Sulfate Mobility in an Acid Danish Forest Soil

Abstract

ABSTRACTA key question in the acidification of soils due to atmospheric deposition of strong acids is the accumulation of SO2−4 in the soil. Sulfate leaching was studied in the laboratory in undisturbed soil cores taken in a Typic Udipsamment under spruce forest. The soil cores were watered with natural throughfall water (pH 3.6) or with throughfall acidified to pH 3.3 or 2.8 with a mixture of sulfuric and nitric acid (1:1, molar basis). A total of 660 mm of throughfall was applied during 11 months. The technique of using soil cores produced increased nitrification and mineralization in the soil, causing a continuous acid production in the soil cores and decrease in the leachate pH during the experiments. Soil solution concentrations of SO2−4 were in the range 0.4 to 1.5 mmol L−1, total Al 0.2 to 1.5 mmol L−1, and pH 3.5 to 4.1. Stability diagrams using Al and SO2−4 activities and pH indicate that precipitation of a solid phase with the composition of AlOHSO4 may control the Al and sulfate activities in the soil solutions in the Bw horizon but not in the Ap horizon. The equilibrium constant for this basic Al sulfate was estimated at 20°C, pKs = 17.36 and at 5°C, pKS = 17.05. The 35S labeled SO2−4 was added once to the same soil cores followed by 15 mm throughfall per day (Natural or adjusted to pH 2.8). After application of 75, 150, or 225 mm of throughfall water the distribution of 35S‐SO2−4 in the soil cores was determined. From 38 to 100% of the applied 35S‐SO2−4 was retained in the soil cores and of this amount 32 to 63% was found in the top of the Bw horizon, which was highest in free Al and Fe oxides. The studied soil did not accumulate SO2−4 under ambient field conditions. In the laboratory, however, SO2−4 accumulation was demonstrated when the soil was exposed to increased SO2−4 load and temperature as well as decreased pH, showing that SO2−4 sorption/desorption may buffer variations in SO2−4 concentrations in soil solutions, and thereby delay effects of acidification on cation leaching.