Abstract
In this study, we aimed to understand the influence of plant type on the monthly variations of diel CH 4 fluxes from Spartina alterniflora and Suaeda salsa of coastal salt marshes at three growth stages (July, August and September). Dissolved CH 4 concentrations in porewater and sediment redox potentials were monitored, as were aboveground plant biomass and stem densities. CH 4 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 CH 4 flux in the S. salsa mesocosm, probably due to its weak gas transport capacity. By contrast, notable diel variations of CH 4 flux with the peak of 1.42 and 3.67 mg CH 4 m −2 h −1 at 12:00 and the lowest of 0.75 and 2.11 mg CH 4 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 CH 4 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 CH 4 concentration were 1.31, 1.39 and 1.17, respectively. CH 4 flux significantly correlated with CH 4 concentration in porewater, and both were significantly related to air temperature. These findings indicate that CH 4 production and CH 4 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 CH 4 flux in August than in September was likely due to the higher proportion of CH 4 oxidized during diffusion within the aerenchyma system. Our results suggest that the extent of diel variations in CH 4 flux may have depended on the gas transport capacity of plants, and the highest diel variation occurred at the middle growth stage.
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