CH4 and N2O emissions from a forest-alas ecosystem in the permafrost taiga forest region, eastern Siberia, Russia

'Alases' are mature thermokarst depressions covered by grassland distributed in taiga forests in central Yakutia, eastern Siberia, following thermokarst formation initiated in early Holocene. Alases are important land-cover class in the central Yakutia lowland occupying 17% of the total land area. CH4 and N2O fluxes were measured temporally in a typical alas which have a pond at the center during the growing season of two years. Seven monitoring plots represented various vegetation types: a larch forest (F), a dry grassland (DG), four wet grasslands (WG) flooded temporarily or continuously, and a pond surface (P) flooded continuously without vegetation. The pond flooding area reached its maximum just after snow-melting and decreased during the summer. The F and DG plots were small CH4 sinks. The wet plots (including WG and P) were large CH4 sources (cumulative value of growing season: 17 to 864 kg CH4-C ha(-1)), and emission rates vary drastically depending on flooding conditions. All plots were slight sources or sinks of N2O except for the WG plots (0.16 to 1.7 kg N2O-N ha(-1)) where peak emissions were observed after the flooding ended. The global warming potentials (time horizon of 100 years) of the cumulative N2O emissions from the plots were lower than those of each CH4 emissions. Estimated total CH4 emission from the whole WG area (7.93 ha) accounted for 88% of the emission from the whole alas (28.9 ha). The current permafrost-taiga region in central Yakutia could be a net CH4 source owing to the presence of alases.