Methane dynamics in an estuarine brackish Cyperus malaccensis marsh: Production and porewater concentration in soils, and net emissions to the atmosphere over five years

Wetlands can potentially affect global climate change through their role in modulating the atmospheric concentrations of methane (CH₄). Their overall CH₄ emissions, however, remain the greatest uncertainty in the global CH₄ budget. One reason for this is the paucity of long-term field measurements to characterize the variability of CH₄ emissions from different types of wetlands. In this study, we quantified CH₄ emissions from a brackish, oligohaline Cyperus malaccensis marsh ecosystem in the Min River Estuary in southeast China over five years. Our results showed substantial temporal variability of CH₄ emissions from this brackish marsh, with hourly fluxes ranging from 0.7 ± 0.6 to 5.1 ± 3.7 mg m⁻² h⁻¹ (mean ± 1 SD) during the study period. The inter-annual variability of CH₄ emissions was significantly correlated with changes in soil temperature, precipitation and salinity, which highlighted the importance of long-term observations in understanding wetland CH₄ dynamics. Distinct seasonal patterns in soil CH₄ production rates and porewater CH₄ concentrations also were observed, and were both positively correlated with CH₄ emissions. The seasonal variations of CH₄ emissions and production were highly correlated with salinity and porewater sulfate levels. The mean annual CH₄ efflux from our site over the five-year period was 23.8 ± 18.1 g CH₄ m⁻² yr⁻¹, indicating that subtropical brackish tidal marsh ecosystems could release a large amount of CH₄ into the atmosphere. Our findings further highlight the need to obtain high-frequency and continuous field measurements over the long term at multiple spatial scales to improve our current estimates of wetland CH₄ emissions.