Mechanisms responsible for high N2O emissions from subarctic permafrost peatlands studied via stable isotope techniques

Recent field studies have shown that there are habitats in the subarctic tundra emitting N2O at exceptionally high rates. In this study, stable isotope techniques were applied to characterize the processes responsible for these high N2O emissions which have been found from bare peat surfaces in permafrost peatlands. The results include the first data on the nitrogen and oxygen isotopic composition of N2O emitted from arctic tundra. The emission-weighted average δ15Nbulk value for N2O of −13.0‰ ± 2.0‰ (mean ± SD; n = 8) from the bare peat surfaces falls within the range of the emission-weighted average values from other natural ecosystems but is distinct from those for managed/agricultural ecosystems. This implies that if in the future, a smaller rate in the overall decreasing trend of δ15Nbulk N2O tropospheric isotopic composition is found, it cannot be attributed only to agricultural N2O emission reductions from mitigation actions but also to soils in natural ecosystems that may be emitting more N2O to the atmosphere due to warmer conditions. The site preference (SP) values from emitted N2O range from −30‰ to 58‰, indicating a temporal shift of microbial production and consumption of N2O during the sampling period. Soil emission SP data suggest that the N2O emission in subarctic tundra are more likely to be produced by nitrifier denitrification in the drier study year, but due to variable published SP values for N2O production processes in soils, this interpretation has to be taken with caution. According to SP values at depth, denitrification was the main N2O production pathway. To better address the usefulness of SP in partitioning microbial mechanisms in soils, further studies in soils mesocosms are required.