Diel methane emissions in stands of Spartina alterniflora and Suaeda salsa from a coastal salt marsh

In this study, we aimed to understand the influence of plant type on the monthly variations of diel CH4 fluxes from Spartina alterniflora and Suaeda salsa of coastal salt marshes at three growth stages (July, August and September). Dissolved CH4 concentrations in porewater and sediment redox potentials were monitored, as were aboveground plant biomass and stem densities. CH4 fluxes exhibited clear monthly variations and peaked in September in the S. alterniflora and S. salsa mesocosms. However, no discernible diel variation was observed in the CH4 flux in the S. salsa mesocosm, probably due to its weak gas transport capacity. By contrast, notable diel variations of CH4 flux with the peak of 1.42 and 3.67 mg CH4 m−2 h−1 at 12:00 and the lowest of 0.75 and 2.11 mg CH4 m−2 h−1 at 3:00 or 6:00 were observed in the S. alterniflora mesocosm on 11 August and 11 September, respectively, but not in July mainly due to low plant biomass masking diel variations in the porewater CH4 concentration. The ratios of the maximum flux to minimum flux over the course of the day in the S. alterniflora mesocosm on 10 July, 11 August and 11 September were 1.28, 1.89 and 1.76, respectively, and corresponding values for porewater CH4 concentration were 1.31, 1.39 and 1.17, respectively. CH4 flux significantly correlated with CH4 concentration in porewater, and both were significantly related to air temperature. These findings indicate that CH4 production and CH4 flux at the middle growth stage (August) exhibited greater responses to changes in air temperature, which in turn induced the higher diel variation. The higher diel cycle for CH4 flux in August than in September was likely due to the higher proportion of CH4 oxidized during diffusion within the aerenchyma system. Our results suggest that the extent of diel variations in CH4 flux may have depended on the gas transport capacity of plants, and the highest diel variation occurred at the middle growth stage.

► CH4 fluxes exhibited clear seasonal variations and peaked in September in the Spartina alterniflora and Suaeda salsa mesocosms.
► The weak gas transport capacity of Suaeda salsa masked diel variations in the CH4 concentration in porewater resulting in no notable diel fluctuation in the CH4 flux.
► The highest extent of diel variation in the CH4 flux appeared at the middle growth stage rather than at the peak emission stage probably due to higher proportion of CH4 oxidized during the diffusion through plants.
► The extent of diel variation in the CH4 flux might depend on the gas transport capacity of plants.