CH₄ transport in wetland plants under controlled environmental conditions – separating the impacts of phenology from environmental variables

Background and Aims: Methane (CH₄) fluxes at peatland plant surfaces are net results of transport of soil-produced CH₄ and within-plant CH₄ production and consumption, yet factors and processes controlling these fluxes remain unclear. We aimed to assess the effects of seasonality, environmental variables, and CH₄ cycling microbes on CH₄ fluxes from characteristic fen species. Methods: Four species (Carex rostrata, Menyanthes trifoliata, Betula nana, Salix lapponum) were selected, and their CH₄ fluxes determined in climate-controlled environments with three mesocosms per growing season per species. Microbial genes for CH₄ cycling were analysed to check the potential for within-plant CH₄ production and oxidation. Two extra experiments were conducted: removal of C. rostrata leaves to identify how leaves constrain CH₄ transport, and a labelling experiment with S. lapponum to distinguish between plant-produced and soil-produced CH₄ in the plant flux. Results: All species showed seasonal variability in CH₄ fluxes. Higher porewater CH₄ concentration increased fluxes from C. rostrata and M. trifoliata, decreased fluxes from S. lapponum, and did not affect fluxes from B. nana. Air temperature only and negatively affected CH₄ flux from C. rostrata. Light level did not impact CH₄ fluxes. Both methanogens and methanotrophs were detected in shoots of S. lapponum and M. trifoliata, methanotrophs in B. nana, and neither in C. rostrata. Conclusion: Our study demonstrates that the seasonal phase of the plants regulates the CH₄ fluxes they mediate across species. The detection of methanogens and methanotrophs in herbs and shrubs suggests that microbial processes may contribute to their CH₄ fluxes.