Process-Level Measurements of Methane Production, Oxidation, and Efflux Across Geomorphic Gradients in Arctic Polygon Tundra

Abstract:

Arctic wetlands are currently net sources of atmospheric CH₄. CH₄ emissions in Arctic tundra vary widely in space and time, with proximate controls—competing decomposition processes, substrate availability, CH₄ oxidation, and CH₄ transport—dependent on local climate, soil, hydrology, and biology. These complex controls and high spatial and temporal variability make it difficult to characterize current CH₄ processes and predict their responses to climate change. We ask: (1) how do metabolic CH₄ production pathways and oxidation rates vary across spatial and temporal gradients in the Arctic, and (2) how do these subsurface processes relate to surface greenhouse gas fluxes? In polygonal tundra in Barrow, Alaska, we sampled from a gradient of topographic features and subsurface ice properties, from areas with large, intact ice wedges and ice-rich permafrost (low polygons) and areas with smaller, degraded ice wedges and lower permafrost ice contents (flat/high polygons). 5 times in 2012 and 2013, we measured surface CH₄ and CO₂ fluxes, soil pore space concentrations of CH₄, DIC, and N₂O at 3 depths, stable isotope abundances of CH₄ and DIC, and soil moisture and temperature. We find that surface greenhouse gas fluxes and CH₄ production pathways vary between geomorphic features. Low polygons have high CH₄ fluxes (~200 nm/m2s) and carbon isotope compositions typical of acetate cleavage, and flat/high polygons have low or negative CH₄ fluxes and isotopic signatures typical of CO₂ reduction. High dissolved CH₄ concentrations at depth and ¹³C enrichment of shallow CH₄ suggest CH₄ oxidation controls surface efflux in upland features, while dual isotope measurements (D/H and ¹³C/ ¹²C) indicate shifting CH₄ production pathways with depth, with acetate cleavage more important at shallower depths. Among distinct sub-features within each polygon type, we find that CH₄ processes, relative oxidation rates, and surface emissions depend on subsurface ice properties in ways that cannot be explained by surface soil moisture and temperature measurements alone. As part of the Next Generation Ecosystem Experiment (NGEE-Arctic), this study’s results will be integrated with a broad range of measurements, with the goal of improving regional-scale models.