The dynamics of sulfur, phosphorus and iron in flooded soils as affected by changes in Eh and pH

Abstract

The dynamics of Fe, P and S were studied in three soils (prairie, podzolic and krasnozem) with varying S sorption capacity (13, 32 and 132 µg S sorbed g soil-1 at 5 µg S mL-1). The soils were incubated as suspensions for 14 days at 25°C and the pH and Eh adjusted independently to four levels in the range of pH 4-8-70 and Eh from + 350 to -150 mV. In the prairie and krasnozem soils, the concentration of S in the soil solution at high pH increased as Eh was lowered to +200 mV and then decreased as Eh was lowered further. At low pH, Eh changes did not affect the concentration of S in the soil solution. The changes in the concentration of sorbed S extracted by KH2PO4 were generally similar to those for S in the soil solution. Sulfate sorption increased with a decrease in pH, with the pH effect being greater at higher Eh. At high pH and Eh, the soils released S into the solution but sulfate sorption capacity increased when Eh was lowered. At low pH, the sulfate sorption capacity of the prairie soil first increased with a lowering in Eh from +350 to about +200 mV, but then decreased with a further lowering of Eh. In the krasnozem soil, sulfate sorption capacity increased with a lowering in Eh from the highest (+350 mV) to the lowest (-150 mV) level only at the lowest pH (4.8). In the podzolic soil, the effect of pH on sulfate sorption capacity was similar at all Eh levels. The Eh effects on sulfate sorption capacity of the three soils were different.