Effect of monocrotophos and quinalphos on soil population and nitrogen-fixing activity of Azospirillum sp.

The effects of monocrotophos and quinalphos on population and nitrogen-fixing activity of Azospirillum sp. in four agricultural soils were determined in a laboratory study. Concentrations of the two insecticides up to a 5 kg ha-1 level were either stimulatory or innocuous to the population of Azospirillum in the soils. Four successive applications of the insecticides to soils resulted in a significant increase in the population density. Cultures of Azospirillum sp., isolated from insecticide-treated soils, exhibited greater nitrogen-fixing activity. Three consecutive subculturings of the isolates from insecticide-treated soils had no effect on their nitrogen-fixing activity.