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There has been an active debate among ecologists about the mechanisms of ephemeral response of soil respiration to climate change. The key questions are whether soil microbes would reduce their physiological rates (thermal acclimation) under a warmer temperature regime?and what are the mechanisms. Using a long-term warming field experiment, we explored the thermal acclimation of soil heterotrophic respiration (R-h) in a temperate steppe ecosystem in Inner Mongolia, China. A thermal acclimation mechanism, namely change in lipid composition of soil microbes, was also tested in this study. Our results showed that 6 years of warming treatment had significantly decreased soil mass-specific respiration (substrate R-mass), indicating soil R-h would acclimate to long-term warming. In addition, we found that this thermal acclimation presented type II acclimation, because experimental warming significantly decreased substrate R-mass but not the Q(10) values. Furthermore, we also found that long-term experimental warming had already increased the carbon numbers of the fatty acids in soil microbial membranes at the community level. In summary, our results indicated that soil R-h could acclimate to global warming via modifying cell membrane constitutions and these mechanisms would play vital roles in the near future. Highlights

Soil heterotrophic respiration (R-h) acclimated to long-term warming in a steppe ecosystem. Soil R-h followed type II acclimation as warming decreased substrate R-mass but did not change Q(10). Experimental warming increased the carbon number of the fatty acids in soil microbial membranes. Soil R-h can acclimate to global warming via shifting the lipid constitute of microbial membrane.