Combined Effects of Earthworms and Plant Growth-Promoting Rhizobacteria (PGPR) on the Phytoremediation Efficiency of Acacia mangium in Polluted Dumpsite Soil in Bonoua, Côte d’Ivoire

The impact of earthworms and plant growth-promoting rhizobacteria (PGPR) on the remediation in polluted dumpsite soil was performed in a greenhouse pot culture with Acacia mangium inoculated or not (control: T0) with Pontoscolex corethrurus (T1) and with Bradyrhizobium (T2); and inoculated with Pontoscolex corethrurus and Bradyrhizobium (T3). Our results showed the presence of Bradyrhizobium and/or earthworms significantly increase (P < 0.05) in the height (2-fold), total dry biomass weight (7- to 15-fold) and metal uptake of the plant (2 to 10-fold), as compared with the non-inoculated plant. The presence of both inoculants (Bradyrhizobium and earthworm) enhanced soil Pb/Ni/Cr mobility and bioavailability in metal-contaminated soil, and increased 15-fold the total plant biomass and 10-fold metal accumulation in plant biomass, as compared with plant inoculated with earthworms or Bradyrhizobium. In addition, the presence of earthworms and/or Bradyrhizobium promoted the phytoimmobilization process of Ni, Cr and Pb preferentially in Acacia mangium roots than in shoot tissue. Our experiments highlight the importance of soil organisms on the phytoremediation efficiency. It appears that earthworms and/or Bradyrhizobium have the potential to enhance the phytoextraction efficiency of plants in metal-contaminated soil.