Microbial communities and soil greenhouse gas fluxes from subtropical tidal and supratidal wetlands

Coastal wetlands are sinks of atmospheric carbon, but they can also emit greenhouse gases (GHGs) such as carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) from aerobic and anaerobic microbial soil respiration. The amount and direction of fluxes (emissions or uptakes) are variable and likely to depend on the soil microbial community and other environmental factors, such as salinity. Yet few studies have simultaneously measured GHG fluxes, microbial communities and environmental drivers in wetlands across an intertidal gradient. In this study, we sampled fringe mangroves (FM), basin mangroves (BM), saltmarsh (SM), and supratidal forests (SF) in the subtropical east coast of Australia. We found that CO₂ and N₂O emissions decreased landwards, from the FM to the SF, while CH₄ fluxes increased. The beta diversity of bacterial and archaea communities differed significantly among wetland types. Desulfobacterota were common in mangroves, suggesting sulphate reduction, which is responsible for damping CH₄ emissions, whereas the presence of Nitrososphaeria on the SF suggests nitrogen cycling, such as nitrification-denitrification, associated with N₂O emissions. Salinity was strongly associated with the variation in microbial communities and the N₂O and CH₄ fluxes. Thus, interstitial salinity and wetland type can explain and potentially predict microbial community composition and their associated GHG fluxes within the intertidal.