Persistence and biodegradation of selected organophosphorus insecticides in flooded versus non-flooded soils

Summary The persistence of parathion, methyl parathion and fenitrothion in five tropical soils of varying physicochemical characteristics was compared under flooded and non-flooded conditions. The degradation of all the three insecticides was more rapid under flooded conditions than under non-flooded conditions in four out of five soils. Degradation of these insecticides proceeded by hydrolysis under non-flooded conditions and essentially by nitro group reduction and to a minor extent by hydrolysis under flooded conditions. Kinetic analysis indicated that degradation of the three insecticides followed a first-order reaction irrespective of the soil and water regime. The degradation of these organophosphorus insecticides was accelerated after repeated applications to flooded alluvial soil. Nitro group reduction was the major pathway of degradation for all the three insecticides after the first addition while the rate of hydrolysis increased after each successive addition.