Improved quantification of microbial CH<sub>4</sub> oxidation efficiency in arctic wetland soils using carbon isotope fractionation

Abstract

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 (αox) and diffusion (αdiff) 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 (αtrans = 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 αdiff = 1.001 ± 0.000 (n = 3). CH4 stable isotope fractionation during diffusion through air-filled pores of the investigated polygonal tundra soils was αdiff = 1.013 ± 0.003 (n = 18). Furthermore, it was found that αox differs widely between sites and horizons (mean αox = 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.

Distribution of methane in the Lena Delta and Buor-Khaya Bay, Russia

“…However, the observed concentrations are within the lower range of boreal rivers for which average concentrations between 10 and 1400 nM have been reported (Middelburg et al, 2002). Isotopic data of methane in creek water and in the Olenekskaya Channel (−42 and −39 ‰) showed a rather heavy methane signature when compared to other Arctic lakes (−58 ‰, Walter et al, 2008) and is much more closer to water samples from polygons at near-by Samoylov Station (−45 ‰, Preuss et al, 2013). This unusual signal could indicate that the organic matter used for methane production probably also had a heavy signature.…”

“…Thus, the methane flux from the Lena River and its various channels is about 3-10 times higher than the terrestrial emissions. One reason for the lower terrestrial emission could be that for the terrestrial environment an intense microbial methane oxidation was observed (Preuss et al, 2013;Liebner and Wagner, 2007), which seems to be absent in the river. However, for an assessment of the regional methane fluxes from the whole Lena Delta, it would be important to take into account the different areas of river channels, thaw ponds and terrestrial environment.…”

Methane-oxidizing seawater microbial communities from an Arctic shelf

“…Isotopic fractionation factors of methane oxidation (α ox = k 12 k 13 ) were determined as described in Preuss et al (2013), using the isotope fractionation approach (Coleman et al, 1981).…”

Year-round simulated methane emissions from a permafrost ecosystem in Northeast Siberia

“…f CH4anox = 0.5. We used this value in the reference simulation because it was previously reported in the literature as characteristic of water-saturated polygon centers (Preuss et al, 2013), and it is similar to the value reported for unsaturated zones in boreal bogs (Wahlen and Reeburgh, 2000). However, in wetland areas, CH 4 is still subject to oxidation after its production and the CO 2 :CH 4 ratio is 890 expected to increase and to vary among types of wetlands (Bridgham et al, 2013).…”