Forest soil solutions: Acid/base chemistry and response to calcite treatment

Soil solution chemistry was investigated at a forested watershed draining into Woods Lake, N.Y. as part of the Experimental Watershed Liming Study (EWLS). The objective of this study was to assess the response of soil water to watershed treatment of calcite (CaCO₃). This material was applied in an effort to mitigate the effects of acidic atmospheric deposition. Soil solutions draining Oa and Bs horizons in reference subcatchments were characterized by low pH and acid neutralizing capacity (ANC) due to elevated concentrations of SO₄^2-, NO₃^- and organic anions relative to the sum of base cation (C_B: Ca²⁺, Mg²⁺, Na⁺, K⁺) concentrations. Seasonal and spatial variation of pH and ANC in soil solutions appeared to be largely controlled by variations in the concentrations of dissolved organic acids which, in turn, were regulated by reactions of Al with soil organic matter. Nitrate was positively correlated and SO₄^2- was negatively correlated with Ca²⁺ and Al concentrations in reference soil solutions, indicating that changes in NO₃^- influences spatial and seasonal variations in Ca²⁺ and Al concentrations. On this basis, NO₃^- appears to be important in soil acidification and the dynamics of drainage water acidity. Comparison of our results with historical data for the site showed declines in concentrations of SO₄^-, which are consistent with decreases in emissions of SO₂ in the eastern U.S. and atmospheric deposition of SO₄^2- to the Adirondack region. Mineral soil solutions have shown large increases in concentrations of NO₃^-. Declines in concentrations of C_B and increases in concentrations of Al have occurred over the last ten years, suggesting depletion of soil pools of exchangeable basic cations and increased sensitivity to acidic deposition. Calcite (CaCO₃) treatment of 6.89 Mg/ha resulted in a significant increase of Ca²⁺, ANC and pH in both Oa and Bs horizon soil solutions. Soil water response to CaCO₃ addition was most evident during the first year after treatment, apparently due to macropore transport of particulate and dissolved CaCO₃. However, increases in ANC and pH in the mineral soil waters were not sustained and appeared insufficient to result in substantial improvement in surface water quality over the 43 month study period.