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pH Buffering in Forest Soil Organic Horizons: Relevance to Acid Precipitation¹

ABSTRACT

Samples of organic surface horizons (Oi, Oe, Oa) from New York State forest soils were equilibrated with 0 to 20 cmol HNO₃ kg^–1 soil in the laboratory by a batch technique designed to simulate reactions of acid precipitation with forest floors. Each organic horizon retained a constant percentage of added H⁺ (67–96%) regardless of quantity added, and buffer capacities of the samples ranged from 18 to 36 cmol (H⁺) kg^–1 (pH unit)^–1, an order of magnitude higher than those measured in underlying mineral horizons. Calcium was the dominant cation in unacidified equilibrium solutions, and its concentration changed the most in response to acidification of the soils. Aluminum contributed <1% to total cationic charge in solution and Fe concentration decreased upon addition of up to at least 5 cmol (H⁺) kg^–1. From 38 to 79% of H⁺ removed by the soils was not balanced by increases in soluble Ca, Mg, K, Mn, Fe, Zn, Al, and Cu, and buffering by soluble organic and inorganic constituents accounted for 8 to 58% of H⁺ removal. Protonation of anions of weak organic acids in solution and solid phases probably accounted for this H⁺ removal without increasing cation concentrations. Because of H⁺ removal by forest floor organic horizons and the dominance of Ca in equilibrium solutions, potential impacts of acid precipitation on aluminum solubility of underlying mineral horizons of forest soils will be due more to Ca(NO₃)₂ and CaSO₄ in leaching water, rather than to direct effects of HNO₃ and H₂SO₄.

Key Words: buffer capacity • protonation of organic acids • proton neutralization • forest floor

¹ Contribution from New York State Agric. Exp. Stn., Ithaca, NY.

² Research Associate and Assistant Professor, respectively, Agronomy Dep., Cornell Univ., Ithaca, NY 14853.

Received for publication March 18, 1985.

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