Effect of insecticides on nitrogen mineralization and nitrifying organisms in a black vertisol soil

Three insecticides viz., monocrotophos, quinalphos and cypermethrin all at 0, 5, 10 and 25 μg g⁻¹ were applied either singly or in combination to a black vertisol soil to study their effects on ammonification and nitrification, and on the population of nitrifiers in a laboratory. All three insecticides individually enhanced the rates of ammonification and nitrification and the proliferation of nitrifiers in the soil even at the highest level (25 μg g⁻¹) of insecticides used in the study. Monocrotophos or quinalphos in combination with cypermethrin interacted significantly at all levels to yield additive, synergistic and antagonistic responses towards the processes of ammonification and nitrification, and the population of nitrifiers. Antagonistic interaction was more pronounced when the two (monocrotophos or quinalphos + cypermethrin) insecticides were present together at the highest level (25 + 25 μg g⁻¹) in the soil, whereas synergistic or additive response occurred at lower levels (5 and 10 μg g⁻¹). These insecticide interactions were persistent in the soil even after 30 days of application.