Sea level rise will expose many coastal wetland soils to higher salinities and longer periods of inundation than previously experienced, with the potential to significantly alter soil microbial communities, physiochemical properties, and biogeochemical cycles. Of particular concerns is the effect of saltwater intrusion on soil organic carbon (C) cycling, because the balance between C inputs and losses are a key factor in determining the likelihood a coastal wetland will be able to accrete vertically at a pace sufficient to persist with rising sea levels. Moreover, changes in the soil C balance could initiate a significant climate feedback due to the vast size of this C reservoir. A growing body of literature on the effects of salinity on soil CO2, CH4, and dissolved organic C (DOC) production has produced sometimes contradictory results, with big questions remaining, such as: 1) what is the salinity threshold at which sulfate reduction inhibits CH4 production, and 2) under what conditions does salinity stimulate microbial respiration, versus suppress it? The findings of several process-based laboratory and mesocosm studies conducted using coastal wetland soils from varying locations and salinity regimes will be synthesized to answer some of these vital lingering questions.
The majority of studies investigating salinity impacts of soil organic C cycling have focused on the effects to freshwater tidal soils, and have found with fair consistency that low-level saltwater intrusion (~3.5 to 7.5 ppt) can stimulate CO2 production, while higher saltwater concentrations (> ~10 to 15 ppt) can suppress CH4 production. However, when saltwater intrusion occurs in coastal systems that have already been exposed to salinity (i.e., brackish and saline wetlands) the effects of increasing salt is minimal, and biogeochemical processes respond most to changes in the depth and duration of inundation. This introduces an important theory for the study of sea level rise effects on coastal wetlands: the nature of the biogeochemical response is dependent upon the current salinity regime of the system, with an important distinction being made between freshwater wetlands and those already saturated with enough sulfate that electron acceptors are no longer the limiting factor for microbial metabolism.