Below-ground Geochemistry Underlying Within-Site Variation of Methane Emissions in a Northern Boreal Fen

Spatial variability of methane emissions within peatlands have been linked to water table depth, vegetation, and other parameters, but relatively few studies have investigated how these differences are relate to below-ground conditions like pore-water chemistry. The Lopolojänkkä fen shows strong spatial variation in surface methane fluxes. A previous study showed highest emissions found halfway (30-60m) from the central stream to the edge of the peatland. Emissions decrease towards the edge of the peatland due to deeper water table depth, but also towards its center due to greater convective oxygen transport into the peat (Zhang et al., 2020). It remains unknown, however, if these difference in methane emissions are driven by changes in methane production and/or consumption ratesHere, we report on an intensive survey to understand the below-ground geochemistry underlying these spatial variations in methane emissions. We established a transect from the center to the edge of the peatland, where we collected porewater samples at 20-100 cm depth from a total of 15 points. We find a strong correlation of surface methane fluxes with porewater dissolved organic matter concentrations. Dissolved methane concentrations revealed that the decrease of methane emissions towards the edge were likely caused by lower methane production rates, represented by lower porewater methane concentrations throughout the peat profile. Decreasing methane emission towards the central stream, in contrast, were likely driven by greater methane oxidation, as evidenced by high methane concentration a great depth which decreased steeply towards the peat surface.