Belowground methane cycling along a stream-to-edge transect in the Lompolojänkkä fen

Lompolojänkkä is a nutrient-rich fen located in western Lapland. The site has been the focus of detailed methane flux measurements, which revealed high spatial variability along a transect from the central stream to the edge of the peatland. Surprisingly, the highest fluxes did not occur in the center of the peatlands, but rather at the halfway point between the center and the edge of the peatland, likely due greater oxygen transport by turbulent water flow at the center of the peatland. In this study, we aim to quantify the contribution of hydrogenotrophic and acetoclastic methanogenesis, the fraction of methane oxidized prior to emission to the atmosphere, and the location (depth) of these processes in the peat profile. We further investigate if these processes differ in space along a the stream-to-edge transect and time with the progress of the growth season. To quantify these processes, we collected pore water samples from 15 depth profiles at 20 to 100 cm depth. In these samples we quantified concentrations of dissolved methane, its carbon and hydrogen isotope values, and a suite of geochemical measures. We find that locations close to the central stream are characterized by high methane concentrations at depth, which decrease steeply towards the surface, indicating that high rate of methane are produced at depth but oxidized prior to reaching the surface. Sites located at the edge of the peatland, in contrast, show low methane concentrations throughout the peat profiles, indicating that small amounts of methane are produced relatively close to the surface. Stable carbon and hydrogen isotope values add additional complexity to our understanding of the methane dynamics. Methane oxidation is associated with strong increases in both δ13C and δ2H values in the residual methane and would therefore be indicated by an increase in both isotope values from deep to shallow peat layers. Such a pattern, however, was only detected close to the central stream, where approximately 50% of methane was oxidized prior to reaching the surface. In most other transect points, we found that δ13C increased from deep to shallow layers, whereas δ2H showed the opposite trend, indicating the mixing of hydrogenotrophic methane produced in deep peat layers with acetoclastic methane produced in the rooting zone. An isotope mixing model indicated that that the fraction of hydrogenotrophic methane increased from center to edge of the site (from 45 to 30% at 100 cm depth in June) and with the advancing growth season (32 to 0% in September). In contrast, we typically find less than 15% hydrogenotrophic methane in shallow layers. We note that these numbers are associated with significant external uncertainty stemming from poor certainty about mixing model parameters. Overall, our data demonstrates high and temporal spatial heterogeneity of methane production and oxidation within a single site. We demonstrate the additional information gained methane dual isotope analysis, and reveals how δ13C profiles alone can be ambiguous and misleading.