Integrated Effect of Heavy Metal-Tolerant Rhizobacteria and Phosphorus on Maize Growth and Phosphorus Bioavailability in Contaminated Soil

Toxic metals significantly influence agricultural crop yields and adversely affect animals and humans. Hence, in the current study impact of different strains of phosphorus-solubilizing bacteria (PSB) and levels of phosphorus on the solubility of phosphorus and the stabilization of heavy metals in contaminated soil were studied in a greenhouse pot experiment in 2022. The PSB strains included Bacillus CUM6, Bacillus DUM7, and Bacillus PIS7. Phosphorus was applied at rates of 40, 60, and 80 kg P2O5 ha-1 using single superphosphate, with a basal dose of 120 kg N and 60 kg K2O ha-1. The experiment followed a randomized, two-factorial design. Maize plants (variety "Azam") were inoculated with the PSB strains. Consequently, combining Bacillus PIS7 with 80 kg P2O5 ha-1 significantly increased both fresh and dry maize biomass (35.33 and 24.56 g pot-1) compared to the control (unspiked soil). Phosphorus bioavailability (7.07 and 5.14 mg kg-1) significantly improved with 80 kg ha-1 phosphorus and Bacillus PIS7. Heavy metal concentrations in soil (Cd, Pb, Cr, and Ni) decreased significantly at 80 kg ha-1 phosphorus and Bacillus PIS7, and Cd concentrations in plants decreased to 3.31 mg kg-1 with Bacillus DUM7 and to 2.96, 0.42, and 1.33 mg kg-1 with Bacillus PIS7. The application of PSB strains and phosphorus fertilizer reduced heavy metal concentrations. Notably, 80 kg P2O5 ha-1 with Bacillus PIS7 showed the best performance. Phosphorus uptake increased significantly (0.106 mg pot-1), while heavy metal uptake (Cd, Pb, and Cr) decreased linearly with increasing phosphorus levels and PSB strains. Bacillus PIS7 with 80 kg P2O5 ha-1 had the lowest heavy metal translocation, doing better than the control and other PSB strains (Bacillus CUM6 and Bacillus DUM7). Soil characteristics indicated increased organic matter content (0.73%) and decreased pH (7.61) and electrical conductivity (0.17 dSm-1) with applied phosphorus and PSB strains, suggesting enhanced phosphorus bioavailability and reduced heavy metal concentrations. In conclusion, adding 80 kg P2O5 ha-1 with Bacillus PIS7 helps plants grow better when they are stressed by heavy metals and keeps Cd, Pb, Cr, and Ni stabilized in the soil.