The magnitude of the feedback between soil microbial respiration and increased mean temperature may decrease (a process called thermal adaptation) or increase over time, and accurately representing this feedback in models improves predictions of soil carbon loss rates. However, climate change entails changes not only in mean temperature but also in temperature fluctuation, and how this fluctuation regulates the thermal response of microbial respiration has never been systematically evaluated. By analysing subtropical forest soils from a 2,000 km transect across China, we showed that although a positive relationship between soil microbial biomass-specific respiration and temperature was observed under increased constant incubation temperature, an increasing temperature fluctuation had a stronger negative effect. Our results further indicated that changes in bacterial community composition and reduced activities of carbon degradation enzymes promoted the effect of temperature fluctuation. This adaptive response of soil microbial respiration suggests that climate warming may have a lesser exacerbating effect on atmospheric CO2 concentrations than predicted.
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