Effect of Benomyl and Paraquat on Bacterial Population Growth of Azospirillum irakense

All members of the genus Azospirillum are soil inhabitant and known to be incredibly efficient nitrogen fixers. High populations of these bacteria in the soil are important to increase plant growth. Various factors can affect the natural balance of the soil ecosystem, and reduce soil populations of beneficial bacteria, which indirectly reduces the growth and beauty of the plant. One of the major factors affecting soil microbial populations is agricultural pesticides. This study was conducted to evaluate the effect of five different concentrations of benomyl and paraquat (250-5000 mg/L) at various temperatures (25-45 Ċ) on indigenous bacterial strains of Azospirillum irakense as a nitrogen fixing bacteria in laboratory and glasshouse conditions. Experiment was performed in a factorial design with four replications. The results showed that both pesticides at a concentration of 5000 mg/L are completely lethal to the bacterium and caused complete cessation of growth. At lower concentrations, both pesticides could significantly reduce bacterial population growth but it didn’t stop bacterial growth. In the glasshouse conditions, both pesticides at any concentration could not remove bacteria from soil but significantly reduced the population of bacterium in the soil. A strong correlation was observed between the temperature and inhibitory effects of pesticides and the rate of inhibition was significantly increased with increasing temperature.