Soil Phosphorus Dynamics and Saltwater Intrusion in a Florida Estuary

Soils in riparian wetlands are thought to stifle eutrophication in coastal water bodies by entraining and transforming nutrients. Altered hydrology and sea level rise underscore the need to evaluate the role of salinity in nutrient cycling within these settings. We studied three intertidal wetlands along the St. Johns River estuary, Florida in order to relate soil phosphorus composition with the distribution of saltwater. Sites represented a tidal freshwater forest, a brackish marsh, and a saltmarsh. Total phosphorus and organic composition decreased along the axis of the estuary as ambient salinity increased. At upstream wetlands, porewater revealed an inverse correlation between PO4 (3-) and the presence of saltwater (Cl- and SO4 (2-)) suggesting soil efflux. During experiments, aerobic soils did not liberate PO4 (3-) when exposed to seawater. During anaerobic experiments, SO4 (2-) amended soils from a saltmarsh mobilized substantial amounts of PO4 (3-) but the same effect was not observed in low-salinity wetland soils. Results indicate that P availability is shaped by saltwater access in the estuary and that SO4 (2-) reduction may regenerate PO4 (3-) in the saltmarsh. We suggest this is relevant to the St. John's river in light of ongoing eutrophication and planned surfacewater withdrawals, which may draw saline water farther upstream.