Transport Behavior of Cd2+ in Highly Weathered Acidic Soils and Shaping in Soil Microbial Community Structure

The mining and smelting site soils in South China present excessive Cd pollution. However, the transport behavior of Cd in the highly weathered acidic soil layer at the lead–zinc smelting site remains unclear. Here, under different conditions of simulated infiltration, the migration behavior of Cd²⁺ in acid smelting site soils at different depths was examined. The remodeling effect of Cd²⁺ migration behavior on microbial community structure and the dominant microorganisms in lead–zinc sites soils was analyzed using high-throughput sequencing of 16S rRNA gene amplicons. The results revealed a specific flow rate in the range of 0.3–0.5 mL/min that the convection and dispersion have no obvious effect on Cd²⁺ migration. The variation of packing porosity could only influence the migration behavior by changing the average pore velocity, but cannot change the adsorption efficiency of soil particles. The Cd has stronger migration capacity under the reactivation of acidic seepage fluid. However, in the alkaline solution, the physical properties of soil, especially pores, intercept the Cd compounds, further affecting their migration capacity. The acid-site soil with high content of SOM, amorphous Fe oxides, crystalline Fe/Mn/Al oxides, goethite, and hematite has stronger ability to adsorb and retain Cd²⁺. However, higher content of kaolinite in acidic soil will increase the potential migration of Cd²⁺. Besides, the migration behavior of Cd²⁺ results in simplified soil microbial communities. Under Cd stress, Cd-tolerant genera (Bacteroides, Sphingomonas, Bradyrhizobium, and Corynebacterium) and bacteria with both acid-Cd tolerance (WCHB 1-84) were distinguished. The Ralstonia showed a high enrichment degree in alkaline Cd²⁺ infiltration solution (pH 10.0). Compared to the influence of Cd²⁺ stress, soil pH had a stronger ability to shape the microbial community in the soil during the process of Cd²⁺ migration.