Biochar and urea enhance phytoremediation of PAH-Pb co-contaminated karst soil: bacterial and metabolic insights.

The remediation of polycyclic aromatic hydrocarbon and lead co-contaminated soils in karst regions remains challenging because of strong soil buffering capacity, complex metal speciation, and the distinct transformation mechanisms of organic pollutants and heavy metals. Although urea and biochar are widely used as soil amendments, their interactive effects on plant-assisted remediation and the associated rhizosphere microbial and metabolic mechanisms remain unclear. In this study, urea and biochar were co-applied in a ryegrass pot experiment to evaluate their synergistic effects on the remediation of PAH and Pb co-contaminated karst soil. The combined treatments significantly enhanced PAH degradation within 80 days, with PBCN and PN achieving the highest degradation efficiencies of 49.5 and 38.4%, respectively. Biochar-plant integration also markedly reduced Pb availability, with PBCN showing the greatest reduction, reaching 24.52 mg kg-1, which was 36.8% lower than that of the control. The improvement in Pb remediation was closely associated with changes in soil pH, aggregate distribution, and organic matter fractions. The combined application of biochar, urea, and ryegrass increased dehydrogenase, urease, and phosphatase activities, as well as the abundances of 16S rDNA and functional genes. Urea reduced bacterial α-diversity, whereas bacterial community composition was mainly shaped by urea and ryegrass treatments. Network analysis indicated that biochar and urea decreased bacterial network complexity and stability, while ryegrass enhanced rhizosphere network connectivity. Metabolomic analysis further revealed that urea strongly regulated rhizosphere metabolites, particularly organic acids, amino acids, and low-molecular-weight compounds associated with Pb chelation and mobilization. Overall, this study provides new insights into the synergistic remediation of PAH and Pb co-contaminated karst soil by biochar, urea, and ryegrass, and supports the development of green and cost-effective remediation strategies for contaminated karst soils.