Forest soil response to acid and salt additions of sulfate: II. Aluminum and base cations

Reconstructed Spodosol and intact Alfisol soil columns were used to examine the effects of 52 weeks of additions of various simulated throughfall solutions on base cation, Al, acid neutralizing capacity, and pH levels in soil leachates. Our purpose was to determine the effects of acid and salt additions of SO₄²⁻on cation leaching in two forest soils as well as to investigate the influence of episodic events of “seasalt” and pCO₂. For leachates collected from the forest floor after H₂SO₄additions, H⁺exchange for Ca²⁺provided the major buffering mechanism of acid inputs. Salt inputs (Na₂SO₄) increased leachate pH in Spodosol columns with mineral soil due to specific adsorption of added SO₄²⁻and release of OH⁻(e.g., Forest floor + 40 cm B column leachates had pH values of 5.15 and 4.70 at week 52 in salt and control treatments, respectively). Increases in leachate SO₄²⁻and other anion concentrations from all H₂SO₄columns were generally balanced by increased Ca²⁺and Mg²⁺concentrations (e.g., FF + 40 cm B column leachates had 154 and 85 μeq Ca²⁺L⁻¹at week 52 in acid and control treatments, respectively), whereas Na₂SO₄leachate anions were balanced by Na⁺. Large additions (1000 μeq · L⁻¹) of salt (NaCl) decreased soil pH and acid neutralizing capacity in short-term experiments (1 day) and interacted with pCO₂levels to control closed and degassed pH as well as acid neutralizing capacity. Our work illustrates the importance of soil cation exchange (especially in the forest floor), anion concentrations, and pCO₂levels in controlling the leachate chemistry in response to acidic and “seasalt” deposition events.