Microbial sulfate and iron reductions have the potential for the remediation of soils contaminated by cadmium (Cd), but the effectiveness may be limited by the availability of organic matter (OM) because most iron- and sulfate-reducing bacteria are heterotrophic. In addition, Cd mobility has hardly been investigated in soils with a realistic concentration of total Cd. We successfully accounted for the dynamics of Cd solubility in a paddy soil that contained 0.36 mg kg^-1 total Cd after flooding and re-drying by monitoring multiple soil chemical and microbial properties. After flooding, the addition of rice straw promoted greater microbial iron reduction and greater increase in soil pH compared to the control and treatments with added gypsum or hematite. Consequently, dissolved soil Cd decreased from 34 μg kg^-1 to under the detection limit in a week when straw was added but remained high even after eight weeks in the control (13 μg kg^-1) and treatments with added hematite (8 μg kg^-1) or gypsum (12 μg kg^-1). After re-drying, dissolved Cd in the straw-amended soil increased but was still lower than other three treatments after five weeks. The results suggest OM addition is efficient to immobilize soil Cd at a realistically low concentration by stimulating microbial reductions of soil native oxidized components after flooding, while adding extra iron oxide or sulfate alone is not effective because it cannot remove the limitation by OM availability.