Effects of fire intensity on carbon dioxide exchange in an arctic dry heath tundra

• Post-fire soil temperatures and moisture increased with increasing fire intensity. • Fire intensity had no immediate effects on ecosystem respiration. • Increased microbial respiration offset decreased plant respiration. • High-intensity fire increased post-fire CO2 losses compared with low-intensity fire. • C losses after high-intensity fire exceeded the C losses during the fire. The frequency and intensity of wildfires in the Arctic has been increasing due to climate change. However, little is known about the effects of fire intensity on carbon dioxide (CO 2 ) exchange in arctic tundra ecosystems. To investigate this, we conducted an experimental fire with different burn intensities (low intensity, high intensity, and unburned control) and measured surface daytime CO 2 fluxes over four growing seasons in a dry heath tundra in West Greenland. We found that post-fire soil temperatures and moisture increased with increasing fire intensity, by up to 2.2 °C and 18 vol%, respectively. Fire had no effects on soil microbial biomass independent of intensity. The high-intensity fire increased soil nitrate concentrations only immediately after the fire. The ecosystem shifted from a net CO 2 sink to a net CO 2 source immediately after the fire, due to the reduced gross ecosystem production. One year after the fire, the low-intensity burned plots became a net CO 2 sink, while the high-intensity burned plots remained a net CO 2 source throughout the study period. This suggests that the time needed for the ecosystem to become a net CO 2 sink increases with fire intensity. Fire intensity had no effect on soil respiration, but the high-intensity fire significantly reduced ecosystem respiration (ER) rates one and three years after the fire. This suggests that decreased ER was mainly driven by the reduced aboveground plant respiration. Over four growing seasons after the high-intensity fire, cumulated post-fire C losses exceeded the C losses during the fire. Thus, it is essential to consider the long-term C losses following fire to improve understanding of wildfire impacts on arctic tundra C dynamics. Overall, this study highlights that high-intensity fires prolong the duration of burned areas as a net CO 2 source, leading to increased post-fire CO 2 emissions, when compared to low-intensity fires.