Due to anthropogenic activities, rural coastlines around the world are undergoing major changes and disruptions in nutrient cycling. Tidal salt marshes, riparian forests, and farmland converge on coastlines, forming ecotones, or unique transitional ecosystems. Effective nutrient management requires a better understanding of phosphorus (P) and nitrogen (N) cycling in these intersecting ecosystems. With centuries of farming and fertilization, N and P in excess of plant demand can accumulate in soils (known as legacy nutrients). In conjunction with storm events, sea level rise and associated saltwater intrusion can remobilize legacy nutrients years or even decades after application, supplying a persistent but unpredictable source of nutrients to downstream waterways. In this study, I examine this phenomenon and its potential impact on P and N loading in the Chesapeake Bay. Study sites are located along Maryland's Lower Eastern Shore in Somerset County. Soil and soil porewater were collected from active fields, salt-damaged farms and marshes. Soil and water were analyzed for phosphate, nitrate, ammonium, and other parameters related to the mechanisms of saltwater ion exchange. In order to tease apart the effects of ion constituents of saltwater on phosphate mobility, soils from these sites were also treated with varying salt concentrations in both aerobic and anaerobic microcosm systems.
Field results showed that concentrations of phosphate, nitrate, and ammonium in soil porewater were significantly higher in salt marshes than in other coastal ecotones. Furthermore, porewater nutrient concentrations differed by land use type. Salt marsh soils with a prior history of heavy fertilizer application contained phosphate concentrations up to ten times that of nearby forest soils. There was also a positive correlation between soil porewater salinity and phosphate and ammonium concentrations. In microcosm experiments, anaerobic soils released significantly more phosphate over a two-week incubation period than aerobic soils in the deionized water control treatment. This suggests that anaerobic soil conditions may encourage phosphate release. Saltwater intrusion may greatly enhance the potential of soil nutrient release due to ion exchange with sea water in combination with reduced oxygen levels characteristic of hydric soils. By quantifying these inputs and identifying some of the biogeochemical processes that spur nutrient release into waterways, this study will provide crucial information for land managers and conservation groups attempting to improve water quality in coastal agricultural areas.