Relating sulfate adsorption to soil properties in Japanese forest soils

The relationship of sulfate adsorption capacity to the presence of aluminum, iron oxides, and organic carbon in four Japanese forest soils was investigated. Sulfate adsorption capacities were higher in subsoils than in surface soils, and their profiles were the inverse of those of organic carbon content. The black soil derived from volcanic ash had high sulfate adsorption capacity and showed no release of sulfate, while some of the surface horizons of the other soils desorbed sulfate. Oxalate extractable aluminum (Al₀) and iron (Fe₀) and ditionite-citrate-bicarbonate (DCB) extractable iron (Fe_d) were positively correlated with the sulfate adsorption capacity, and their correlation coefficients were almost identical (0.58–0.61). The lack of significant correlation between Al_d and the adsorption capacity suggests that DCB is unsuitable for extracting aluminum compounds which have the ability to adsorb sulfate. In contrast with oxides, organic carbon correlated with sulfate adsorption capacity negatively, and had a negative coefficient in a multiple regression equation, insignificantly. Furthermore, H₂O₂-treated samples containing decomposed organic matter adsorbed more sulfate than untreated samples. These results clearly indicated that organic matter hinders sulfate adsorption. A comparison of theβ value (standard regression coefficient) of organic carbon with that of Al₀ showed that the negative contribution of organic matter to sulfate adsorption capacity was slightly less than the positive contribution of oxides.