Sulphate reduction in waterlogged soils

To study the factors that influence the release of free H2S, flooded soils were incubated with plant material and either sodium sulphate or elemental sulphur. Residual sulphate, FeS and H2S were measured throughout the reaction. Less free H2S was evolved from a periodically waterlogged soil than from a well‐drained soil of comparable HC1‐ and dithionite‐soluble Fe content. With a readily soluble synthetic Fe2O3 as the source of Fe, a slight excess of Fe over the SO42– equivalent sufficed to prevent loss of H2S. The native organic matter in the well‐drained Rothamsted top‐soil seemed to have no effect in promoting loss of H2S, but the FeS:H2S ratio was strongly influenced by the nature of the plant material used as the source of energy. With either Na2SO4 or S, considerably more free H2S was evolved with lucerne than with rice blade. With S, the pH of the incubated mixtures was close to that of the nominally S‐free system, whereas with Na2SO4 the mixture became quite alkaline. More free H2S was evolved under the more alkaline conditions obtained with Na2SO4, which suggests that it is the initial mobilization of Fe that governs the distribution of the reduced S, rather than the pH‐solubility relations of FeS. An oxidized surface layer is very effective in retaining sulphur in the soil. RESP-595