High methane flux from an arctic floodplain (Indigirka lowlands, eastern Siberia)

Methane fluxes from arctic tundra soils on a river terrace and floodplain in northeastern Siberia, measured with flux chambers, show a high spatial variability. The methane fluxes on the river terrace compare well with fluxes reported in other studies on tundra methane fluxes. The average methane flux is 4.3 mg CH4 m(-2) hr(-1), and the average flux for wet sites is 7.2 mg m(-2) hr(-1) and for dry sites 0.18 mg m(-2) hr(-1), with negative fluxes occurring locally. The fluxes from floodplain sites are considerably higher, with an average flux of 12.5 mg m(-2) hr(-1), an average for wet sites of 23.4 mg m(-2) hr(-1), and an average for dry sites of 1.6 mg m(-2) hr(-1). Multilinear regression demonstrates that the methane flux depends on both water table and active layer thickness. However, these factors alone cannot explain the large floodplain methane fluxes. We hypothesize that the large fluxes from floodplain sites are caused by (1) higher primary productivity of the floodplain vegetation and a high plant-mediated transport of methane, and (2) enhanced supply of substrate for methanogens by sedimentation of particulate organic matter from flood water. Our results indicate that changes in discharge and organic matter transport of arctic rivers may have a considerable effect on arctic methane fluxes.