Influence of insecticides alone and in combination with fungicides on enzyme activities in soils

Abstract

Two insecticides, monocrotophos and chlorpyrifos alone and in combination with two fungicides, mancozeb and carbendazim, respectively, were assessed for their effects on the activities of arylamidase (as glucose formed from sinigrin), dehydrogenase (in terms of triphenyl formazan formed from triphenyl tetrazolium chloride) and myrosinase (as β-naphthylamine formed from l-leucine β-naphthylamide) in vertisol and laterite soils collected from a fallow groundnut (Arachis hypogaea L.) field. The influence of selected pesticides, alone and in combination on enzyme activities was concentration dependent; the activities increased with increasing concentration of the pesticides up to 2.5 kg ha−1, whereas application of monocrotophos alone showed maximum enzyme activities up to 5.0 kg ha−1, in both soils. However, higher concentrations (7.5 and 10 kg ha−1) of the pesticides were either innocuous or toxic to the enzyme activities. The significant stimulation in the activities of arylamidase, dehydrogenase and myrosinase, was associated with 2.5 kg ha−1. The maximum stimulation in arylamidase and myrosinase activity was observed at 20-day incubation, and the enzyme activities decreased gradually at 30 and 40 days of incubation. Significant increase in dehydrogenase activity was observed at 21-day incubation, and the enzyme activity decreased gradually at 28 and 35 days of incubation in both vertisol and laterite soils. The results of the present study thus, clearly, indicate that application of the insecticides alone or in combination with fungicides, in cultivation of groundnut, at field application rates improved the activities of arylamidase, dehydrogenase and myrosinase in soils.