Winter and summer nitrous oxide and nitrogen oxides fluxes from a seasonally snow-covered subalpine meadow at Niwot Ridge, Colorado

The soil emission rates (fluxes) of nitrous oxide (N(2)O) and nitrogen oxides (NO + NO(2) = NO (x) ) through a seasonal snowpack were determined by a flux gradient method from near-continuous 2-year measurements using an automated system for sampling interstitial air at various heights within the snowpack from a subalpine site at Niwot Ridge, Colorado. The winter seasonal-averaged N(2)O fluxes of 0.047-0.069 nmol m(-2) s(-1) were similar to 15 times higher than observed NO (x) fluxes of 0.0030-0.0067 nmol m(-2) s(-1). During spring N(2)O emissions first peaked and then dropped sharply as the soil water content increased from the release of snowpack meltwater, while other gases, including NO (x) and CO(2) did not show this behavior. To compare and contrast the winter fluxes with snow-free conditions, N(2)O fluxes were also measured at the same site in the summers of 2006 and 2007 using a closed soil chamber method. Summer N(2)O fluxes followed a decreasing trend during the dry-out period after snowmelt, interrupted by higher values related to precipitation events. These peaks were up to 2-3 times higher than the background summer levels. The integrated N(2)O-N loss over the summer period was calculated to be 1.1-2.4 kg N ha(-1), compared to similar to 0.24-0.34 kg N ha(-1) for the winter season. These wintertime N(2)O fluxes from subniveal soil are generally higher than the few previously published data. These results are of the same order of magnitude as data from more productive ecosystems such as fertilized grasslands and high-N-cycling forests, most likely because of a combination of the relatively well-developed soils and the fact that subnivean biogeochemical processes are promoted by the deep, insulating snowpack. Hence, microbially mediated oxidized nitrogen emissions occurring during the winter can be a significant part of the N-cycle in seasonally snow-covered subalpine ecosystems.