Methane oxidation potential of the arctic wetland soils of a taiga-tundra ecotone in northeastern Siberia

Abstract. Arctic wetlands are significant sources of atmospheric methane and the observed accelerated climate changes in the arctic could cause the change in methane dynamics, where methane oxidation would be the key process to control methane emission from wetlands. In this study we determined the potential methane oxidation rate of the wetland soils of a taiga-tundra transition zone in northeastern Siberia. Peat soil samples were collected in summer from depressions covered with tussocks of sedges and Sphagnum spp. and from mounds vegetated with moss and larch trees. A bottle incubation experiment demonstrated that the soil samples collected from depressions in the moss- and sedge-dominated zones exhibited active methane oxidation with no time lag. The potential methane oxidation rates at 15 °C ranged from 94 to 496 nmol h⁻¹ g⁻¹ dw. Methane oxidation was observed over the depths studied (0–40 cm) including the water-saturated anoxic layers. The maximum methane oxidation rate was recorded in the layer above the water-saturated layer: the surface (0–2 cm) layer in the sedge-dominated zone and in the middle (4–6 cm) layer in the moss-dominated zone. The methane oxidation rate was temperature-dependent, and the threshold temperature of methane oxidation was estimated to be −4 to −11 °C, which suggested methane oxidation at subzero temperatures. Soil samples collected from the frozen layer of Sphagnum peat also showed immediate methane consumption when incubated at 15 °C. The present results suggest that the methane oxidizing bacteria in the wetland soils keep their potential activities even under anoxic and frozen conditions and immediately utilize methane when the conditions become favorable. On the other hand, the inhibitor of methane oxidation did not affect the methane flux from the sedge and moss zones in situ, which indicated the minor role of plant-associated methane oxidation.