In-situ Studies of Microbial CH4 Oxidation Efficiency in Arctic Wetland Soils – Application of Stable Carbon Isotopes

Arctic wetland soils are significant sources of the climate-relevant trace gas methane (CH 4 ).The observed accelerated warming of the Arctic is expected to cause deeper permafrost thawing followed by increased carbon mineralization and CH 4 formation in water-saturated permafrost-affected 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.This study improved the in-situ quantification of microbial CH 4 oxidation efficiency in arctic wetland soils in Russia's Lena River Delta based on stable isotope signatures of CH 4 .In addition to the common practice of determining the stable isotope fractionation during oxidation, additionally the fractionation effect of diffusion, an important gas transport mechanism in tundra soils, was investigated for both saturated and unsaturated conditions.The isotopic fractionation factors α ox and α diff were used to calculate the CH 4 oxidation efficiency from the CH 4 stable isotope signatures of wet polygonal tundra soils of different hydrology.Further, the method was used to study the short-term effects of temperature increase with a climate manipulation experiment.For the first time, the stable isotope fractionation of CH 4 diffusion through water-saturated soils was determined with α diff = 1.001 ± 0.0002 (n = 3).CH 4 stable isotope fractionation during diffusion through air-filled pores of the investigated polygonal tundra soils was α diff = 1.013 ± 0.003 (n = 18).For the studied sites the fractionation factor for diffusion under saturated conditions α diff = 1.001 seems to be of utmost importance for the quantification of the CH 4 oxidation efficiency, since most of the CH 4 is oxidized in the saturated part at the aerobic-anaerobic interface.Furthermore, it was found that α ox differs widely between sites and horizons (mean α ox = 1.018 ± 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 diffusivity under saturated and unsaturated conditions and potential oxidation rates.The predominant water table determines the magnitude of CH 4 oxidation efficiencies in arctic wetland soils: submerged organic-matter-rich soils indicated CH 4 oxidation efficiencies of Summary VIII 10 to 70 %, while polygon centers and rims with an aerobic surface layer showed capacity of complete oxidation.Temperature increase might affect CH 4 oxidation efficiencies of saturated sites in the long term, however short-time effects were not observed.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.