Atmospheric CH₄ oxidation by Arctic permafrost and mineral cryosols as a function of water saturation and temperature

The response of methanotrophic bacteria capable of oxidizing atmospheric CH₄ to climate warming is poorly understood, especially for those present in Arctic mineral cryosols. The atmospheric CH₄ oxidation rates were measured in microcosms incubated at 4 °C and 10 °C along a 1-m depth profile and over a range of water saturation conditions for mineral cryosols containing type I and type II methanotrophs from Axel Heiberg Island (AHI), Nunavut, Canada. The cryosols exhibited net consumption of ~2 ppmv CH₄ under all conditions, including during anaerobic incubations. Methane oxidation rates increased with temperature and decreased with increasing water saturation and depth, exhibiting the highest rates at 10 °C and 33% saturation at 5 cm depth (260 ± 60 pmol CH₄ gdw^-1 d^-1 ). Extrapolation of the CH₄ oxidation rates to the field yields net CH₄ uptake fluxes ranging from 11 to 73 μmol CH₄  m^-2 d^-1 , which are comparable to field measurements. Stable isotope mass balance indicates ~50% of the oxidized CH₄ is incorporated into the biomass regardless of temperature or saturation. Future atmospheric CH₄ uptake rates at AHI with increasing temperatures will be determined by the interplay of increasing CH₄ oxidation rates vs. water saturation and the depth to the water table during summer thaw.