Tidal and seasonal effects on sediment methane emissions from three different mangrove species

The anaerobic environment of mangrove sediments due to periodic tides is conducive to methane (CH₄) production, but processes and mechanisms of CH₄ emission from mangrove sediments are not yet well understood. We used in situ field monitoring and laboratory experiments to investigate the effects of tides and seasons on CH₄ emissions from the sediments of Sonneratia apetala (SA), Kandelia obovata (KO), and Avicennia marina (AM), respectively. Methane emissions from the sediments of all mangrove species were significantly higher in summer than in winter, with overall CH₄ fluxes being 2.14 times higher during the after-ebb tide compared to the pre-flood tide. Among the mangrove species, AM (16.77 ± 13.73 mg m^-2 h^-1) exhibited the highest emissions, followed by SA (1.45 ± 0.90 mg m^-2 h^-1) and KO (0.14 ± 0.16 mg m^-2 h^-1). CH₄ emissions in three mangrove species were mainly driven directly by abiotic factors, including sediment organic carbon (SOC) that could provide substrate for methanogens to generate CH₄, and dissolved CH₄ concentration in porewater likely served as a carbon source or turnover state for CH₄ to eventually enter the atmosphere. Also, sediment CH₄ emissions were suppressed by the α-diversity of methanogenic communities. In addition, pH, CH₄ flux, SOC, and redox potential significantly shaped structure of the methanogenic communities, potentially regulating sediment CH₄ emissions. This study result highlights that abiotic factors can greatly influence CH₄ emissions from mangrove sediments, as well as emphasizes the important role of the sediment-porewater-atmosphere pathway on CH₄ emissions.