Forest soil response to acid and salt additions of sulfate: I. Sulfur constituents and net retention

We used soil columns constructed from a Maine Spodosol and Illinois Alfisol to investigate the retention of SO₄²⁻added as Na₂SO₄or H₂SO₄. Both organic and inorganic S pools were examined to determine how retention of added SO₄²⁻was influenced by both mineralization/immobilization and adsorption/desorption processes. Forty columns were leached weekly for a year with simulated throughfall solutions containing base cations (Ca²⁺, Mg²⁺, K⁺, Na⁺), NH₄⁺, NO₃⁻, Cl⁻, and either 80, 280, or 1080 μeq SO₄²⁻L⁻¹for varying periods. At the conclusion of the experiment acid and control columns were destructively sampled by depth increments to examine organic (C-bonded S and ester sulfate) and extractable SO₄²⁻concentrations, as well as soil pH. For all Spodosol columns, SO₄²⁻adsorption by the B horizon was the dominant process of SO₄²⁻retention; no changes in organic S pools were observed. Soils in acid columns retained greater SO₄²⁻than salt columns, most likely due to pH dependent adsorption. However, all Spodosols retained large amounts of SO₄²⁻. In the Alfisol, SO₄²⁻retention was lower than in the Spodosols (<25%) due to a limited SO₄²⁻adsorption capacity; mineralization of C-bonded S resulted in S inputs nearly equaling outputs. Although organic S was the dominant S pool in both soils, there was little mineralization overall, and inorganic adsorption appeared to be the primary process of SO₄²⁻retention.