Synergistic rhizosphere processes enhance cadmium and lead stabilization by Phragmites australis: Microbial community succession and metal speciation shifts.

This study investigated the accumulation of cadmium and lead in the root system of Phragmites australis and the associated structural shifts in the rhizosphere microbial community under metal stress. In a pot experiment, soils were individually contaminated with either cadmium at concentrations of 5 to 25 mg/kg or lead at concentrations of 40 to 400 mg/kg, with an untreated control receiving only deionized water. The results demonstrated a significant positive correlation between root accumulation of both metals and their concentrations in the soil, increasing over a 60-day period. Maximum enrichment factors reached 53% for cadmium and 275% for lead compared to low-concentration treatments. Sequential extraction indicated that cadmium in roots was primarily bound to pectin and proteins in the cell wall, while lead was immobilized through chelation and precipitation in cell walls and vacuolar sequestration via phytochelatins. In the rhizosphere soil, phytoremediation progressively transformed cadmium and lead into more stable fractions such as residual fraction, carbonate-bound fraction and Fe-Mn oxide-bound fraction, reducing the bioavailable exchangeable fraction. Soil pH and organic matter content were the dominant factors controlling metal speciation and bioavailability. High activities of urease, sucrase and catalase were correlated with enhanced metal stabilization, reflecting robust microbial-mediated processes. High-throughput sequencing revealed that initial cadmium and lead stress reduced bacterial and fungal diversity. However, diversity recovered and community structure restructured as phytoremediation progressed. Metal-tolerant organisms, exemplified by Actinobacteria and Xylariales, showed significant enrichment, which implicates them in key adaptive roles. The findings highlight that Phragmites australis effectively stabilizes cadmium and lead through root accumulation and metal speciation alteration, facilitated by synergistic rhizosphere processes involving physicochemical changes and adaptive microbial communities, underscoring its potential for phytoremediation of soils contaminated with cadmium or lead.