| Popis: |
Permafrost-affected tundra soils are significant sources of the climate-relevant trace gas methane (CH 4 ). The observed accelerated warming of the arctic will cause deeper permafrost thawing, followed by increased carbon mineralization and CH 4 formation in water-saturated tundra soils, thus creating a positive feedback to climate change. Aerobic CH 4 oxidation is regarded as the key process reducing CH 4 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 CH 4 oxidation efficiency in arctic wetland soils. The aim of the current study is to quantify CH 4 oxidation efficiency in permafrost-affected tundra soils in Russia's Lena River delta based on stable isotope signatures of CH 4 . Therefore, depth profiles of CH 4 concentrations and δ 13 CH 4 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 CH 4 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 CH 4 diffusion through water-saturated soils was determined with an α diff = 1.001 p 0.000 ( n = 3). CH 4 stable isotope fractionation during diffusion through air-filled pores of the investigated polygonal tundra soils was α diff = 1.013 p 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 CH 4 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 CH 4 oxidation efficiency of 50% at the anaerobic–aerobic interface in the upper horizon. The improved in situ quantification of CH 4 oxidation in wetlands enables a better assessment of current and potential CH 4 sources and sinks in permafrost-affected ecosystems and their potential strengths in response to global warming. |
