Regulation of methane production, oxidation, and emission by vascular plants and bryophytes in ponds of the northeast Siberian polygonal tundra

Abstract

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

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

“…> 90 % of the produced CH 4 is oxidized before it 900 is emitted to the atmosphere). This value has been estimated in polygonal ponds without vascular plants, empirically supporting the relevance of CH 4 oxidation below the water table in these types of environments (Knoblauch et al, 2016). A lower CH 4 oxidation fraction occurs in the presence of vascular plants that are effective at bypassing the aerobic areas in the soil.…”

“…Values of f ox have been previously 895 reported as ranging between 0.6-0.7 for sites with vascular plants. On the other hand, it can be nearly equal to 1 in sites with, for example, a layer of Sphagnum moss, where the majority of the produced CH 4 is oxidized, or in bottom soils in pond centers where slow molecular diffusion of CH 4 takes place through the water (Knoblauch et al, 2016). Under the latter conditions, f ox can be approximated to > 0.9 (i.e.…”

“…equilibrium conditions). Both h snow and φ are parameters in the model used to restrict the transport of gas via diffusion through snow during the tic permafrost tundra ecosystems, plant-mediated transport is the dominating methane transport pathway over ebullition and diffusion (Knoblauch et al, 2016), and this transport pathway can contribute from 30 to 100 % to the total CH 4 flux (Bridgham et al, 2013).…”

“…However, the resulting total CH 4 emissions with the three tested snow threshold depths were not statistically different. The JSBACH-methane model does not explicitly consider some mechanisms related to the 930 carbon decomposition and thaw in Arctic permafrost and wetland ecosystems, such as: CH 4 production in the soil from root exudates (Knoblauch et al, 2016;Ström et al, 2012), vertical transfer of carbon and its vertically resolved decomposition (Braakhekke et al, 2011(Braakhekke et al, , 2013(Braakhekke et al, and 2014Koven et al, 2015), and microbial community dynamics (McCalley et al, 2014) involved in anoxic CH 4 oxidation or the production of CH 4 in anaerobic microsites confined 935 in oxic soils. Although these processes might contribute substantially to the dynamics of CH 4 , research on these processes in soil-permafrost and wetland environments is still lacking or poorly understood with controversial results so far (Bridgham et al, 2013).…”

“…However, ebullition is a rapid process that is highly variable temporally and spatially. Observations have shown that CH 4 emitted through ebullition occurs sporadically and mainly in undisturbed natural water bodies, such as lakes and ponds, where gas bubbles formed during 1270 active methanogenesis ascend rapidly through the water and into the atmosphere, circumventing aerobic soil layers (Klapstein et al, 2014;Knoblauch et al, 2016). Thus, by leaving this process in the last position we ensured that the small amounts of CH 4 transported by molecular diffusion can work properly.…”