Hot spots for nitrous oxide emissions found in different types of permafrost peatlands

摘要

Recent findings on large nitrous oxide (N2O) emissions from permafrost peatlands have shown that tundra soils can support high N2O release, which is on the contrary to what was thought previously. However, field data on this topic have been very limited, and the spatial and temporal extent of the phenomenon has not been known. To address this question, we studied N2O dynamics in two types of subarctic permafrost peatlands, a peat plateau in Russia and three palsa mires in Finland, including also adjacent upland soils. The peatlands studied have surfaces that are uplifted by frost (palsas and peat plateaus) and partly unvegetated as a result of wind erosion and frost action. Unvegetated peat surfaces with high N2O emissions were found from all the studied peatlands. Very high N2O emissions were measured from peat circles at the Russian site (1.40 +/- 0.15 g N(2)Om(-2)yr(-1)). Elevated, sparsely vegetated peat mounds at the same site had significantly lower N2O release. The N2O emissions from bare palsa surfaces in Northern Finland were highly variable but reached high rates, similar to those measured from the peat circles. All the vegetated soils studied had negligible N2O release. At the bare peat surfaces, the large N2O emissions were supported by the absence of plant N uptake, the low C:N ratio of the peat, the relatively high gross N mineralization rate and favourable moisture content, together increasing availability of mineral N for N2O production. We hypothesize that frost heave is crucial for high N2O emissions, since it lifts the peat above the water table, increasing oxygen availability and making it vulnerable to the the physical processes that may remove the vegetation cover. In the future, permafrost thawing may change the distribution of wet and dry surfaces in permafrost peatlands, which will affect N2O emissions.