Boreal forest soil nitrous oxide production and consumption responds positively to temperature and a warmer climate

Abstract:

The greenhouse gas nitrous oxide (N₂O) is both produced and consumed in soils. In strict denitrification, nitrogen (N)-oxides are reduced stepwise by microbial enzymes until dinitrogen (N₂) is produced. N₂O may be lost and/or consumed during this process, by microbes that may or may not possess the enzyme required for the final step of the reduction chain. In cold, N-limited, boreal forest soils consumption may dominate, resulting in low net N₂O flux to the atmosphere, though the ubiquity of this pattern and the temperature response of these processes are unclear. To accurately predict the temperature response of net soil efflux of N₂O in the boreal biome, we must understand fundamental, temperature responses of the boreal soil denitrifier community.

Using boreal forest organic and mineral soils collected from three climate regions along the Newfoundland and Labrador Boreal Ecosystem Latitudinal Transect, Canada, we measured soil N₂O fluxes during ~60 h lab incubations, in which conditions for denitrification were optimized. Organic soils from the warmer climate and incubated at the warmest temperature produced the most N₂O. These soils also experienced net consumption of N₂O earlier in the incubation than other soils. All mineral soils, regardless of location along the climate gradient, exhibited net N₂O consumption earliest in the incubation at the warmest incubation temperature. Ongoing analyses of headspace N₂O and N₂ d¹⁵N and relative abundances of denitrifying communities during the incubation will reveal mechanisms driving changes in [N₂O] over time, but these data suggest warmer temperatures in these boreal soils promote enhanced rates of both N₂O production and consumption, with ‘hot moments’ of net N₂O efflux during which production far exceeds consumption. If these results are applicable in situ, a warming climate may result in greater net N₂O flux from boreal forest soils such as these.