Transformations of iron and manganese under controlled Eh, Eh-pH conditions and addition of organic matter

Iron and manganese transformations were studied in a suspension of rice soil at controlled redox potentials (E_h) of − 300, − 150, 0, + 150 and + 300 mV only or in combination with three pH values of 6, 7 and 8. These experiments were carried out with and without addition of 1% (W/W) ground raw wheat straw. The E_h of theE_h-pH combination was kept on a constant level for 5 days, after which the different fractions of iron and manganese were determined. The incubation was carried out at 25 °C. Under controlled redox potential conditions, lowering in E_h caused an increase in pH. The soil suspension was kept at − 300 mV contained approximately a double amount of water-soluble plus exchangeable iron as compared with the other controlled redox potentials. The obtained data also showed that the concentration of water-soluble manganese was slightly affected by the redox potential. Meanwhile, the exchangeable manganese was strongly influenced by the E_h value. The threshold E_h value for the total soil (water-soluble + exchangeable) of iron and manganese was about − 150 to 0 mV. The residual iron and manganese decreased when the easily reducible iron and manganese increased with decreasing soil redox potential value and vice versa. The inflection point for the two fractions is about 0 mV. The addition of 1% organic matter led to a slight decrease in pH at different controlled E_h values. Generally, it increases water-soluble iron and manganese, exchangeable manganese and easily reducible manganese, meanwhile, slightly decreasing the exchangeable and reducible iron. Under controlled E_h-pH conditions, the redox potential and pH values affected the concentrations of water-soluble and exchangeable iron and manganese. Substantial amounts of both fractions were found under relatively low pH low redox potential conditions. Threshold E_h value for total water-soluble and exchangeable iron was at about − 150 mV and pH 8 and 7 while it increased to + 200 mV at pH 6. However, E_h was + 150 mV at pH 7 and 8 for manganese. Addition of organic matter slightly increased the amount of water-soluble iron at both pH 8 and 7 but it sharply decreased the amount of water-soluble iron at pH 6. The addition also decreased the exchangeable iron fraction at pH 8, while it increased the same fraction at both pH 7 and 6. The amount of reducible iron generally decreased, but no difference was found in the inflection point at pH 7 and 6 between the control and the addition of organic matter. Addition of organic matter only slightly decreased the water-soluble manganese fraction at pH 8, the decrease was important at pH 7. However, there was a sharp increase at pH 6. The effect of the addition of organic matter on the exchangeable manganese showed the same trend as observed for the water-soluble fraction. The easily reducible manganese fraction increased at pH 8 and 7, while it decreased at pH 6. The residual manganese fraction was also affected by the addition of 1% organic matter as a consequence of the effect on the other different fractions