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 SO4 retention capacities of the sites. The SO4 adsorption capacity was determined by sequential equilibration of air‐dried soil samples with a percolating solution of 0.25 mM CaSO4, and adsorption reversibility by leaching both SO4 saturated and untreated soil samples with deionzed water. Phosphate extractable SO4 was measured at the end of each series of equilibrations as a means of estimating irreversibly adsorbed SO4. A total of 32 out of 36 soil horizons studied showed net SO4 adsorption, ranging from a desorption of 0.4 to an adsorption of 7.2 mmol SO4 kg−1. Thus, most of these soils were not yet saturated with respect to a solution concentration of 0.25 mM SO4. Most soil horizons showed irreversible SO4 adsorption (an average of 36% of adsorbed SO4 was retained irreversibly) as evidenced both by changes in phosphate extractable SO4 and by a comparison of desorption of SO4 before and after saturation by the 0.25 mM SO4 solution. When adsorption and adsorption reversibility observations were compared with measured soil properties, the presence of hydrous oxides of Al and native SO4 were the best indicators of SO4 adsorption potential. Most adsorption/desorption/extraction sequences showed mass conservation of SO4, indicating inorganic SO4 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.

imposition of drought through roofs in the forest subcanopy to five forest ecosystems in Europe failed to sup-

In the 1981 water year, bulk precipitation was primarily a solution of dilute H2SO4, and SO42‐ was the dominant anion in tbroughfall and soil leachates in two eastern Tennessee deciduous forests. Ecosystem inputs of SO42‐, which included dry deposition of forest canopies, may have been up to 40% greater than input estimates based on atmospheric deposition sampling in open areas. Volume‐weighted mean annual pH of bulk precipitation was 4.3; of throughfall 4.8; and of leachates from O2, A1, and B21 soil horizons about 6.0. At both sites, strong acids in precipitation were largely neutralized prior to rainwater's infiltration into mineral soil.Base cations that exchanged with H+ (hydrogen ions) in acid precipitation were almost entirely supplied by forest canopies and litter layers, and did not come directly from exchangeable mineral soil pools. Annual fluxes of HCO3− alkalinity from B21 horizons, about 0.42 and 0.51 kmol (−) ha−1 (keq ha−1) at the two sites, indicated that the natural H+ input from the partial ionization of H2CO3 was of similar magnitude to H+ input in bulk precipitation in 1981 [0.60 kmol(+)ha−1]. However, even in these infertile soils with low cation exchange capacities (CEC) [<7.2 cmol (NH4+) kg−1 (meq/100 g) in surface 50‐ or 80‐cm soil], exchangeable bases were more than two orders of magnitude greater than annual H+ input in bulk precipitation, and represented a substantial reserve for base cations in canopies and litter layers that exchanged with H+ in acid rain. Furthermore, inputs of H+ from acid precipitation were equal to about 0.4% of the base cations that are biologically cycling and immediately available in these ecosystems. Although poorly quantified, mineral weathering and deep rooting will supply, over time, substantial amounts of additional base cations for biologic cycling. Both soils are base‐poor Udults and classified as sensitive to acid rain, but the deposition, cycling, and soil data presented in this report indicate that leaching remains a process affecting cation reserves and soil development only over the very long term.

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