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Aluminum chemistry of selected podzols in Southwestern British Columbia Yuan, Guodon


Three related aspects of A1 chemistry of selected Humo-Ferric Podzols in Southwestern British Columbia were investigated: mechanisms of podzolization, acidity transfer from soils to surface waters, and phosphate status in relation to soil acidification. Distributions of A1 and Fe in horizons and results of column leaching studies revealed that Al behaves differently from Fe in podzolization in terms of ease of eluviation from the top horizon and illuviation into underlying horizons. This may be thermodynamically explained by the solubility products of Al-bearing minerals and stability constants of A1-organic complexes. This result differs from the traditional belief that A1 and Fe behave similarly in podzolization. Both organic and inorganic mechanisms of podzolization are proposed in the literature. Studies reported are in support of either one or the other mechanism. Chemical extraction results in this study showed that the content of A1 in proto-imogolite was significant in some Bf horizons. This may indicate the contribution of an inorganic mechanism in podzolization. The competition among A1-O-Si, A1-O-C, and A1-O-H bonds, which determined the proportion of inorganic and organic processes, was related to annual precipitation and pH, but not influenced directly by dissolved organic carbon content. Podzol formation is a natural process of soil acidification. This study suggested that the amorphous A1 (A1₀) content in soil might be used as a capacity factor to characterize acidity transfer from soils to surface waters. A1 activity in neutral salt extracts of soils, which represents the amount of A1 subject to transfer to surface waters in situations with little organic and mineral acid input to soils, correlates significantly with A1₀. This A1 activity can be described by an exchange model. A parameter in the model has an empirical relationship with organicbound A1 content. Al concentration in simulated leachates, indicating the amount of A1 transferable to surface waters in situations where soils receive acidic inputs from natural and anthropogenic sources, is much higher than that in neutral salt extracts and is related to A1₀. Exposure of Bf horizons to the land surface may contribute to accelerating transfer of acidity to surface waters. Reduction in bioavailability of phosphorus is a phenomenon during soil acidification. Phosphate sorption parameters were strongly dependent on amorphous A1, Fe, and other constituents. Amorphous A1 and Fe can be used to predict phosphate sorption capacity of the soils. Two European-derived models were shown to be applicable in the areas of this study. Modification was done to include native sorbed phosphate in the sorption capacity of the models.

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