Pyrene is one of the polycyclic aromatic hydrocarbons that has carcinogenic, mutagenic and teratogenic effects for living organisms. Landfill leachate is another environmental pollutant that covers a wide range of pollutants, especially heavy metals. The simultaneous presence of two types of pollutants with organic and inorganic structures can increase their toxicity. Methods: In this experimental study, the single and simultaneous effect of Landfill leachate and pyrene on plant growth

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... s and the number of heterotrophic soil bacteria was investigated. The study was conducted for 90 days at concentrations of 150, 300, 500, 750 and 1000 mg/kg-1 of pyrene and percentages 0, 30, 50, 70 and 100% of landfill leachate. Results: The results showed that after 90 days, the highest dry stem and root biomass were obtained in irrigation treatment with pyrene and Landfill leachate (Blank)-non-contaminated municipal water with quantities of 8.2 and 3.5 g, respectively; moreover, the lowest stem and root biomass related to the treatment were observed in the simultaneous presence of 30% leachate and pyrene with a concentration of 300 mg/kg-1 with quantities of 5 and 1.8 g, respectively. Leachate did not produce any biological toxicity at any of the surfaces used, but the use of pyrene at concentrations of 1500 mg/kg-1 and above reduced the number of heterotrophic bacteria. Conclusion: According to the results, the simultaneous presence of the two pollutants, pyrene and leachate, exacerbates the phytotoxicity due to possible interactions between them. Pyrene as a carbon source is decomposed by bacteria at low concentrations, but it inhibits metabolism and growth at high concentrations.