Enhanced net formations of nitrous oxide and methane underneath the frozen soil in Sanjiang wetland, northeastern China

In temperate climates, up to ∼50–70% of the total annual nitrous oxide (N2O) flux occurs in winter, which is mainly attributed to freezing‐thawing processes of frozen soils. To investigate the nature of the enhanced N2O emission in winter, we monitored gas fluxes at two plots of freshwater marshes in northeastern China (47°35′N, 133°31′E), using a static chamber method at weekly to monthly intervals from 2002 to 2004. The results show that the big emission burst of N2O coupled with peak emission of CH4 occurred in late July to early August, which is about 1 week after the soil thawed out completely. Further observations show that the N2O and CH4 were continuously produced underneath the frozen soil. The gas‐producing layer was continuously depressed downward during the progressive freezing. The highest concentrations of N2O and CH4 were first observed when the frozen layer reached its maximum depth of ∼90 cm in early February and remained there with roughly constant concentrations until June. We propose that at the beginning the frozen soil limits the transportation of oxygen from atmosphere to soil, while freezing increases the nutrient supplies, producing an anoxic organic‐rich environment underneath the frozen layer, which promotes the net formation of N2O and CH4 through denitrification and anoxic digestion of organic matters, respectively. In contrast, after the frozen layer reached the organic‐poor gley layer and there is no further supply of nutrients through freezing, the formation of N2O and CH4 stopped shortly after.