Sustained increase in soil respiration after nine years of warming in an alpine meadow on the Tibetan Plateau

•Soil respiration was still increased after 9 years of warming.•Microbial biomass only decreased at the 0–10 cm.•Both the bacterial and fungal composition had no change.•Abundance of Actinobacteria decreased at 20–30 and 30–50 cm, which negatively correlated with Rs. Soil microbes are key determinants of soil carbon (C) dynamics. The response of the soil microbial community to climate warming modulates the feedback between ecosystem C cycling and future climate change. We conducted a long-term manipulative warming (1.6 °C increase of the soil temperature at 5 cm) experiment to examine the soil respiration, microbial biomass, and community composition at an alpine meadow site on the Qinghai-Tibetan Plateau. After nine years of warming, soil respiration (3.5 μmol m2 s−1 in control in the growing season) increased in the warmed plots. In the early growing season, the increase in heterotrophic respiration (Rh) accounted for more than 90% of the increase in soil respiration. The warming effect gradually decreased during the mid and late growing season (46%). Microbial biomass C and nitrogen declined significantly in the 0–10 cm to the 30–50 cm layer. Warming did not significantly affect microbial biomass C and N in any soil depth layer. Metabolic activity of microbes in terms of Rh per unit microbial biomass C significantly increased by 66% in warmed plots. The bacterial and fungal community composition did not significantly change in the warmed plots. The relative abundance of Actinobacteria decreased at 20–30 cm and 30–50 cm soil depths, but that of Cercozoa increased in all four soil layers. The relative Actinobacteria abundance was negatively correlated with Rh and metabolic activity in the 10–20 and 20–30 cm layers. Our results indicate a decrease in Actinobacteria abundance, increases in metabolic activity, and no substrate limitation sustained the positive warming effect on soil respiration throughout the last 9 years. This implies that climate warming could trigger a substantial loss of soil C to the atmosphere in the alpine meadow on the Qinghai-Tibet Plateau.