While boreal mires are known to be significant natural source of methane (CH4), the seasonal variation of the processes involved in emission 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 together with CH4 concentration in pore water (pCH4). We found the vertical profiles of pCH4 and δ13C-CH4 in the summer, with higher values near the peat surface for both, to be typical for fens, whereas the profiles reversed in the winter to resemble those of bogs. 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 between in shoulder seasons. Our comprehensive dataset provided invaluable insight into the dynamic interplay of multiple processes related to CH4 emission in boreal mires, especially in the rarely-studied winter and shoulder seasons, the incorporation of which into Earth System Models will allow more accurate prediction of wetland responses to ongoing climate change.