Microbial thermal adaptation will lead to a weakening of the positive feedback between climate warming and soil respiration. The thermal adaptations of microbial communities and fungal species has been widely proven. However, studies on the thermal adaptation of bacterial species, the most important decomposers in the soil, are still lacking. Here, we isolated six species of widely distributed dominant bacteria and studied the effects of constant warming and temperature fluctuations on those species. The results showed that both scenarios caused a downregulation of respiratory temperature sensitivity (Q10) of the bacterial species, accompanied by an elevation of the minimum temperature (Tmin) required for growth, suggesting that both scenarios caused thermal adaptation in bacterial species. Fluctuating and increasing temperatures are considered an important component of future warming. Therefore, the inclusion of physiological responses of bacteria to these changes is essential the prediction of global soil-atmosphere C feedbacks.

The feedback between soil carbon (C) and climate has the potential to decrease in magnitude over time due to the thermal acclimation of microbial respiration, while, whose strength is highly uncertain, partly because the response of microbial respiration is regulated by multiple environmental factors simultaneously rather than by temperature alone. Using a 9-year two-way factorial experiment involving warming and multilevel nitrogen enrichment treatments from an alpine grassland, we show that microbial respiration acclimates to warming only under nitrogen enrichment and that the strength of thermal acclimation increases as nitrogen enrichment. We identified two contrasting pathways—via an enhancement of acclimation by soil acidification and a weakening of acclimation by the inhibition of soil C availability and stimulation of soil C-degrading enzymes—with a net positive effect of nitrogen enrichment on microbial thermal acclimation. Our findings emphasize the importance of considering multiple environmental factors in shaping the strength of thermal acclimation.