Sulfate Adsorption Relationships in Forested Spodosols of the Northeastern USA

Abstract Soil SO 2− 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 SO 2− 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 SO 2− 4 in Bs horizons predominately in an insoluble (nonwater‐extractable) form and were capable of adsorbing additional SO 2− 4 given higher SO 2− 4 concentrations than currently found in situ. Concentrations of insoluble SO 2− 4 in Bs horizons ranged from 0.3 to 2.1 × 10 −3 mol SO 2− 4 Kg −1 by H 2 PO − 4 extraction. Fractionation of free Fe and Al suggest that both elements were present largely in organically complexed forms. Insoluble SO 2− 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 SO 2− 4 adsorption capability. These soils may not presently be accumulating SO 2− 4 through the adsorption process on an annual basis. Their SO 2− 4 adsorption capability, however, indicates that the adsorption process may well buffer seasonal variations in SO 2− 4 concentration or serve as a net source or sink of SO 2− 4 subsequent to conditions which either influence concentrations of SO 2− 4 in the soil solution (eg. atmospheric inputs) or affect the adsorbent phase.