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

Summary Methane (CH 4 ) fluxes at 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 conducted high-frequency automated CH 4 flux measurements from shoots of Carex rostrata (sedge), Menyanthes trifoliata (forb) and shrubs ( Betula nana , Salix lapponum ) during early, high and late summer in a climate-controlled environment to assess the effects of environmental variables, seasonality and CH4 cycling microbes in the CH4 flux. Measurements were conducted from intact plant-soil samples collected throughout growing seasons 2020 and 2021 from Lompolojänkkäfen, northern Finland. All studied species showed seasonal variability in CH 4 fluxes. The CH4 fluxes were not impacted by light level, while out of the studied species, porewater CH 4 concentration increased fluxes from all but B. nana. Air temperature only and negatively affected CH4 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 . Our study demonstrates that the seasonal phase of the plants regulates CH4 flux 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 flux.