Effect of HCH and fenvalerate on growth and distribution of microorganisms in relation to persistence of the insecticides in the rhizosphere soils of wetland rice

摘要

Insecticides are frequently applied to crop fields to combat insect pests for better crop production. After application, a large portion of insecticides accumulates in the top soil layer (0-10 cm) where maximum microbiological activities occur. As microorganisms are scavengers in soil and possess physiological variability, they degrade a great variety of chemical substances including the insecticides. Some insecticides are degraded by soil microorganisms to derive energy and nutrients (Bhuyan et al. 1993) with the resultant increase in the population of insecticide-degrading organisms. On the other hand, there are some insecticides that exert deleterious effect on microorganisms (Moorman 1989; Martinez-Toledo et al. 1992). Therefore, no definite conclusion can be made on the effect of insecticides on microorganisms and their activities in soil since different groups of insecticides exhibit manifold variations in toxicity (Simon-Sylvestre and Fournier 1979). The present experiment has been conducted to investigate the effect of HCH (1, 2, 3, 4, 5, 6 - hexachlorocyclohexane) and fenvalerate [(RS)-cyano-(3-phenoxyphenyl)(RS) methyl- 4 - chloro - alpha - (1- methylethyl) benzeneacetate] at their recommended field rates, on the population and distribution of bacteria, actinomycetes and fungi as well as the persistence of the insecticides in the rhizosphere soils of wetland rice.