Apparent thermal acclimation of soil heterotrophic respiration mainly mediated by substrate availability

Multiple lines of existing evidence suggest that increasing CO2 emission from soils in response to rising temperatures could accelerate global warming. However, in experimental studies, the initial positive response of soil heterotrophic respiration (RH) to warming often weakens over time (referred to apparent thermal acclimation). If the decreased RH is driven by the thermal adaptation of soil microbial community, the potential for soil carbon (C) losses would be reduced substantially. In the meanwhile, the response could equally be caused by substrate depletion, and would then reflect the gradual loss of soil C. To address uncertainties regarding the causes of apparent thermal acclimation, we carried out sterilization and inoculation experiments using the soil samples from an alpine meadow with 6-years of warming and nitrogen (N) addition. We demonstrate that substrate depletion, rather than microbial adaptation, determined the response of RH to long-term warming. Furthermore, N addit...