Abstract Wetlands are a significant source of methane (CH 4 ), but their contributions to the global CH 4 budget remain uncertain. Wetland CH 4 production may be better constrained by addressing uncertainty in plant‐mediated CH 4 transport, which is highly variable, motivating the current work. We measured whole‐plant, plant‐adjacent and non‐vegetated carbon dioxide (CO 2 ) and CH 4 fluxes throughout the day approximately monthly for 1 yr in a euhaline tidal salt marsh and a mesohaline, non‐tidal impounded wetland. To assess mechanisms, gas fluxes were accompanied by measurements of environmental variables and soil chemical and biological properties. We found that plants facilitated greater CH 4 fluxes across diurnal and seasonal scales but via distinct mechanisms at our two study sites. We observed daytime peaks in whole‐plant and plant‐adjacent CH 4 fluxes at the salt marsh, but no diurnal patterns in any CH 4 fluxes at the impounded wetland. Daytime peaks could indicate more efficient pressurized flow of CH 4 through plant biomass or greater sensitivity of aboveground fluxes to temperature or plant productivity. At the annual scale, peak whole‐plant CH 4 fluxes were greater at the impounded wetland than at the salt marsh, while peak plant‐adjacent CH 4 fluxes were greater at the salt marsh than at the impounded wetland. Differences between sites were attributed to plant‐soil‐microbe interactions and abiotic conditions such as temperature, water column depth and salinity that determine root‐adjacent porewater CH 4 concentrations and the potential for plant‐mediated CH 4 transport. These results increase our understanding of plant‐mediated CH 4 transport and the mechanisms underlying high variability in these fluxes across coastal wetlands.
Seyfried et al. (Sun,) studied this question.