Atmospheric Sulfur Deposition, Neutralization, And Ion Leaching In Two Deciduous Forest Ecosystems

In the 1981 water year, bulk precipitation was primarily a solution of dilute H₂SO₄, and SO₄^2-was the dominant anion in tbroughfall and soil leachates in two eastern Tennessee deciduous forests. Ecosystem inputs of SO₄^2-, 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.