Combining biochar, plant growth-promoting rhizobacteria, and annual ryegrass for the recovery of a PTEs multi-contaminated soil

Phytoremediation is a sustainable, environmentally friendly and low-cost technology for the restoration of soils contaminated with potentially toxic elements (PTEs). We hypothesized that the effectiveness of the recovery process can be improved by associating phytoremediation with the application to the soil of amendments and/or the inoculum of plant growth-promoting rhizobacteria (PGPR). The aim of this study was therefore to assess the suitability of combining a new microbial consortium with biochar for the remediation of a soil contaminated with Cd (22.6 mg·kg⁻¹), Pb (4473 mg kg⁻¹) and Zn (3147 mg kg⁻¹) and cultivated with annual ryegrass (Lolium rigidum). In the biochar treated-soil, the amounts of exchangeable PTEs were reduced by 56, 60 and 81 % for Cd, Pb and Zn respectively, compared to the untreated soil. Dehydrogenase and urease activities increased in the amended soil compared to the control, while β-glucosidase decreased in the presence of biochar. Soils treated with PGPR alone or in combination with biochar showed the lowest bacterial and fungal diversity, showing that the microbial inoculum was the main factor influencing the structure of the soil microbiota. The highest biomass production of ryegrass was detected in plants grown in soil treated with biochar and inoculated with PGPR (+31 % vs. control), suggesting a synergistic effect of the treatments in improving plants growth. A reduced PTEs uptake was also recorded in plants grown in soil treated with biochar and inoculated with PGPR. These results highlighted that PGPR significantly increased the effectiveness of biochar in the assisted phytoremediation.