Effects of fire intensity on CO2 exchange in an arctic tundra ecosystem

Recently, the frequency and intensity of wildfires has been increasing in the Arctic as a result of climate change. However, there is still little knowledge on the effects of fire intensity on carbon dioxide (CO2) exchange in arctic tundra ecosystems. We conducted an experimental fire of different burn intensity (i.e., low intensity, high intensity and unburned control) to investigate effects of fire intensity on soil biogeochemical cycles and surface CO2 fluxes over four growing seasons in an arctic heath tundra, West Greenland. Post-fire soil temperatures and soil moisture increased with increasing fire intensity by up to 2.2 ℃ and 18 vol%, respectively. The high-intensity fire also significantly increased soil nitrate concentrations 1 day post fire, but this effect disappeared 1 year post fire. There were no significant effect of fire intensity on soil carbon and phosphorus availability or microbial biomass. The ecosystem shifted from a net CO2 sink to a net CO2 source immediately after the fire, because of the reductions in photosynthetic activity. 1 year post fire the low-intensity burned plots have turned into a net CO2 sink, while the high-intensity burned plots were a net CO2 source for the entire study period. This suggests that the time needed for the burned ecosystem to turn into a net CO2 sink increases with increasing fire intensity. Fire intensity had no effect on ecosystem respiration (ER) immediately after the fire, likely because the increases in microbial respiration caused by elevated soil temperatures and moisture and soil nitrogen availability have offset the decreases in plant respiration. However, 1-3 years after the fire, the high-intensity fire significantly reduced ER rates, suggesting that the moderate increases in microbial respiration only caused by elevated soil temperatures and moisture could not balance out the decreases in plant respiration. Overall, compared with low-intensity fire, high-intensity fire not only combusts more biomass or soil organic matter and releases more CO2 during the fire, but also prolongs the duration of the burned areas as a net CO2 source and consequently enhances post-fire CO2 losses.