Identifying the Salinity Thresholds that Impact Greenhouse Gas Production in Subtropical Tidal Freshwater Marsh Soils

Increasing salinity due to sea level rise is an important factor influencing biogeochemical processes in estuarine wetlands, with the potential to impact greenhouse gas (GHG) emissions. However, there is little consensus regarding what salinity thresholds will significantly alter the production of GHGs or the physiochemical properties of wetland soils. This study used a fine-scale salinity gradient to determine the impact of seawater concentration on the potential production of CH4, CO2 and N2O and associated soil properties using bottle incubations of tidal freshwater marsh soils from the Min River estuary, SE China. Potential CH4 production was unaffected by salinities from 0 to 7.5 parts per thousand, but declined significantly at 10 parts per thousand and above. Potential CO2 production was stimulated at intermediate salinities (5 to 7.5 parts per thousand), but inhibited by salinities >= 15 parts per thousand, while potential N2O production was unaffected by salinity. In contrast, soil dissolved organic carbon and NH4+-N generally increased with salinity. Overall, this research indicates salinities of similar to 10-15% represent an important tipping point for biogeochemical processes in wetlands. Above this threshold, carbon mineralization is reduced and may promote vertical soil accretion in brackish and salinity wetlands. Meanwhile, low-level saltwater intrusion may leave wetlands vulnerable to submergence due to accelerated soil organic carbon loss.