Saltwater Intrusion Releases Iron and Phosphorus from Agricultural Soils

Abstract

Saltwater intrusion (SWI) associated with global sea-level rise pervasively occur in agricultural lands, with potential to alter biogeochemical cycles and reduce agricultural yields. As SWI encroaches on coastal areas, agricultural fields can release large quantities of phosphorus (P) into nearby waterways due to interactions between sulfate (SO42-) and iron (Fe), that make Fe less available to sequester P. This study investigates key biogeochemical mechanisms that mediate the interaction between saltwater, Fe and P in coastal agricultural soils. We collected agricultural soil on the Lower Eastern Shore of Maryland and exposed it to eight different salt treatments under aerobic and anaerobic conditions over a period of thirty days. We analyzed the PO43- and Fe concentration in soil-water and total Fe in treated soil at days 0, 15, and 30. The results show that soil-water Fe and PO43- concentrations increase under anaerobic conditions over time in all eight salt treatments, suggesting that SWI can stimulate Fe and PO43- release under anaerobic conditions despite variation in saltwater compositions. The high levels of PO43- in sodium (Na)-containing treatments indicate that increases in sodicity can facilitate P release; the high levels of Fe in treatments consisting CaSO4 show that the presence of Ca potentially prompts the interaction between Fe and SO42-.