Petroleum hydrocarbon concentration as the primary driver of soil bacterial community dynamics during mycorrhizal fungi and rhizobacteria assisted phytoremediation

Petroleum hydrocarbon (PHC) contamination poses widespread environmental risks in Canada, necessitating effective remediation strategies in the boreal ecozone due to the effects of the cold climate on soil nutrient cycling and weathering processes. This study analyzed rhizobacterial biodiversity and composition during phytoremediation of contaminated field soil (25,700 mg/kg total petroleum hydrocarbons - TPH) collected from a Canadian boreal site where PHCs had weathered in place for 25+ years. Additionally, a 1:1 contaminated: background (diluted) soil (12,600 mg/kg TPH), was used in conjunction with the field-contaminated soil, to investigate the effects on rhizobacterial community composition. In two-year greenhouse experiments, soils were planted with native or naturalized plant species and inoculated with i) arbuscular mycorrhizal fungi (AMF), ii) the biosurfactant-producing and plant-growth-promoting rhizobacterium (PGPR) - Bacillus subtilis ATCC 21332, or iii) both AMF and B. subtilis . PHC contamination exerted a dominant influence on rhizobacterial community composition. Alpha and beta diversity analyses revealed that neither plant species nor microbial inoculants significantly altered bacterial diversity or community structure beyond the overriding effect of PHC concentration. Proteobacteria , Actinobacteriota , Acidobacteriota , and Chloroflexi dominated across all soils, with hydrocarbon-degrading genera such as KCM-B-112 and Sphingomonas highly abundant in the 25,700 mg/kg TPH soil. Exogenously introduced B. subtilis ATCC 21332 established only in the background soil, suggesting competitive dominance of indigenous PHC-degraders. These findings underscore the magnitude of PHC concentration as the primary driver of rhizobacterial dynamics during remediation. • PHC concentration significantly influenced microbial community during remediation • Higher (33%) bacterial abundance was observed in 25,700 mg/kg TPH soil than control • Sphingomonas and KCM-B-112 are abundant indigenous PHC-degraders in boreal soils • Bacillus subtilis ATCC 21332 inoculant was outcompeted in 12,600 mg/kg TPH soil