Overwinter Accumulation of Nitrous Oxide in Snow-Covered Sub-Arctic Boreal Soils

Abstract:

Nitrogen (N) is limiting to primary production in the sub-arctic boreal biome, which should constrain losses of N from ecosystems via streams or via gaseous pathways. However, large nitrous oxide (N₂O) emissions observed in some high-latitude ecosystems may be a result of permafrost thaw. Thaw is hypothesized to cause N₂O emission by introducing more N into the actively cycling pool or due to increased rates of N cycling at elevated temperatures. Unmeasured emissions of N₂O have significance for climate warming, because N₂O has a global warming potential 265 times that of carbon dioxide (CO₂), and losses of N from N-limited ecosystems further constrain the storage of carbon in plant biomass. We measured in-situ N₂O flux and concentration in soils adjacent to a thermokarst lake that is actively undergoing permafrost thaw and known to produce high CO₂ and methane (CH₄) emissions in the sub-arctic boreal forest of Interior Alaska. Fluxes from the surface were measured during the snow-free period, and indicated that some sites produced low, but consistent emissions of N₂O, though most sites did not emit N₂O. However, N₂O concentration measured in soils and within the snowpack during winter was greater than atmospheric concentration. Concentration increased over the winter and reached a maximum of 40 ppm at 25 cm and 13 ppm at 50 cm depths. These observations suggest active microbial cycling and gaseous release of N even when soils are frozen. It remains unclear whether the N₂O accumulated overwinter is evaded to the atmosphere upon thaw or is denitrified and released as nitrogen gas. In either case, these observations suggest large gaseous losses of N from ecosystems with unstable permafrost over winter that might maintain N limitation of plant growth.