The effect of the herbicide atrazine on the diazotrophic microorganisms associated with rhizosphere soil and roots of a corn cultivated on a clay-loam soil in a greenhouse, Ustropet Fluvaquentic from Saldana (Tolima) was evaluated. 61% of soil isolates and 83% of root isolates presented characteristic cultural growth of whitish pellicle under the surface of the semisolid NFb-malic acid medium. The soil population of diazotrophic microorganisms and the nitrogenase activity were not significantly

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... different between atrazine treatments. The highest values were obtained 45 days after the highest dose ofatrazine (3.0 Kg/ha). Corn roots without treatment had greater values throughout the time tested, but those were not significantly different between the treatments. As a conclusion, there were not effects on the population and the nitrogenase activity of the diazotrophic microorganisms up to 90 days after the application of atrazine.