Transformation of Manganese in a Waterlogged Soil as Affected by Redox Potential and pH

Abstract The distribution of different forms of manganese in waterlogged soil was studied over a wide range of closely controlled E h ‐pH conditions. At pH 5 almost all of soil manganese was converted from the reducible to the water soluble plus exchangeable fraction even at a redox potential as high as +500 mV. In sharp contrast, at pH levels between 6 and 8 most of the conversion took place at relatively lower redox potentials of +200 to +300 mV. When the water soluble plus exchangeable fraction was further divided into its two components, low pH, and low E h were found to increase water soluble manganese at the expense of the exchangeable form. Cation exchange reactions were apparently important in regulating the equilibria between water soluble and exchangeable manganese. Labelled manganese added to the soil showed an almost identical distribution among the various manganese fractions to that of native soil manganese. The observed pMn/pH slope and Eo were much lower at pH 6 to 8 than those calculated for pure equilibrium systems. It may be concluded from this study that the E h and pH of flooded soils provide general control of manganese transformation which probably include both chemical and biological processes, the complexity of which make it difficult to explain the reactions in terms of simple chemical systems.