Assessment of the Effects of Longer Thaw Seasons on Subsurface Methane Production and Consumption in the Arctic

Methane is a highly potent greenhouse gas that is produced by microorganisms in anaerobic soil horizons. This methane can escape to the atmosphere or be consumed converted to CO₂ by methane oxidizing organisms. The net balance of these two processes in Arctic soils is highly dependent on a complex interaction between factors including the depth of the active layer, soil moisture content, soil organic matter, plant cover and the types of methanogens and methane oxidizers present in the soil. To assess the importance of these factors and quantify production and oxidation rates of methane in Arctic soils, we are monitoring the concentrations and isotopic compositions of methane and dissolved inorganic carbon (DIC)/CO₂ in different soil environments at the Barrow Experimental Observatory (BEO) as part of the Next Generation Ecosystem Experiment (NGEE) Arctic project. Gas/water samplers have been installed at depths of ~30 cm, 20 cm and 10 cm in the centers, edges and troughs of polygonal soil features on the BEO. In addition, surface flux samples are collected with each set of soil gas/water samples. During 2012, a limited set of surface flux samples and gas/water samples were collected early in the thaw season (late June/early July) and more complete sets were collected during mid August and late September/early October (at the beginning of freeze up). In general, the early season samples had much lower concentrations of dissolved methane and CO₂/DIC than samples taken during the mid and late thaw season (concentrations of dissolved methane as high as 950 μM were measured in deep, water-saturated soils in the latter part of the thaw season). The isotopic compositions of methane indicate that acetoclastic methanogenesis is the dominant mode of microbial methane production in the low-centered, water-saturated polygons whereas CO₂ reduction is more common in transitional and high-centered polygons. Increased δ¹³C values of methane at shallower depths in aerobic, unsaturated soils of the transitional and high-centered polygons indicates that oxidation of methane produced at depth is occurring. Similar shifts were smaller to non-existent in the low-centered polygons, which also had the highest surface methane fluxes, suggesting that oxidation is not mediating the flux of methane to the atmosphere. For this year, we are collecting more frequent samples from a higher number of sites in order to quantify the effect of increasing thaw depth during the early part of the season and freeze up at the end of the season.