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 ofAzospirillum sp.in four agricultural soils were determined in a laboratory study.Concentrationsof the two insecticides up to a 5 kg ha-1level were either stimulatory or innocuous to the popula-tion of Azospirillum in the soils.Four successive applications of the insecticides to soils resultedin a significant increase in the population density.Cultures of Azospirillum sp.,isolated frominsecticide-treated soils,exhibited greater nitrogen-fixing activity.Three consecutive subcultur-ings of the isolates from insecticide-treated soils had no effect on their nitrogen-fixingactivity.1989 Academic Press,Inc.