Soils in transition: saltwater intrusion alters soil chemistry in agricultural fields

As global sea-levels rise, low-lying coastal lands are subject to shallow coastal flooding and saltwater intrusion, affecting the productivity of farmlands worldwide. Soil biogeochemistry can be dramatically altered as saltwater intrudes agricultural fields. We selected three farm fields in Somerset Co., Maryland affected bysaltwater intrusion and established transects from the ditch bank to the center of the cropped field and collected soils (to similar to 140cm) at five points along this transect. The three fields in this study have different soil types, are located along different tributaries in the county, and receive different fertilizer rates, yet they all showed similar biogeochemical responses to saltwater intrusion. We found an increase in electrical conductivity and concentrations of chloride, sulfate, and forms of phosphorus (P) from the center of the field (low) to the ditch banks (high). As inundation increased, the structure of iron (Fe) changed from crystalline to non-crystalline forms, possibly due to dissolution under saturated conditions. Near the edges of the fields, the formation of organometallic complexes was positively associated with increases in soil carbon and organic soil P concentrations. Compared to areas of the fields where crops were actively growing, soil P concentrations are 2-3 higher on field edges, suggesting that saltwater intrusion may be transporting P to the edges of agricultural fields. These field edges are frequently saturated, thus reduction of Fe could lead to P release into solution potentially harming water quality. As climate change pushes saltwater further inland, it is important to understand the biogeochemical consequences for ecosystems up- and downstream. Understanding the how fractions of P move and change across fields affected by saltwater intrusion will be crucial for planning current and future management of coastal agricultural lands.