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. Funding Agencies|National Natural Science Foundation of China [41801070, 41877335, 41671088]; Program for Innovative Research Team at Fujian Normal University [IRTL1205]; Key Sciences and Technology Project of Fujian Province [2014R1034-1]; Study-Abroad Grant Project for Graduates of the School of Geographical Sciences; Graduated Student Science and Technology Innovation Project of the School of Geographical Science, Fujian Normal University [GY201601]; Research Grants Council of the Hong Kong Special Administrative Region, China [CUHK458913, CUHK14302014, CUHK14305515]; Chinese University of Hong Kong [SS16914]; Swedish Research Council VR; European Research Council ERC [725546]; Linkoping University