Leveraging PGPR-enriched biochar for enhanced canola growth in heavy metal-contaminated soil

Abstract

The reuse of polluted drainage water for irrigation is increasingly unavoidable in arid and semi-arid regions, yet it poses serious risks due to the accumulation of toxic heavy metals in soils and crops. Although biochar and plant growth-promoting rhizobacteria (PGPR) have individually shown potential to alleviate metal stress, field-scale evidence elucidating their synergistic and mechanistic effects under realistic, combined soil- and irrigation-derived contamination remains limited. This study addresses this gap by evaluating the effectiveness of PGPR-enriched biochar in mitigating lead (Pb), cadmium (Cd), and nickel (Ni) stress in canola (Brassica napus L.) grown under open-field conditions. A naturally contaminated clay soil was continuously irrigated with polluted drainage water from the Kitchener drain (Egypt), creating chronic heavy metal stress. Biochar was applied at 5 and 10 ton ha⁻¹, alone or enriched with defined PGPR consortia composed of Bacillus circulans NCAIM B.02324, Azospirillum brasiliense SARS 1001, and Pseudomonas koreensis MG209738, applied via seed inoculation. The combined application of 10 ton ha⁻¹ biochar with the three-strain consortium (10BC+PGPR3) produced the strongest responses. This treatment (10BC+PGPR3) significantly enhanced soil microbial respiration and key enzyme activities, indicating improved soil biological functioning, while reducing extractable Pb, Cd, and Ni by 55–65% relative to the untreated control. These soil-level improvements translated into marked reductions in metal uptake and translocation to shoots and seeds, alongside enhanced plant water status, membrane stability, and oxidative stress tolerance. Consequently, seed yield and oil content increased by ~ 60% and ~ 90%, respectively. Overall, this study demonstrates that PGPR-enriched biochar acts through coupled soil biochemical and plant physiological mechanisms to immobilize heavy metals and restore crop productivity under real contaminated irrigation scenarios. The findings provide robust field-based evidence supporting this integrated strategy as a practical and sustainable solution for improving soil health, crop performance, and food safety in heavy metal-affected agroecosystems.