Bioinoculant potential of heavy metal- and diesel-tolerant bacteria in enhancing soil and plant health

In this study, the bioinoculant potential of four plant growth-promoting rhizobacteria (PGPR; Pseudomonas sp. NL3, Bacillus sp. NL6, Bacillus sp. LBY14, and Priestia sp. TSY6) was examined. Their indole-3-acetic acid (IAA) production, tolerance to heavy metals (Cu, Pb, and Cd), and diesel degradability were assessed. Moreover, the effect of PGPR inoculation on the germination of tall fescue seeds and seedling growth in soil contaminated with metals and diesel was also examined. The findings highlighted significant IAA productivity across the strains, with Priestia sp. TSY6 demonstrating the highest capacity (291.1 mg-IAA·L⁻¹). Notably, Priestia sp. TSY6 exhibited the highest resistance to Pb stress (IC₅₀ of 164.5 mg·L⁻¹), while Bacillus sp. NL6 showed the highest resistance under Cu stress (IC₅₀ of 29.4 mg·L⁻¹). Overall, the strains showed low diesel degradability and did not improve seed germination in co-contaminated soil with metals and diesel. In contrast, all strains significantly improved tall fescue seed germination and seedling growth in soil contaminated only with metals, with Bacillus sp. NL6 notably enhancing the seedling vigor index. The study identified Bacillus sp. NL6 and Priestia sp. TSY6 as promising candidates for bioinoculants in heavy metals-contaminated soils and provides insights for developing sustainable biostimulation strategies to support environmental remediation.