Nitrogen availability regulates topsoil carbon dynamics after permafrost thaw by altering microbial metabolic efficiency

Input of plant-derived labile carbon (C) may induce soil C loss by accelerating the mineralization of soil organic matter (SOM) ('priming effect'), strengthening the permafrost C-climate feedback. The degree to which the priming effect alters soil C dynamics might be regulated by soil nitrogen (N) availability after permafrost thaw. Despite this recognition, experimental evidence for the linkage between the priming effect and post-thaw N availability is still lacking in permafrost ecosystem. Particularly, the role of microbial metabolic efficiency in this C-N interaction is poorly understood. Here, we used a thermokarst-induced natural N gradient on the Tibetan Plateau in combination with an isotope-labelled glucose and N addition experiment to test the hypothesis that elevated N availability after permafrost collapse inhibits the priming effect by increasing the microbial metabolic efficiency. We found that the priming intensity was negatively correlated with the concentration of soil total dissolved nitrogen (TDN) along the thaw gradient but was not significantly associated with other environmental variables. Addition of mineral N significantly decreased priming intensity, which confirmed the observed negative N effect on the priming intensity along the thaw gradient. In contrast to the prevailing assumption, this N-regulated priming intensity was independent of C- or N-acquiring enzyme activities and negatively associated with the microbial metabolic efficiency, i.e., lower microbial metabolic efficiency corresponded to greater priming intensity under low N conditions. Taken together, these findings demonstrate that post-thaw N availability regulates topsoil C dynamics through its modification of microbial metabolic efficiency.