Year-Round δ13C-CH4 Reveals Seasonal Transition in Methane-Related Processes in a Boreal Mire

While boreal mires are known to be a significant natural source of methane (CH4), the seasonality of the related processes and their controls are still poorly understood. Here we aim to characterize CH4 production, oxidation and transport, and their drivers in a boreal mire using year-round continuous measurements of stable carbon isotope composition (δ13C-CH4) in dissolved and emitted CH4. We found reversed vertical profiles of δ13C-CH4 in the summer (higher values at surface) and in the winter (higher values at bottom). The 13C enriched emitted CH4, as compared to pore water CH4, indicated methane oxidation at the peat-snow interface by sphagnum mosses in the winter. The observed hysteretic δ13C-CH4 - pCH4 relation indicated the importance of substrate availability for methane production in addition to soil temperature, and their time-lagged seasonal cycles. Our data also demonstrated the dominance of plant-mediated transport in the summer, the dominance of diffusion through peat and moss matrix (with associated microbial methane oxidation) in the winter and a transition in the spring and autumn. In general, the measured δ13C values of emitted CH4 at this and other northern mires are considerably lower than the values used in atmospheric inversion models. Our comprehensive data set provided invaluable insight into wetland δ13C-CH4, the dynamic interplay of multiple processes related to CH4 emission in boreal mires, especially in the rarely studied winter, spring, and autumn, the incorporation of which into Earth System Models will allow more accurate prediction of wetland responses to ongoing climate change.