Microbial climate feedback driven by both mean temperature and its variability

The climate feedback from soil microbial respiration is a critical yet uncertain determinant of global soil carbon cycle and its vulnerability to climate change. Current assessments predominantly rely on responses to mean temperature shifts, largely neglecting the ecological importance of climate variability, particularly short-term temperature fluctuations. How such fluctuations modulate microbial respiratory feedback remains unresolved. Here, using soils collected along a 5,000-km forest transect spanning a broad mean annual temperature (MAT) gradient, we disentangle the effects of fluctuating versus constant temperatures on microbial respiration while accounting for substrate availability and microbial biomass. We find that microbial respiration decreased by 0.11 µg C g −1 soil h −1 for every 1 °C increase in mean temperature, consistent with the expected thermal adaptation to mean temperature changes. Crucially, temperature fluctuations systematically reduced this thermal response to 0.09 µg C g −1 soil h −1 °C −1 , indicating an 18.2 % reduction in the compensatory thermal adaptation. Consequently, under realistic warming scenarios in which both mean temperature and variability increase, soil microbial respiration substantially exceeds predictions derived from mean temperature responses alone. This fundamental discrepancy recalibrates projections of soil carbon loss in a warmer and more variable world.