Sulfate Adsorption Relationships in Forested Spodosols of the Northeastern USA

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Abstract
Soil SO2−4 adsorption has important implications in the study of acidic deposition effects on terrestrial and aquatic systems, because this process results in the release of alkalinity or prevents leaching of H+ and cations. Forested Spodosols of the northeastern USA have generally been considered poor adsorbers of SO2−4, in part due to the interfering effects of soil organic matter. Soils from both the Adirondack Mountains, NY and the Hubbard Brook Experimental Forest, NH (Typic and Lithic Haplorthods and Aquic Fragiorthods, respectively), however, contained SO2−4 in Bs horizons predominately in an insoluble (nonwater‐extractable) form and were capable of adsorbing additional SO2−4 given higher SO2−4 concentrations than currently found in situ. Concentrations of insoluble SO2−4 in Bs horizons ranged from 0.3 to 2.1 × 10−3 mol SO2−4 Kg−1 by H2PO−4 extraction. Fractionation of free Fe and Al suggest that both elements were present largely in organically complexed forms. Insoluble SO2−4 was highly correlated with several forms of free Fe and Al, but crystalline Fe and organic Al content appeared to explain most of the variability in SO2−4 adsorption capability. These soils may not presently be accumulating SO2−4 through the adsorption process on an annual basis. Their SO2−4 adsorption capability, however, indicates that the adsorption process may well buffer seasonal variations in SO2−4 concentration or serve as a net source or sink of SO2−4 subsequent to conditions which either influence concentrations of SO2−4 in the soil solution (eg. atmospheric inputs) or affect the adsorbent phase.