Sea-Level Rise Unlocks Agricultural Legacies on Maryland's Lower Eastern Shore.

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 Maryland, 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).

Soil physical and chemical properties vary vertically and horizontally and along an intrusion gradient. Plant-available P moves horizontally in shallow soil layers from the center of fields to toward agricultural ditches. Soil Cl^- and SO₄^2- concentrations were 95% higher in ditches compared to the center of fields, but soil Cl^- levels in fields were still above the toxicity threshold for most row crops. Further, by comparing soil Cl^- and SO₄^2- 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 discuss the potential for large pulses of nutrient release with saltwater intrusion in coastal farm fields. We will show how our findings can inform land management practices for resilient agroecosystems in the face of sea level rise.