Biochar-based metal tolerating plant growth promoting bacterial inoculants enhanced the ability of ryegrass for phytostabilization.

Metal-tolerant microbes with plant growth-promoting traits represent a promising biological amendment for enhancing the phytostabilization of contaminated soils. However, the relationship between phytostabilization efficiency and microbial consortium composition and diversity remains unclear. This study selected three cadmium (Cd) resistant plant growth promoting bacteria (PGPB) from Bacillus, Pseudomonas, and Rhodopseudomonas were selected as candidates for biochar-based microbial inoculants. In our pot experiment with single, dual, and triple inoculations, a more diverse microbial consortium significantly increased root Cd accumulation and aboveground biomass. Triple inoculation boosted root Cd accumulation by 56.4 % to 121.5 % and belowground biomass by 4.8 % to 46.2 %, compared to dual and single inoculations. However, this trend was not observed in the aboveground parts of the plants, resulting in a decrease in the translocation factor of Cd in ryegrass. Microbial inoculation altered the structure of the rhizosphere bacterial community, especially the triple microbial inoculation treatment, which showed significant differences compared to the other treatment groups. However, there were no significant changes in alpha diversity. Increased soil pH and its positive interaction with soil enzymes significantly contributed to the phytostabilization efficiency of biochar-based microbial inoculation, whereas the contribution of rhizosphere bacterial communities was much less significant. In summary, metal-tolerant PGPB inoculation can promote phytostabilization efficiency and enhance metal immobilization in soil, reducing their threat to organisms and the environment.