Red mud causes dynamic changes in the soil microbial community and cadmium fractions in a slightly cadmium-contaminated paddy soil

Red mud is a highly alkaline industrial by-product rich in iron oxides with great potential for soil cadmium remediation. Although the stabilization of Cd by red mud is well reported in rice potted and field experiments, the influence of red mud on microbial communities in paddy soil and the contribution of soil microbial communities subjected to red mud in Cd stabilization remain unknown. This study used high-throughput sequencing and bioinformatics, combined with a sequential extraction procedure, to determine the microbiological mechanisms of rice Cd reduction by red mud and information on the corresponding soil Cd fraction. The results showed that red mud significantly increased the soil pH and iron and manganese oxide-bound Cd fractions. Red mud application influenced the microbial beta diversity rather than the alpha diversity, especially for bacteria. Unique taxa associated with iron reduction (e.g., phylum Firmicutes and genus Anaeromyxobacter) were enriched at the rice-filling stage, which may contribute to the stabilization of Cd. Red mud application caused little difference in the fungal communities. A 2 % red mud amendment successfully decreased the grain Cd content in a high Cd-accumulating rice cultivar by 72 %. Red mud effectively reduces Cd accumulation in the short-term and demonstrates potential for remedial applications. This study provides microbiological evidence for stabilizing Cd in red mud, but its long-term environmental impact and field applicability require further research.