Sulfate retention and cation leaching of forest soils in response to acid additions

Abstract

A vacuum extractor was used to examine the effects of increased SO42− deposition on net S retention and cation leaching in three Maine Spodosols (Berkshire, Dixfield, and Rawsonville series) and an Illinois Alfisol. Columns (leached daily for 30 days with either a simulated throughfall solution containing 80 μequiv. SO42−•L−1 (pH = 4.77) or a simulated throughfall solution plus 200 μequiv. H2SO4•L−1 (pH = 3.66)) were constructed using O horizons over upper B (Bh or Bhs) and lower B (Bhs or BC) horizons for the Spodosols and two depth increments of bulked soil (0–12 and 12–25 cm) for the Alfisol. Leachate concentrations of base cations were dominated by Ca2+ and were generally greater in the Alfisol than in the Spodosol leachates. Declining concentrations of base cations and NH4+ with time led to an increase in Al3+ concentrations and a decrease in pH for some of the Spodosol leachates (e.g., leachate Al3+ increased from 19 μequiv. Al3+ •L−1 (day 2) to 194 μtequiv. Al3+ •L−1 (day 30), and pH decreased from 5.53 to 4.41, respectively, for the Dixfield high-S treatment). Columns that received the high-S treatment retained a greater percentage of the added S than those that received the low-S treatment because of increased SO42− adsorption in the former. High rates of net organic S mineralization were found for all soil types (e.g., 169 μg organic S•g−1 over 30 days for the Rawsonville Bhl horizon, 15% of the total S); no treatment effects were found for the amount of S mineralized. Trends in net S retention across soil type (within the same treatment) reflected increases in soil SO42− from adsorption, as well as decreases in organic S from mineralization. When net S mineralization was removed from leaching losses of S, trends in net S retention closely reflected SO42− adsorption differences (50, 23, 8, and 1% of the added S retained by the Rawsonville, Dixfield, and Berkshire series, and the Alfisol, respectively, for the high-S treatment). Because of the low SO42− adsorption capacity of the Alfisol relative to the Spodosols, organic processes were more important in affecting net S retention for the Alfisol. Inorganic processes of S retention were more important for the three Spodosols studied.