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Methane (CH 4 ) production, oxidation, and emission were studied in ponds of the permafrost-affected polygonal tundra in northeast Siberia. Microbial degradation of organic matter in water-saturated soils is the most important source for the climate-relevant trace gas CH 4 . Although ponds and lakes cover a substantial fraction of the land surface of northern Siberia, data on CH 4 fluxes from these water bodies are scarce. Summer CH 4 fluxes were measured with closed chambers at the margins of ponds vegetated by vascular plants and in their centers without vascular plants. Furthermore, CH 4 and oxygen concentration gradients, stable carbon isotope signatures of dissolved and emitted CH 4 , and microbial CH 4 production and CH 4 oxidation were determined. Mean summer fluxes were significantly higher at the margins of the ponds (46.1 ± 15.4 mg CH 4 m À2 d À1 ) than at the centers (5.9 ± 8.2 mg CH 4 m À2 d À1 ). CH 4 transport was dominated by diffusion in most open water sites, but substantial ebullitive fluxes (12.0 ± 8.1 mg CH 4 m À2 d À1 ) were detected in one pond. Plant-mediated transport accounted for 70 to 90% of total CH 4 fluxes above emerged vegetation.In the absence of vascular plants, 61 to 99% of the CH 4 produced in the anoxic bottom soil was consumed in a layer of the submerged moss Scorpidium scorpioides, which covered the bottoms of the ponds. The fraction of CH 4 oxidized was lower at sites with vascular plants since CH 4 was predominantly transported through their aerenchyma, thereby bypassing the CH 4 oxidation zone in the moss layer. These results emphasize the importance of moss-associated CH 4 oxidation causing low CH 4 fluxes from the studied Siberian ponds. soil surface forming polygonal ponds [Muster et al., 2012;Zibulski et al., 2013]. The vegetation in these shallow ponds is dominated by mosses, sedges, and grasses, such as Carex spp. or Arctophila fulva [Walker, 1985;Zibulski et al., 2013]. With increasing pond water depth, the abundance of vascular plants generally decreases, and their growth remains restricted to the shallow ponds' margins. In case of the wet and nonacidic tundra in northern Siberia and Alaska, the dominant moss species at these water-saturated soils and ponds are not Sphagnum mosses but Bryopsida species like Scorpidium scorpioides, Meesia triquetra, and Drepanocladus KNOBLAUCH ET AL. METHANE TURNOVER IN SIBERIAN PONDS 2525 PUBLICATIONS

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N2 is one of the major gaseous nitrogen compounds released by soils due to N-transformation processes. Since it is also the major constituent of the earth's atmosphere (78.08% vol.), the determination of soil N2 release is still one of the main methodological challenges with respect to a complete evaluation of the gaseous N-loss of soils. Commonly used approaches are based either on a C2H2 inhibition technique, an artificial atmosphere or a 15N-tracer technique, and are designed either as closed systems (non-steady state) or gas flow systems (steady state). The intention of this work has been to upgrade the current gas flow technique using an artificial atmosphere for a 15N-aided determination of the soil N2 release simultaneously with N2O. A 15N-aided artificial atmosphere gas flow approach has been developed, which allows a simultaneous online determination of N2 as well as N2O fluxes from an open soil system (steady state). Fluxes of both gases can be determined continuously over long incubation periods and with high sampling frequency. The N2 selective molecular sieve Köstrolith SX6 was tested successfully for the first time for dinitrogen collection. The presented paper mainly focuses on N2 flux determination. For validation purposes soil aggregates of a Haplic Phaeozem were incubated under aerobic (21 and 6 vol.% O2) and anaerobic conditions. Significant amounts of N2 were released only during anaerobic incubation (0.4 and 640.2 pmol N2 h(-1) g(-1) dry soil). However, some N2 formation also occurred during aerobic incubation. It was also found that, during ongoing denitrification, introduced [NO3]- will be more strongly delivered to microorganisms than the original soil [NO3]-.

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