Oxidation and reduction of Mn in acidic soils: Effect of temperature and soil pH

The effects of temperature and liming on microbial Mn oxidation and chemical Mn reduction in acidic soils were studied. Sodium azide was used to inhibit the oxidation of Mn, and changes in water-soluble plus exchangeable (WS + exch) Mn in soils treated with azide therefore represent changes due to chemical Mn reduction. By comparing untreated soils with azide-treated soils rates of microbial Mn oxidation were also estimated. Reduction and oxidation of Mn were detected under all experimental conditions. The rate of reduction increased with increasing temperature and decreased with increasing pH, as predicted by theory. However, in both soils tested the rate of oxidation was inhibited at the highest pH, indicating that the organisms responsible for Mn oxidation in these soils were well adapted to their acidic environment. Both oxidation and reduction were detected in soils which showed little overall change in WS + exch Mn indicating that in these soils the redox reactions of Mn were in quasi-equilibrium. The rates of oxidation and reduction were sensitive to changes in the concentration of WS + exch Mn. Oxidation was faster when concentrations of WS + exch Mn were high, while the rate of reduction increased after a period of rapid oxidation, probably in response to a greater surface area of Mn oxides then present. This suggests that Mn oxidation and reduction can occur simultaneously in close proximity to each other.