Atmospheric impact of nitrous oxide uptake by boreal forest soils can be comparable to that of methane uptake.

Aims: Environmental factors controlling nitrous oxide (N2O) uptake in forest soils are poorly known, and the atmospheric impact of the forest N2O sink is not well constrained compared to that of methane (CH4). Methods: We compared nitrous oxide (N2O) and CH4 fluxes over two growing seasons in boreal forest soils located in Eastern Finland. Within a spruce forest, we compared plots with long-term nitrogen (N) fertilization history and non-fertilized plots, and additionally pine forest plots without a fertilization history. The flux data was complemented with measurements of climatic conditions and soil physical and chemical characteristics, in order to identify factors affecting N2O and CH4 fluxes. Results: Non-fertilized spruce forest soils showed the highest cumulative N2O uptake among the sites, whereas the pine forest site displayed low cumulative N2O emission. Nitrous oxide uptake was favored by high soil silt and water content. The low temperature seasons, spring and autumn, had the highest N2O uptake, likely associated with high soil water content typical for these seasons. In the spruce forest the N2O uptake was seasonally decoupled from the CH4 uptake. Conclusions: Applying the Global Warming Potential (GWP) approach, the cooling effect of N2O uptake in the spruce forest was on average 35% of that of CH4 uptake showing that N2O uptake should be considered when evaluating the atmospheric impact of boreal forests. [ABSTRACT FROM AUTHOR]