Long-term changes in soil and stream chemistry across an acid deposition gradient in the northeastern United States

Declines in acidic deposition across Europe and North America have led to decreases in surface water acidity and signs of chemical recovery of soils from acidification. To better understand the link between recovery of soils and surface waters, chemical trends in precipitation, soils, and streamwater were investigated in three watersheds representing a depositional gradient from high to low across the northeastern United States. Significant declines in concentrations of H⁺ (ranging from −1.2 to −2.74 microequivalents [μeq] L⁻¹ yr⁻¹), NO₃⁻ (ranging from −0.6 to −0.84 μeq L⁻¹ yr⁻¹), and SO₄²⁻ (ranging from −0.95 to −2.13 μeq L⁻¹ yr⁻¹) were detected in precipitation in the three watersheds during the period 1999 to 2013. Soil chemistry in the A horizon of the watershed with the greatest decrease in deposition showed significant decreases in exchangeable Al and increases in exchangeable bases. Soil chemistry did not significantly improve during the study in the other watersheds, and base saturation in the Oa and upper B horizons significantly declined in the watershed with the smallest decrease in deposition. Streamwater SO₄²⁻concentrations significantly declined in all three streams (ranging from −2.01 to −2.87 μeq L⁻¹ yr⁻¹) and acid neutralizing capacity increased (ranging from 1.38 to 1.60 μeq L⁻¹ yr⁻¹) in the two streams with the greatest decreases in deposition. Recovery of soils has likely been limited by decades of acid deposition that have leached base cations from soils with base-poor parent material.