A Selective Dissolution Analysis Optimized for Measurement of Weathering Products in a Soil
1997
Benoit, Gaboury | Rozan, Tim F. | April, Richard H.
Soil weathering is an important source of nutrient cations and P for development of forested ecosystems, but the rates at which these nutrients are released are not well known. Measurement of weathering products can be used in long-term (5–10 yr) mesocosm studies to evaluate soil weathering rates. In this study, a method was developed to optimize extraction of nutrient cations in weathering products (organic matter, transition-metal oxyhydroxides, and amorphous Al and Si compounds) while dissblving minimal amounts of primary minerals. For organic matter destruction, dry ashing was compared with peroxide oxidation. Reducing agents (hydroxylamine hydrochloride, oxalate, dithionite) were evaluated for efficacy in extracting transition-metal oxyhydroxides. Treatment with hydroxide solutions (NaOH, NH₄OH) was tested for dissolving amorphous and poorly crystalline Al and Si compounds. Two methods were used to test for undesirable destruction of unweathered minerals: (i) parallel extraction tests on pure primary minerals, and (ii) release of Rb. The greatest variability in effectiveness of the extractants involved the reduction step. For most elements, the completeness of extraction followed the sequence dithionite > oxalate > hydroxylamine. A wash with HCl was needed to redissolve any carbonates that precipitated in the dithionite step. The best overall protocol involves (i) dry ashing, (ii) extraction with a mixture of citrate, bicarbonate, and dithionite, (iii) washing with HCl, and (iv) treatment with NaOH. This last step was found to release mainly Al and Si and only negligible amounts of cations (K, Ca, Mg, Fe, Mn). With this procedure, destruction of primary minerals can be held to ≈0.4%, well below the quantity of weathering products.
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