SOLUBLE ALUMINUM IN ACIDIFIED ORGANIC HORIZONS OF FOREST SOILS

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Abstract
Concentrations of labile and total Al in soil extracts were measured as a function of equilibrium solution pH in six forest soil organic horizons acidified with HNO3 (0–20 cmol H+∙kg−1) under controlled conditions of ionic strength (0.05 M NaNO3), temperature (23 °C), and solution:soil ratio (25:1). Decreases of 0.1–0.2 in solution pH in the range of pH 2.4–4.5 resulted in increases and decreases in concentrations of labile and total Al. Organic acids and soluble C were the dominant Al-complexing ligands in solution, and acidification of each horizon decreased their concentration. Silicon and F concentrations in the equilibrium solutions were lower than that of C, but levels were similar to those of nonlabile Al. Nonlabile complexes were the dominant forms of soluble Al in horizons above pH 4, and labile Al comprised an increasing percentage of total Al as pH decreased, especially below pH 3. The results showed pH alone to be a poor predictor of log nonlabile (r2 = 0.06) or total Al (r2 = 0.33) concentration in these soils, but a good predictor of log labile forms (r2 = 0.77). Solution pH, however, was positively correlated with the log of the ratios of C-to-total Al (r2 = 0.78), C-to-nonlabile Al (r2 = 0.68), and C-to-labile Al (r2 = 0.88). The results should aid in predicting the changes in speciation and solubility of Al due to inputs of acid precipitation to forest soil organic horizons with variable pH and content of dissolved organic C. The observed variability in response of Al to acidification of organic horizons of forest soils indicated that it is not possible to generalize whether nonlabile and labile forms of Al will increase, decrease, or not change upon addition of mineral acid. Besides pH, concentrations of different Al-complexing ligands should be measured in equilibirum soil extracts. Key words: Acid precipitation, fluoride, silica, fulvic acid