Temperature fluctuation promotes the thermal adaptation of soil microbial respiration

Microbial respiration is the main process contributing to soil carbon (C) loss and is simultaneously regulated by changes in mean temperature and temperature fluctuation. The magnitude of the feedback between soil microbial respiration and increased mean temperature may decrease (i.e., thermal adaptation) or increase over time, and accurately representing this feedback within models improves predictions of soil C loss rates. However, climate change entails changes not only in mean thermal conditions but also in the patterns of temperature fluctuation, and whether temperature fluctuation could also cause thermal adaptation has never been addressed. Here, we collected soil samples from 6 sites along a 2,000-km-long west-east transect extending across subtropical forests in China and used them in an incubation experiment involving various temperature regimes to explore how temperature fluctuation influences the thermal response of soil microbial respiration and the underlying mechanisms of this process. We revealed that soil biomass-specific microbial respiration (R_mass) was significantly lower with increasing temperature fluctuation during incubation regardless of the assay temperature, while a positive relationship between R_mass and temperature was observed under increased constant incubation temperature. Structural equation modelling further indicated that increased bacterial species turnover and reduced substrate affinity (K_m) promoted the decrease in R_mass associated with greater temperature variation. Our results demonstrate that if such an adaptive response of soil microbial respiration occurs under greater temperature variation, the stimulatory effect of climate warming may be less than that predicted and thus may not increase atmospheric CO₂ concentrations as much as anticipated.