Can Saltwater Intrusion Accelerate Nutrient Export from Freshwater Wetland Soils? An Experimental Approach

Core Ideas Ammonium that accumulated in the porewater following an influx of seawater took ∼1 wk to flux into the surrounding water. Soluble reactive P concentrations in both the porewater and surrounding water increased with salinity. Nutrient export was influenced by wetland soil type. Saltwater intrusion could catalyze the rapid export of nutrients from freshwater wetland soils into the coastal zone, especially in organic matter‐rich 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. Using intact soil cores from three different freshwater wetlands and subjecting them to control (0), 5, or 15 parts per thousand (ppt) seawater treatments, we investigated how different wetland soils (bayhead swamp, cypress dome, and mineral marsh) and salinities alter the export of soil nutrients [soluble reactive P (SRP), NH4+, and dissolved organic C (DOC)], as well as microbial biomass C and greenhouse gas emissions. The highest salinity treatment increased CO2 production by 30% with no change in methane production. Porewater NH4+ concentrations averaged 8× higher than the control in the 15‐ppt treatment and 5× greater than the control in the 5‐ppt treatment, with a ∼1‐wk lag period between porewater accumulation of NH4+ and export into the surrounding water. The magnitude of NH4+ export was mediated by wetland soil type. Soluble reactive P concentrations averaged 8× and 3.5× higher in the cypress dome and bayhead swamp 15‐ppt treatments respectively than in their controls, though no effect was observed within the mineral marsh. Dissolved organic C concentrations differed by wetland soil type but were unaffected by salinity treatment. This study demonstrates that saltwater intrusion could catalyze the rapid export of nutrients from freshwater wetland soils into the coastal zone, though nutrient export varies by wetland soil type.