Sustained increases in soil respiration accompany increased carbon input under long-term warming across global grasslands

Respiratory effluxes of carbon (C) from the soil to the atmosphere are expected to rise with temperature, potentially intensifying future climate warming. However, whether and how this increase would be sustained under long-term warming is not well understood. Here, we combined a manipulation experiment in an alpine meadow with a global meta-analysis to explore the mechanisms underlying the long-term responses of soil respiration to climate warming. The results from the experiment in the alpine meadow showed that the warming-induced increase in net primary productivity (NPP, 23.6 %) explained 52 % of the increase in soil respiration across 6 years. In contrast, the warming-induced changes in soil moisture, soil temperature, microbial biomass C or nitrogen were not significantly correlated with soil respiration responses. Consistently, in the global meta-analysis, both soil respiration and NPP continually increased over the years by an average of 9.5 % and 15.9 %, respectively. The increases in soil respiration were also primarily correlated with the continued increases in NPP over this period. Notably, the sustained increase in soil respiration was mainly contributed by the response of autotrophic respiration, which was closely correlated with the sustained increase in belowground NPP under warming. The results from both our field experiment and meta-analysis suggest that the increased soil respiration under climate warming was at least partly from the stimulation of C input in grasslands. The simultaneous increases in soil respiration and NPP may counteract the expected positive terrestrial C-climate feedback and should be considered in land models to more accurately predict future climate change.