**Abstract**

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.

**Keywords:** acacia, phytoremediation, inoculation, phytoimmobilization, bioavailability

### **1. Introduction**

*Acacia mangium* Wild. is a tropical plant, which has the capacity to improve soil fertility [1], to interact with soil bacteria and soil fauna, particularly in the rhizosphere [2], and to extract metal from polluted soils [3, 4]. In fact, *A.mangium* can accumulate 93.5 mg kg<sup>1</sup> of copper (Cu) and 79 mg kg<sup>1</sup> of zinc (Zn) in its biomass, was able to tolerate high concentration of cadmium (Cd) [3], and can require 5 and 17 years to remove 79.8 kg ha<sup>1</sup> of Zn and 47 kg ha<sup>1</sup> of Cu, respectively [4]. So, the phytoremediation efficiency of *A.mangium* has been reported in numerous studies [5, 6] in which it has been demonstrated that the success of phytoremediation may not solely depend on the plant itself but also on the interaction of plant roots with soil microorganism and soil fauna and the availability of heavy metals accumulated in soil [2, 7], because the interaction between plants and beneficial rhizosphere bacteria can enhance biomass production and the tolerance of plants to heavy metals.

It has recently been shown that rhizosphere bacteria may improve metal solubility and availability by decreasing the soil pH or by producing chelators and siderophores [8, 9]. Rhizosphere bacteria such as *Bradyrhizobium allorhizobium* stimulate plant growth either directly or indirectly and have been successfully used to reduce plant stress in metal-contaminated soils and to increase phytoremediation efficiency [6, 7, 10]. Moreover, plant growth-promoting rhizobacteria (PGPR) are known to affect heavy metal mobility and availability to the plant through the release of chelating agents, acidification, phosphate solubilization, and redox changes; therefore, they have the potential to enhance phytoremediation processes [2, 11, 12].

Hence, an alternative method to enhance phytoextraction efficiency and to improve plant growth is by using rhizosphere bacteria such as PGPR and rhizobia [7].

However, in recent studies, the action of earthworms, particularly *Pontoscolex corethrurus* to improve plant metal uptake during phytoremediation in contaminated soils, has been demonstrated [13–17]. Furthermore, the beneficial effects of *P. corethrurus* earthworms on *A. mangium* growth and its Pb, Ni, and Cr uptake have also been showed [5]. It was noted that in the presence of *P. corethrurus*, *A. mangium* promoted the phytoimmobilization process for Ni, Cr, and Pb, but its effectiveness depends on the nature of the plant, its behavior toward metals, rhizosphere function, and metal speciation in different soil compartments involved in the phytoremediation process [5].

Thus, it is of interest to study the conjugated actions of *P. corethrurus* earthworm, symbiotic bacteria (*Bradyrhizobium*), and of a metal tolerant plant such as A. *mangium* in the remediation of metal-contaminated dumpsite soil of M'Plouessoue Park at Bonoua, where the previous studies [18, 19] have demonstrated that the concentration of Cr (130 mg.kg<sup>1</sup> ), Cd (81 mg.kg<sup>1</sup> ), Pb (118 mg.kg<sup>1</sup> ), and Ni (119 mg.kg<sup>1</sup> ) are far above the permissible limits such as Canadian environmental quality criteria for contaminated sites (CEPA) recommendation [20] and World Health Organization limit (WHO-limit) recommendation [21].

The principal aims of this research were to evaluate the effects of *P. corethrurus* and/or *Bradyrhizobium* on lead, chromium, and nickel phytoextraction by *A. mangium* in polluted dumpsite soil metal-contaminated soil.

*Combined Effects of Earthworms and Plant Growth-Promoting Rhizobacteria (PGPR)… DOI: http://dx.doi.org/10.5772/intechopen.108825*
