Sulfur constituents and transformations in upland and floodplain forest soils

Forest soils at an upland site (Typic Hapludalfs) and a floodplain site (Cumulic Haplaquolls) were sampled seasonally to examine organic and inorganic sulfur constituents at four depths (0–10, 10–30, 30–60, and 60–103 cm). At both sites, organic sulfur (carbon-bonded and ester sulfate) was >90% of total sulfur concentrations. Low levels of extractable sulfate (<12 μg•g−1 dry mass of soil) were found at all sites and depths. The floodplain soil contained higher sulfur levels (except sulfate) than the upland site (3072 and 1353 kg total S•ha−1, respectively). Sulfur constituents (except sulfate) decreased with depth in the soil profile at both sites. At both sites soils had little capacity to adsorb sulfate at current solution concentrations. In addition to the field study, a laboratory experiment was designed to examine simulated flooding (over 28 days), using bottomland soil and water from the Sangamon River, Illinois. The field and artificial flooding results suggested little reduction of sulfate in the floodplain soil (nonsulfate inorganic sulfur concentrations of <1% of total sulfur). Large sulfur pools in the bottomland soil are thought to be deposited by sedimentation of organic materials during flood events. Sulfate in soils from both sites appears to be rapidly incorporated into organic sulfur and is not readily adsorbed.