Inoculation effect of heavy metal tolerant and plant growth promoting rhizobacteria for rhizoremediation

The utilization of plant growth promoting rhizobacteria (PGPR) is a potential strategy to ameliorate the rhizoremediation effect in the heavy metals (HMs) contaminated soil. In this study, a new heavy metal tolerant PGPR, Sphingomonas sp. PbM2, was isolated from the rhizosphere of maize. The siderophore production and 1-aminocyclopropane-1-carboxylic acid deaminase activity of PbM2 were superior to that of Novosphingobium sp. CuT1, an HM-tolerant PGPR, while the indole-3-acetic acid productivity of PbM2 was inferior to that of CuT1. The inoculation effect of the PGPR (PbM2 alone or a mixture of PbM2 and CuT1) on the rhizoremediation performance of HM-contaminated soil planted with maize was compared. Cu bioavailability was enhanced with PGPR treatment, while the bioconcentration factor significantly increased or remained steady depending on the HM concentration (200, 500, or 1000 mg/kg-soil) and remediation period (20 or 60 d). PGPR inoculation significantly enhanced soil PGP activity except for siderophores but was not statistically associated with improved plant growth. The dynamics change in the bacterial communities during rhizoremediation was similar for all soil conditions regardless of PGPR inoculation. Network analysis revealed that both the inoculated PGPR and indigenous rhizobacteria contributed to Cu bioavailability and soil PGP activity. These results suggest that the inoculation of PGPR is effective in the remediation performance of contaminated soil in which autogenous PGPR is inhibited.