Temperate mangrove and salt marsh sediments are a small methane and nitrous oxide source but important carbon store

Tidal saline wetlands (TSW), such as mangrove and salt marsh systems, provide many valuable ecosystem services, but continue to suffer disturbance, degradation and deforestation. Tropical mangroves perform a critical role in the exchange and storage of terrestrial–marine carbon but can function as a source of methane (CH₄) and nitrous oxide (N₂O). However, little is known of biogeochemical processes in temperate mangrove and salt marsh systems in the southern hemisphere. In this study, the soil/sediment exchange of CO₂, CH₄ and N₂O was measured seasonally along a natural transition from melaleuca woodland, salt marsh and into mangroves along the Mornington Peninsula edge of Westernport Bay, Victoria, Australia. Soil/sediment physiochemical properties and sediment C density were measured concurrently. The melaleuca woodland soil was a constant CH₄ sink of approximately −25 μg C m⁻² h⁻¹ but along the transect this rapidly switched to a weak CH₄ source (<5 μg C m⁻² h⁻¹) in the salt marsh which increased further in the mangrove sediments where emissions of up to 375 μg C m⁻² h⁻¹ were measured in summer. Sediment CH₄ exchange correlated with salinity, pneumatophore number and the redox potential of sediment water at depth. All three ecosystems were a small N₂O source of <10 μg N m⁻² h⁻¹. Soil–atmosphere exchange was dominated by CO₂ which showed a significant response according to ecosystem and season along with soil temperature and salinity. Sediment C density was significantly greater in the salt marsh than the mangrove. Salt marsh sediment C density was 168 Mg C ha⁻¹ which is comparable with that measured globally, whereas the mangrove sediment C density of 145 Mg C ha⁻¹ is among the lowest reported. Contrary to global patterns in terrestrial soil C content and salt marsh sediment C content, data from our study indicate that mangrove sediments from a cooler, drier temperate latitude may store less C than mangroves in warmer and wetter tropical latitudes. Understanding both C storage and the greenhouse gas balance of TSWs will help us to better value these vulnerable ecosystems and manage them accordingly.