N2O emissions from snow-covered soils in the Swiss Alps

N2O emissions were investigated at two sites in the Swiss Alps, Alptal (SZ) and Piora Valley (TI), using the closed, vented chamber method during the snow-free period and the snowpack diffusion method during snow cover. Significant spatial variations in surface N2O effluxes between different sampling subsites were observed in both summer and winter. While in winter N2O fluxes from a raw humus site in Alptal and from a site of neutral pH in the Piora Valley were very low (0.01–0.02 mg N2O-N m−2 d−1) and similar to other published values, N2O emissions for acidic and alder-covered sites were 0.3–0.4 mg N2O-N m−2 d−1 and reached 1.7–1.9 mg N2O-N m−2 d−1 for nutrient-rich sites. This suggests that the soil temperatures close to the freezing point were not limiting microbial activity under snow. Snow-covered soils are insulated from extreme air temperatures and stay constant between −0.3 and −1.1°C. They represent an environment where micro-organisms may remain active while vegetation is dormant. N2O liberation is repressed at neutral and raw humus sites, as nitrification is the main source of N2O. During summer the soil microbial activity is strongly related to water availability and temperature: in contrast, in winter both factors are rather constant and the controlling factors of heterotrophic activity are unknown. Our data show that N2O is a significant component of alpine N cycling and that winter N2O fluxes should not be neglected in the calculation of the annual budget for alpine ecosystems, considering that snow covers (sub-) alpine regions for a large part of the year