Managing Global Resources for a Secure Future

Sea level rise and associated saltwater intrusion can remobilize phosphorus (P) and nitrogen (N) stored in soils years or even decades after fertilizer application on farmland (known as legacy nutrients). This process results in a persistent but unpredictable supply 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. Soil and soil porewater were collected from active farm fields, salt-damaged fields, forests, salt-damaged forests, and salt marshes. Results show that concentrations of phosphate and ammonium in soil and 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 contain 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, soils from these sites were treated with varying combinations of salt solutions under aerobic and anaerobic conditions to tease apart the effects of the ion constituents of saltwater on nutrient mobility. Anaerobic soils released significantly more phosphate over a two-week incubation period than aerobic soils in the deionized water control treatment. Soils treated with higher concentrations of salt solutions released significantly more ammonia than the deionized water control and lower salt concentration treatments. By quantifying these nutrient inputs and identifying some of the biogeochemical processes that enhance nutrient release into waterways, this study will provide crucial information on causes of water quality impairment in coastal agricultural areas. Coastal conservation land management practices can be improved in light of this new picture of nutrient cycling.