Can Saltwater Intrusion Accelerate Nutrient Export from Freshwater Wetland Soils?.

Coastal wetlands are vulnerable to saltwater intrusion, which may alter soil biogeochemical processes, lead to the export of nutrients into the coastal zone, and contribute to eutrophication. We investigated how different wetland soils and salinities alter the export of soil nutrients by collecting intact cores from three freshwater wetlands representing a range of soil organic matter (SOM) contents and randomly assigned them to control (0), 5, or 15 ppt seawater treatments. Samples collected from both soil porewater and surrounding water over 3 weeks and were analyzed for soluble reactive phosphorus (SRP), ammonium (NH₄⁺), and dissolved organic carbon (DOC). Cores were destructively sampled for soil physiochemical properties, microbial biomass C, and greenhouse gas emissions. The highest salinity treatment increased carbon dioxide production by 30% with no change in methane production. Porewater NH₄⁺ concentrations averaged 8x higher than the control in the 15 ppt treatment, and 5x greater than the control in the 5 ppt treatment, with a ~1 week lag period between porewater accumulation of NH₄⁺ and flux into the surrounding water; the magnitude of NH₄⁺ export was mediated by SOM content. Porewater and surrounding water SRP concentrations increased with salinity and flux was negatively correlated to SOM content. Dissolved organic C concentrations differed by wetland type, but were not affected by salinity treatment. This study demonstrates that saltwater intrusion could catalyze a rapid export of nutrients from freshwater wetland soils into the coastal zone, especially in SOM-rich soils.