Nitrogen enrichment causes the thermal adaptation of soil microbial respiration

As the climate warms, the feedback between soil carbon (C) and climate has the potential to decrease in magnitude over time due to the thermal adaptation of microbial respiration. However, the strength of microbial thermal adaptation (i.e., the degree to which microbial respiration adapts to temperature change) is uncertain, partly because the response of microbial respiration is regulated by multiple environmental factors acting simultaneously rather than by temperature alone; however, the combined effects of an environmental factor and warming on the thermal adaptation of microbial respiration have never been assessed. Using a 9-year two-way factorial experiment involving warming (daytime: 1.80℃; nighttime: 0.77℃) and nitrogen (N) enrichment (up to 15 g m^-2 y^-1) treatments in an alpine permafrost on the Tibetan Plateau, we show that microbial respiration adapts to warming only under exogenous N enrichment and that the strength of thermal adaptation gradually increases as N enrichment increases. We identified two contrasting pathways by which N enrichment appears to affect the strength of thermal adaptation—via an increase caused by soil acidification and a decrease caused by the inhibition of soil C availability and stimulation of soil C-degrading enzymes—with a net positive effect of N enrichment on microbial thermal adaptation. Our findings emphasize the importance of considering multiple environmental change factors in shaping the strength of thermal adaptation when predicting future soil C-climate feedbacks.