Warming conditions across Canada's subarctic and arctic regions are causing permafrost landforms to thaw, resulting in rapid land cover change, including conversion of peat plateaus to wetland and thermokarst. These changes have important implications for northern ecosystems, including shifting controls on carbon uptake and release functions, as well as altering evapotranspiration (ET) rates, which form feedbacks with climatic change. Four landforms (peat plateau, sedge lawn, channel fen, and a thermokarst shoreline collapse scar) in the Hudson Bay Lowlands, northern Manitoba, were instrumented for weekly chamber measurements of carbon dioxide (CO2) and water vapor flux over a summer season (May to September 2014). Relative to other landforms, thermokarst CO2 exchange was characterized by high respiration rates early in the season, which decreased and were offset later in the season by CO2 uptake driven by sedge productivity. For all landforms, ET peaked post-snowmelt during rapid active layer thaw, and decreased throughout the growing season, controlled primarily by atmospheric vapor deficits. This work shows distinct differences in CO2 exchange and ET between intact and thawing permafrost features. While representative of small-scale processes in a single study region over one growing season, the results presented in this study have important implications for our understanding of ecohydrological and biogeochemical functioning of subarctic landscapes under future climates.

Geomorphic disturbances to surrounding terrain induced by thermal degradation of permafrost often lead to surface ponding or soil saturation. However, interactions between soil moisture and temperature on belowground carbon processes are not fully understood. We conducted batch incubation for three temperature treatments [constant freezing (CF), constant thawing (CT), and fluctuating temperatures (FTC)] and two soil moisture conditions (ponded and unsaturated). Extracellular enzyme activity was higher under ponded conditions than under unsaturated conditions, resulting in higher dissolved organic carbon (DOC) levels for ponded conditions. More CO2 and less CH4 were emitted under unsaturated conditions than under ponded conditions. Carbon dioxide emission was similar for CT and FTC treatments regardless of moisture conditions. However, CH4 emission was higher under ponded conditions than under unsaturated conditions for CT treatments, but was very low for FTC treatments regardless of moisture conditions. Little CO2 and CH4 were produced in CF treatments. Despite similar CO2 and CH4 emission levels for CT and FTC treatments, lower DOC levels were observed in the latter, indicating slower soil organic carbon (SOC) decomposition. Similar DOC variation patterns between CT and CF treatments indicated that SOC decomposition was considerable and further degradation to CO2 or CH4 was negligible even for CF treatments. The SOC decomposition and CO2 and CH4 emissions were considerable for FTC treatments. Our results suggest that labile-C produced during SOC decomposition in seasonally frozen soils and permafrost may provide supplemental substrates that would enhance the positive feedback to climate change with rising temperatures and wetter conditions.