Synergistic effect of biochar-immobilized plant growth-promoting bacterium on multi-heavy metal phytoremediation by Zinnia elegans L. in landfill soil

This research investigated the capacity of Micrococcus yunnanensis (MY), a heavy metal-resistant plant growth-promoting bacterium (PGPB), in promoting root growth and development of Zinnia elegans L. under heavy metal stress conditions. The study compared the effectiveness of M. yunnanensis applied as biochar-immobilized cells (BC-MY) and as cell suspension (CS-MY) in enhancing Z. elegans phytoremediation of cadmium (Cd), copper (Cu), and lead (Pb) in municipal solid waste landfill soil. The results demonstrated that M. yunnanensis significantly promoted root growth in Z. elegans seedlings under heavy metal stress compared to the uninoculated controls. Pot experiments revealed that both BC-MY and CS-MY inoculation enhanced plant growth and biomass when Z. elegans was cultivated in multi-heavy metal-contaminated landfill soil. The application of BC-MY or CS-MY increased Cd and Cu concentrations in the root and shoot tissues relative to the uninoculated plants, whereas Pb accumulation was not significantly different ( p < 0.05). Micro-X-ray fluorescence analysis confirmed the spatial distribution and accumulation of Cd, Cu, and Pb in root and shoot tissues. Furthermore, both BC-MY and CS-MY treatments significantly improved key phytoremediation indices, including root uptake, shoot uptake, phytoextraction efficiency, and uptake efficiency. Notably, bacterial inoculation enhanced the accumulation and translocation factors for Cd and Cu. These findings demonstrate the promising potential of M. yunnanensis in both BC-MY and CS-MY forms to improve the phytoremediation performance of Z. elegans for Cd and Cu removal from polluted landfill soils.