Thiosulphate and tetrathionate oxidation in arable soils

Oxidation of thiosulphate (S2O3 2−) and tetrathionate (S4O6 2−), added at 300mgSkg−1 soil, was studied in four arable soils at 25°C, over incubation periods of 0, 3, 7, 10, 20 and 30d. Thiosulphate was oxidised rapidly to S4O6 2−, with most of the oxidation occurring within 3d. In contrast, S4O6 2− was oxidised to SO4 2− much more slowly. Oxidation of S4O6 2− varied with soil type, being slower in a loamy soil (pH 5.8) than in three other soils (clay loam, silty clay loam and loamy sand, pH 6.2–7.0). An incubation experiment using autoclaved Rothamsted soil indicated a large abiotic component in the oxidation of S2O3 2− to S4O6 2−, whereas the oxidation of S4O6 2− to SO4 2− was of a biological nature. Production of SO4 2− was more rapid from the S2O3 2− amendment than from the S4O6 2− amendment, which suggested that either S2O3 2− primed the S4O6 2− oxidisers, or microorganisms were present that could oxidise S2O3 2− to SO4 2−, but not S4O6 2− to SO4 2−.