Inducing Rhizosphere Acidification in White Willow with Bacillus sp. ZV6 Enhances Ni Phytoextraction from Soil and Soil Quality

Chelating agents may decrease the extent of Ni phytoextraction by reducing plant growth and soil health due to Ni toxicity during enhanced phytoextraction. Contrarily, inducing acidity in the rhizosphere of Ni-accumulating plants with plant growth-promoting rhizobacteria (PGPR) having rhizosphere acidification ability can enhance Ni phytoextraction by increasing Ni bioavailability in the soil, plant growth, and plant stress tolerance. We investigated the efficacy of a PGPR species with rhizosphere acidification potential, named Bacillus sp. ZV6 (ARB), in enhancing Ni phytoextraction by white willow (Salix alba) from a Ni-affected soil. The plants were grown for 120 days in soil with zero, threshold, and moderate Ni pollution levels (0, 50, and 100 mg Ni kg⁻¹ soil, respectively) with and without ARB inoculation. After harvest, the effects of the treatments on rhizosphere acidification and associated Ni bioavailability in this zone, Ni distribution in plants, and Ni removal from the soil were investigated. Moreover, enzyme activity, count of bacteria, biomass of microbes, and organic C in the soil, together with indices of plant growth and antioxidant defense, were evaluated. The ARB inoculation significantly improved the plant parameters and soil health and reduced plant oxidative stress at each Ni level compared to the treatments lacking ARB. Besides lowering the soil pH and increasing Ni bioavailability in the rhizosphere with respect to the bulk zone, ARB inoculation exerted additional effects. Surprisingly, the Ni 100 + ARB treatment induced the highest decrease in soil pH (0.32 unit) and an increase in DPTA-extractable Ni (0.45 mg kg⁻¹ soil) between that measured in the bulk zones and that obtained in the rhizosphere zone. Ni distribution in plant parts and Ni removal (% of total Ni) from the soil were also significantly improved with ARB inoculation, compared to the Ni treatments without ARB. The extent of Ni removal was similar for the Ni 50 + ARB (0.27%) and Ni 100 + ARB (0.25%) treatments. Concluding, ARB-inoculated Salix alba can remove similar amounts of Ni from the soil, irrespective of the Ni pollution level.