Human-accelerated sea-level rise is facilitating the movement of brackish water into low-lying agricultural lands across the Atlantic Coastal Plain. On the lower eastern shore of the Chesapeake Bay, saltwater intrusion (the landward movement of saltwater from the ocean) is increasing the potential for large pulses of nitrogen (N) and phosphorus (P) release from cultivated lands - with devastating consequences for both agriculture and the environment. Our research was conducted in Somerset County, Maryland, where sea-level rise rates are twice the global average and centuries of farming have dramatically altered soil chemical and physical properties. We examined soil profiles in three farms of differing fertilizer histories experiencing saltwater intrusion. Soils (to 140 cm) were collected along the intrusion transect from the center to the edge of the field. Soils were analyzed for texture, pH, plant-available P, N, chloride, electrical conductivity (EC), iron (Fe) and aluminum (Al), and cation exchange capacity (CEC).
We show how soil physical and chemical properties vary vertically and horizontally and along an intrusion gradient. In heavily fertilized soils, plant-available P moves horizontally in shallow soil layers from the center of fields to build up in agricultural ditches. We found soil Cl- and SO42- concentrations were 95% higher in ditches compared to the center of fields, but soil Cl- levels in fields were still above the toxicity threshold (277 mg/L) for most row crops (150 mg/L). Further, by comparing soil Cl- and SO42- concentrations against the ratio in seawater, we detected zones of sulfate-reduction and farmer amendments of gypsum (CaSO4) along the transects, both of which have consequences for P release. We will show the relationships between soil P, Fe, and Al pools in fields with different fertilizer histories and discuss the potential for large pulses of nutrient release with saltwater intrusion in coastal farm fields. We will show how our findings can inform marsh conservation practices, including the management of the problem species Phragmites australis, prioritize protected land acquisition, and probe the capacity of migrating wetlands and transitioning forests to buffer water quality, which could provide the basis for best management practices in coastal farms.