Nitrous oxide production in boreal soils with variable organic matter content at low temperature – snow manipulation experiment

Abstract. Agricultural soils are the most important sources for the greenhouse gas nitrous oxide (N₂O), which is produced and emitted from soils also at low temperatures. The processes behind emissions at low temperatures are still poorly known. Snow is a good insulator and it keeps soil temperature rather constant. To simulate the effects of a reduction in snow depth on N₂O emission in warming climate, snow pack was removed from experimental plots on three different agricultural soils (sand, mull, peat). Removal of snow lowered soil temperature and increased the extent and duration of soil frost in sand and mull soils. This led to enhanced N₂O emissions during freezing and thawing events. The cumulative emissions during the first year when snow was removed over the whole winter were 0.25, 0.66 and 3.0 g N₂O-N m⁻² yr⁻¹ in control plots of sand, mull and peat soils, respectively. In the treatment plots, without snow cover, the respective cumulative emissions were 0.37, 1.3 and 3.3 g N₂O-N m⁻² yr⁻¹. Shorter snow manipulation during the second year did not increase the annual emissions. Only 20% of the N₂O emission occurred during the growing season. Thus, these results highlight the importance of the winter season for this exchange and that the year-round measurements of annual N₂O emissions from boreal soils are integral for estimating their N₂O source strength. N₂O accumulated in the frozen soil during winter and the soil N₂O concentration correlated with the depth of frost but not with the winter N₂O emission rates per se. Also laboratory incubations of soil samples showed high production rates of N₂O at temperatures below 0°C, especially in the sand and peat soils.