Soil Respiration Responses to Prolonged Warming Vary Seasonally in a Subarctic Grassland.

Northern ecosystems are warming rapidly, but the effects on soil carbon (C) dynamics are not fully understood. In particular, it is unclear whether warming effects on soil respiration (SR) -the primary source of atmospheric CO2 from terrestrial ecosystems- persist throughout the year or vary seasonally. While both summer and cold-season processes shape annual soil C fluxes, the long cold season, which dominates high-latitude regions, has received far less attention. Using a natural geothermal gradient in a subarctic grassland (+0°C to +10°C above ambient soil temperature), we assessed the effects of more than a decade of continuous soil warming on SR. We obtained high-resolution year-round soil CO2 fluxes and separated geothermal from biological sources of CO2 following a stable isotope approach. Our findings revealed seasonally variable responses: warming significantly increased SR during the cold season (October-May) but, surprisingly, had no effect during summer (July-September). The lack of a summer warming response likely resulted from warming-induced soil C depletion and microbial and root adaptations. Overall, prolonged warming altered seasonal C flux dynamics by enhancing the cold-season contribution to total annual SR. This shift led to a 5.8% rise in annual SR per degree Celsius of warming. Furthermore, the apparent temperature sensitivity of SR (Q10) and the seasonal amplitude of SR declined exponentially with warming intensity, with effects stabilizing beyond +3°C of warming, indicating apparent long-term thermal acclimation of SR seasonality. Altogether, our findings highlight the importance of considering the seasonality of SR for accurate predictions of soil C dynamics in northern ecosystems under long-term warming.