Saltwater intrusion in context: soil factors regulate impacts of salinity on soil carbon cycling

Salinization of freshwater ecosystems impacts carbon cycling, a particular concern for coastal wetlands, which are important agents of carbon sequestration. Previous experimental work using salt additions as a proxy for sea level rise, reveals widely divergent effects of salt on soil carbon processes. We performed a laboratory salt addition experiment on two different types of wetland soils (Ponzer muck and Hyde loam, both poorly drained organic soils) from the Coastal Plain of North Carolina. We used a commercial aquarium salt mix to make treatment solutions of 0, 2.5 and 10 ppt salinity and independently manipulated solution pH (5.5, 7.2, 8.8) for a full factorial experimental design. Our goal was to identify the effects of increasing ionic strength and increasing soil solution pH on soil carbon solubility and turnover. Microbial respiration and dissolved organic carbon solubility were depressed by marine salts, while pH manipulation alone had minimal effect. The addition of marine salts substantially reduced rates of carbon mineralization, reduced carbon solubility, and preferentially reduced the abundance of phenolic and aromatic organic molecules in solution. In the more acidic Ponzer muck, where salt additions dropped the pH from > 5 to < 4, we measured more substantial declines in DOC concentrations than in the base saturated Hyde loam. In contrast, in the base saturated Hyde loam, more marine salts remained in solution and the treatment effects on rates of carbon mineralization were more pronounced. Our results provide a clear demonstration of how ion exchange mechanisms result in indirect effects of salinization on the pH of soil solution and the solubility of organic matter. These indirect effects may explain much of the existing variation in reports of salt effects on soil carbon dynamics.