Thermal and Hydrological Controls on Soil Respiration and Subsurface Gas Transport in Arctic Tundra Ecosystems

Arctic tundra ecosystems have experienced marked changes in temperature and precipitation patterns in recent decades. The impact of these changes on the physical and biogeochemical mechanisms governing soil respiration and CO2 emissions remains poorly understood. We investigate how seasonal thermo-hydrological dynamics impact soil freezing/thawing, soil respiration, and O2 and CO2 transport and exchange in a typical well-drained Arctic tundra ecosystem by combining laboratory experiments and field observations with process-based modeling. Our results demonstrate that low temperatures constrain soil CO2 production and efflux during most of the year, but rising temperatures and large snowmelt infiltration in early spring reduce thermal constraints and induce relatively short yet important periods of soil water-saturated conditions. During these periods, depth-resolved measurements show noticeable limitations in oxygen transport and corresponding CO2 production, which leads to the reduction of CO2 effluxes. The snowmelt infiltration also reduces CO2 efflux by promoting downward CO2 migration and delays its emission by limiting diffusive transport to the atmosphere until the water level recedes. Comparing the simulations with a no-snowmelt scenario, we find that these physical and biogeochemical controls during such short periods collectively reduce annual CO2 efflux by 7%, highlighting the importance of climate-driven forcing in modulating soil greenhouse gas emissions.