FOREST SOIL RESPONSE TO ACID AND SALT ADDITIONS OF SULFATE .1. 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 Na2SO4 or 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-mu-eq SO42- L-1 for 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.