Net Ecosystem Methane Exchange at a Subtropical Mangrove Wetland, Hong Kong

Tropical and subtropical mangrove wetlands are highly productive ecosystems sequestrating about 800 Tg greenhouse gas (GHG) carbon dioxide (CO₂) annually. Meanwhile, anoxic carbon-rich sediment facilitates microbial production of another powerful GHG methane (CH₄). CH₄ has global warming potentials (GWPs) 86 and 34 times higher than those of CO₂ over time horizons of 20 and 100 years, respectively; therefore, its emission has the potential to offset net ecosystem production (NEP). However, no long-term quantification of mangrove-atmosphere CH₄ exchange has been available.

Here we report on eddy covariance results of CH₄ fluxes over a subtropical mangrove canopy during Feb 2016 to Oct 2017 at Hong Kong. The dominant species are Kandelia obovata and Acanthus ilicifolius. Daily average CH₄ emission rate was 1375 μmol m^-2 day^-1 which is 55% lower than the previous finding obtained from chamber measurements during wet season at this site. Diurnal CH₄ flux peaked at 1 pm (local time) and maintained relatively high levels until 4 pm. CH₄ emission during wet season was 46% higher than that during dry season. Soil temperature at 40 cm depth, tidal salinity concentration, gross primary production (GPP) and air temperature are among the best predictors of net CH₄ exchange. For example, 75% of large fluxes (> 50 nmol m^-2 s^-1) occurred under low salinity concentration (< 5 ppt), but a negative correlation between flux and salinity was found under high salinity concentration (> 5 ppt) rather than low. High tidal level may inhibit CH₄ transport from sediment to the atmosphere. Daily CH₄ fluxes displayed positive linear relationships with temperature and GPP. Annual CH₄ emission (6 g C m^-2 year^-1) offset 58 % and 23% of NEP (890 g C m^-2 year^-1) using 20- and 100-year GWP, respectively. Hence, understanding CH₄ dynamics is of critical importance when assessing the value of mangrove wetlands in climate change mitigation.