Effects of experimental fire in combination with climate warming on greenhouse gas fluxes in Arctic tundra soils.

The frequency and severity of fire is increasing in Arctic tundra regions with climate change. Here we investigated effects of experimental low-intensity fire and shrub cutting, in combination with warming, on soil biogeochemical cycles and post-fire greenhouse gas (GHG) emissions in a dry heath tundra, West Greenland. We performed in vitro incubation experiments based on soil samples collected for up to two years after the fire. We observed tendency for increased soil nitrate (14-fold) and significant increases in soil ammonium and phosphate (four-fold and five-fold, respectively) two years after the fire, but no effects of shrub cutting on these compounds. Thus, changes appear to be largely due to fire effects rather than indirect effects by vegetation destruction. Two years after fire, nitrous oxide (NO) and carbon dioxide (CO) production was significantly increased (three-fold and 32% higher, respectively), in burned than unburned soils, while methane (CH) uptake remained unchanged. This stimulated NO and CO production by the fire, however, was only apparent under conditions when soil was at maximum water holding capacity, suggesting that fire effects can be masked under dry conditions in this tundra ecosystem. There were positive effects by modest 2.5 °C warming on CO production in control but not in burned soils, suggesting that fire may decrease the temperature response in soil respiration. Methane uptake was neither altered by the modest warming in shrub-cut nor in burned soils after two years, suggesting that the removal of vegetation may play a key role in controlling future temperature response of CH oxidation. Altogether, our results show that post-fire tundra soils have the potential to enhance soil GHG emissions (e.g. NO and CO) especially during episodes with wet soil conditions. On the other hand, the lack of warming responses in post-fire soil respiration may weaken this positive feedback to climate change.