Effective cadmium immobilization in paddy soil by the interaction of sulfate reducing bacteria and manganese fertilizer

Manganese fertilizer (MnSO4) was widely applied to control the Cadmium (Cd) uptake by rice, but the overall process of microbial activities controlling Cd mobilization in paddy soil is poorly understood. This study investigated the stimulation effect of sulfate reducing bacteria (SRB) on Cd bioavailability with the input of different doses MnSO4 (0.5, 1.0, 2.0 g/kg) under the anaerobic paddy soil. The results show that the input of MnSO4 generated soil H+ release. However, the stimulation of SRB remarkedly increased soil pH and reduced the redox potential (Eh) by inhibiting the exchange of Mn2+ and H+, resulting in the available Cd decreased and the amorphous Fe/Mn Oxide-Cd increased significantly. In the co-existed SRB and 1.0 g/kg MnSO4, the available Cd decreased remarkedly by 40.18%, which was transformed to reducible Cd. Meanwhile, the addition of MnSO4 and SRB enhanced the abundance of Cd immobilization related bacteria, including Desulfobacterota, Chloroflexi, Bacteroidota, and Myxococcota. KEGG results showed that MnSO4 and SRB treatment enhanced the ability of microbial sulfur and secondary metabolites. Furthermore, the sulfate reduction related genes (i.e. aprA, sat) obviously enriched in soils. Structural equation modeling showed that Mn, Fe, DOC, Eh, and pH are the key factors affecting available Cd. These findings add to the current knowledge of how MnSO4 and microorganisms affect the mobilization and availability of Cd under paddy soil media, providing new ideas and a theoretical basis for reducing the environmental risk of Cd.