CH4 transport in wetland plants under controlled environmental conditions – untangling the impacts of phenology

Background and Aims Methane (CH 4 ) fluxes at peatland plant surfaces are the net result of transport of soil-produced CH 4 and within-plant CH 4 production and consumption, yet factors and processes controlling these fluxes remain unclear. We aimed to assess the effects of environmental variables, seasonality and CH 4 cycling microbes in CH 4 fluxes from shoots of characteristic fen species. Methods We conducted high-frequency automated CH 4 flux measurements from shoots of a sedge ( Carex rostrata) , a forb ( Menyanthes trifoliata) and two shrubs ( Betula nana , Salix lapponum ) in a climate-controlled environment. Measurements were conducted from intact plant-soil samples collected throughout growing seasons 2020 and 2021 from Lompolojänkkä fen, northern Finland. Results All studied species showed seasonal variability in CH 4 fluxes. The CH 4 fluxes were not impacted by light level, while higher porewater CH 4 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 CH 4 flux from C. rostrata . Both methanogens and methanotrophs were detected in aboveground parts of S. lapponum and M. trifoliata , methanotrophs in B. nana , while neither were detected in C. rostrata . Conclusion Our study demonstrates that the seasonal phase of the plants regulates CH 4 fluxes they mediate across species, which was not observed in the field. The detection of methanogens and methanotrophs in herbs and shrubs suggests that microbial processes may contribute to their CH 4 fluxes.