From short- to long-term warming: microbial metabolic responses control soil carbon and nitrogen losses

Although high-latitude soils are undergoing significant warming, with potential consequences for soil carbon (C) and nitrogen (N) cycling, the way in which warming duration modulates microbial physiological responses and associated changes in soil C and N pools is not well understood. Here, we used a natural geothermal gradient ranging from +0 to +12.3 °C to assess the effects of short-term (1 year), medium-term (5 to 9 years), and long-term (>50 years) soil warming on microbial biomass C, microbial physiology (mass-specific respiration and growth), carbon use efficiency (CUE), and soil C and N pools.Across all warming durations, microbial biomass C and CUE decreased with increasing temperature. Warming consistently accelerated microbial metabolic rates, with mass-specific respiration increasing more than mass-specific growth, thereby explaining the observed reduction in CUE. Warming also reduced plant litter biomass while increasing its N concentration, suggesting accelerated litter decomposition under enhanced microbial activity. The magnitude of these physiological and functional responses was attenuated after nine years of warming, indicating a partial acclimation of microbial metabolism to sustained warming. While cumulative soil C and N losses were not yet detectable after one year of warming, they became evident after several years of exposure. This delayed emergence of C and N losses suggests that microbial communities gradually adjusted to the new thermal conditions, leading to partial acclimation once substrate availability had been substantially altered.These results suggest that warming-induced changes in soil C and N dynamics are governed by the interaction between intrinsic microbial temperature sensitivity and progressive substrate depletion, as mediated by their effects on microbial biomass and physiology. Our findings improve the understanding of how microbial physiological responses shape soil C and N losses over time in a warming climate.