Improved quantification of microbial CH₄ oxidation efficiency in arctic wetland soils using carbon isotope fractionation

Abstract. Permafrost-affected tundra soils are significant sources of the climate-relevant trace gas methane (CH₄). The observed accelerated warming of the arctic will cause deeper permafrost thawing, followed by increased carbon mineralization and CH₄ formation in water-saturated tundra soils, thus creating a positive feedback to climate change. Aerobic CH₄ oxidation is regarded as the key process reducing CH₄ emissions from wetlands, but quantification of turnover rates has remained difficult so far. The application of carbon stable isotope fractionation enables the in situ quantification of CH₄ oxidation efficiency in arctic wetland soils. The aim of the current study is to quantify CH₄ oxidation efficiency in permafrost-affected tundra soils in Russia's Lena River delta based on stable isotope signatures of CH₄. Therefore, depth profiles of CH₄ concentrations and δ¹³CH₄ signatures were measured and the fractionation factors for the processes of oxidation (α_ox) and diffusion (α_diff) were determined. Most previous studies employing stable isotope fractionation for the quantification of CH₄ oxidation in soils of other habitats (such as landfill cover soils) have assumed a gas transport dominated by advection (α_trans = 1). In tundra soils, however, diffusion is the main gas transport mechanism and diffusive stable isotope fractionation should be considered alongside oxidative fractionation. For the first time, the stable isotope fractionation of CH₄ diffusion through water-saturated soils was determined with an α_diff = 1.001 ± 0.000 (n = 3). CH₄ stable isotope fractionation during diffusion through air-filled pores of the investigated polygonal tundra soils was α_diff = 1.013 ± 0.003 (n = 18). Furthermore, it was found that α_ox differs widely between sites and horizons (mean α_ox = 1.017 ± 0.009) and needs to be determined on a case by case basis. The impact of both fractionation factors on the quantification of CH₄ oxidation was analyzed by considering both the potential diffusion rate under saturated and unsaturated conditions and potential oxidation rates. For a submerged, organic-rich soil, the data indicate a CH₄ oxidation efficiency of 50% at the anaerobic–aerobic interface in the upper horizon. The improved in situ quantification of CH₄ oxidation in wetlands enables a better assessment of current and potential CH₄ sources and sinks in permafrost-affected ecosystems and their potential strengths in response to global warming.