Methane emission in sub-tropical wetlands along a tidally inundated river

Abstract

Methane is a potent greenhouse gas. Among all possible sources, wetlands are estimated to account for more than one third of the overall methane emission, ranking the greatest natural contributor. The exact quantity of methane released from such environment to the atmosphere is, however, controlled by the interplay between microbial production and consumption. While tidal wetlands represent a critical transition witnessing intensive material and energy exchange between terrestrial and marine realms, uncovering methane production and consumption impacted by tidal inundation would provide mechanistic constraints to refine the estimates of methane emission on various temporal and spatial scales. In this study, field campaigns combined with laboratory experiments were employed to investigate the factors controlling methane emission in wetlands distributed along the Dan-Tsui river in northern Taiwan under different degrees of tidal inundation. Flux measurements indicated that methane emission was negatively and positively correlated with sulfate/chloride concentration and temperature, respectively. Methane emission was also the greatest during the lowest tide. Porewater geochemistry further indicated that while metabolic zonation (sulfate reduction, anaerobic methanotrophy, and methanogenesis) prevailed, methane production was apparently confined at great depths by high sulfate during high tide but stimulated at shallow depths during low tide. Abundant methane (mM scaled) produced during low tide fueled anaerobic and aerobic methanotrophy near surface. While a majority of methane produced was consumed by anaerobic methanotrophy, methane production apparently surpassed methanotrophy, leading to higher emission when compared with that during high tide. In addition, potential rates of aerobic methane oxidation were neither correlated with methane emission fluxes nor oxygen penetration, and pmoA gene abundances during different tidal phases, suggesting that methane emission is not controlled by initial population size, oxygen availability, and methane oxidation and production capacity individually. Overall, temperature and sulfate availability related to tidal inundation appear to be the first order control of methane emission. Factors modulating microbial physiological capacities and activities for methane production and consumption are also important to constrain the mechanistic bases for methane emission.