Sulfate Adsorption and Desorption Reversibility in a Variety of Forest Soils

Abstract

Abstract Subsurface soil samples were collected from 20 forest sites of differing soil and cover type, atmospheric deposition history and physiographic location. Samples were analyzed for S pools, sulfate adsorption capacity, adsorption reversibility, and associated soil properties in order to compare the relative S chemistry and SO 4 retention capacities of the sites. The SO 4 adsorption capacity was determined by sequential equilibration of air‐dried soil samples with a percolating solution of 0.25 m M CaSO 4 , and adsorption reversibility by leaching both SO 4 saturated and untreated soil samples with deionzed water. Phosphate extractable SO 4 was measured at the end of each series of equilibrations as a means of estimating irreversibly adsorbed SO 4 . A total of 32 out of 36 soil horizons studied showed net SO 4 adsorption, ranging from a desorption of 0.4 to an adsorption of 7.2 mmol SO 4 kg −1 . Thus, most of these soils were not yet saturated with respect to a solution concentration of 0.25 m M SO 4 . Most soil horizons showed irreversible SO 4 adsorption (an average of 36% of adsorbed SO 4 was retained irreversibly) as evidenced both by changes in phosphate extractable SO 4 and by a comparison of desorption of SO 4 before and after saturation by the 0.25 m M SO 4 solution. When adsorption and adsorption reversibility observations were compared with measured soil properties, the presence of hydrous oxides of Al and native SO 4 were the best indicators of SO 4 adsorption potential. Most adsorption/desorption/extraction sequences showed mass conservation of SO 4 , indicating inorganic SO 4 adsorption appeared to be far more important in these procdures than organic S incorporation or mineralization. It is unclear how much air‐drying, sample treatment, and the procedures followed might have changed the inorganic and organic S pools of these soils.