Nitrous oxide emissions from boreal organic soil under different land-use

Nitrous oxide emissions were studied with a static chamber technique during 2 years from a drained organic soil in eastern Finland. After drainage, the soil was forested with birch (Betula pendula Roth) and 22 years later, part of the forest was felled and then used for cultivation of barley (Hordeum vulgare L.) and grass. The annual N2O emissions from the cultivated soil (from 8.3 to 11.0 kg N2O-N ha−1 year−1) were ca. twice the annual emission from the adjacent forest site (4.2 kg N2O-N ha−1 year−1). The N2O emissions from the soils without plants (kept bare by regular cutting or tilling) were also lower (from 6.5 to 7.1 kg N2O-N ha−1 year−1) than those from the cultivated soil. There was a high seasonal variation in the fluxes with a maximum in spring and early summer. The N2O fluxes during the winter period accounted for 15–60% of the total annual emissions. N2O fluxes during the snow-free periods were related to the water table (WT) level, water-filled pore space, carbon mineralisation and the soil temperature. A linear regression model with CO2 production, WT and soil temperature at the depth of 5 cm as independent variables explained 54% of the variation in the weekly mean N2O fluxes during the snow-free periods. N2O fluxes were associated with in situ net nitrification, which alone explained 58% of the variation in the mean N2O fluxes during the snow-free period. The N2O-N emissions were from 1.5 to 5% of net nitrification. The acetylene blockage technique indicated that most of the N2O emitted in the snow-free period originated from denitrification.