Effects of Mn(II) on the Oxidation of Fe in Soils and the Uptake of Cadmium by Rice (Oryza sativa)

Soil properties affect the bioavailability of cadmium (Cd). In general, soils in southern China are rich in iron (Fe) and contain manganese (Mn), which form ferromanganese nodules through biological or chemical reactions. In this study, it is hypothesized that the formation of Fe/Mn oxides can reduce the bioavailability of Cd and play an important role in reducing the uptake of Cd by rice. In this study, the effects of Fe and Mn on the contents of Fe (II)-oxidizing bacteria (FeOB) communities, Fe oxides, and Cd speciation in Cd-contaminated paddy soils were studied in a pot experiment. Results showed that, compared with the CK treatment, the addition of Mn (II) increased the abundance of FeOB (Thiobacillus sp.) in the soil, thereby increasing the content of amorphous Fe oxides in the rhizosphere. Amorphous Fe oxides effectively immobilized Cd in the soil, that is, with an increasing content of Cd bound to amorphous Fe/Mn oxides in the rhizosphere and non-rhizosphere soils, the mobility and bioavailability of Cd were reduced. After the addition of Mn (II) and Fe (II), the Fe content in the root Fe plaque of rice increased significantly, while the Cd content decreased significantly. The increase of Fe in the root Fe plaque inhibited rice uptake of soil Cd; thus, the Cd content in rice shoots was significantly lower than the Cd content in the CK treatment. Compared with the addition of Fe (II), the addition of Mn (II) effectively reduced the Cd content in rice shoots. These findings suggested that the addition of Mn (II) can promote the formation of Fe oxides in soils and reduce the Cd content in rice. These research results provide support for using Mn-based soil remediation materials to remedy Cd-contaminated soils.