbon-climate feedbacks under deterioration of ecological environment.