Effects of warming on carbon and nitrogen cycling in alpine grassland ecosystems on the Tibetan Plateau: A meta-analysis

Anthropogenic activities have caused rapid warming globally, particularly in the alpine regions. The Tibetan Plateau is sensitive to global change and plays an important role in global carbon cycle due to its large area and high carbon density. Although a large number of field manipulative warming experiments have been conducted on the Tibetan Plateau, the general patterns of how warming affects ecosystem carbon (C) and nitrogen (N) cycles remain unclear. Here we conducted a meta-analysis from 65 publications to synthesize the responses of C and N cycles to warming in the alpine grassland ecosystems across the Tibetan Plateau. The results showed that warming significantly increased aboveground (mean 11.0%, 95% CI 1.6-21.4%) and belowground plant biomass (24.3%, 13.7-36.0%), soil dissolved organic C (13.6%, 5.4-22.5%), soil respiration (14.3%, 2.9-27.1%), ecosystem respiration (15.3%, 4.7-27.1%), and soil ammonium N (29.0%, 4.6-59.2%), but did not change soil total organic C (2.4%, -4.4 to 9.7%), soil total N (1.9%, -2.7 to 6.7%), microbial biomass C (5.5%, -4.1 to 16.2%) and microbial biomass N (4.7%, - 7.4 to 18.4%). There were negative relationships between the response ratio of aboveground (P = 0.08, R-2 = 0.05) and particularly below-ground (P < 0.01, R-2 = 0.23) biomass and the duration of warming. Moreover, the effect of warming on ecosystem respiration was positively correlated with the change in soil water content after warming (P = 0.03, R-2 = 0.12). Overall, this study showed that ecosystem C and N cycles were generally stimulated by warming, but soil C and N stocks showed minimal responses to warming in the alpine grassland ecosystems on the Tibetan Plateau. This finding suggests that the faster ecosystem C and N cycles under climate warming may not necessarily lead to the loss of soil C and N stocks in the alpine grassland on the Tibetan Plateau. Future studies should conduct long-term coordinated distributed experiments using consistent methods and pay more attention to subsurface soils and the less-studied central and western regions of the Tibetan Plateau.