Thermal and hydrological controls on subsurface gas transport and soil respiration in Arctic tundra ecosystems

In recent decades, the temperature and precipitation patterns in Arctic ecosystems have been highly affected by climate change. Previous studies suggest that changing air circulation and more evaporation from ice-free Arctic seas could increase snowfall and winter snow accumulation in parts of the Arctic, which in turn can change the onset of the growing season. In combination with ongoing and projected temperature rise, such shifts will alter the physical and biogeochemical processes that are associated with soil respiration and production/release of greenhouse gases like CO₂ from Arctic tundra soils.

Arctic tundra soils experience strong seasonal hydrological dynamics, ranging from frozen conditions in winter to near water saturated and partially water saturated conditions following snowmelt infiltration in early spring. These conditions exert controls (i) on the transport behavior and delivery of O₂ into the soil, (ii) on the kinetics of soil respiration and (iii) on the release of CO₂ to the atmosphere. Despite the importance of these complex interactions for Earth’s climate, there is still a considerable limitation on the accurate quantification of the interplay between thermo-hydrological, transport and microbial respiration in controlling CO₂ emissions from tundra ecosystems under transient field conditions.

We investigated how physical and biogeochemical processes, including oxygen transport, soil respiration and CO₂ emissions respond to seasonal thermo-hydrological dynamics in a typical well-drained Arctic tundra ecosystems by combining lab experiments and field observations with process-based modelling. Our results show that respiration and CO₂ emissions are strongly constrained by low temperatures during most of the year as oxygen concentration remains close to atmospheric levels and therefore oxygen availability is not a limiting factor. The onset of spring is accompanied by a gradual increase in temperature and melting of snowpack, which reduces the thermal limitation on soil respiration. However, the resulting snowmelt infiltration exerts a series of biochemical and physical controls on soil respiration dynamics and CO₂ emission by (i) inducing water saturated conditions in soil; (ii) limiting oxygen transport into the soil and CO₂ migration toward the atmosphere due to slow gas diffusivity in water and (iii) reducing oxygen concentration to values close to half saturation constant of oxygen, thereby exerting metabolic constrains. These results highlight the importance of considering the impact of climate forcing (e.g., thermal and hydrological dynamics) on physical and biogeochemical processes that regulate carbon dynamics in Arctic tundra ecosystems.