Biologically-induced hydrolysis of parathion in soil: Kinetics and modelling

The hydrolysis of the organophosphorus insecticide, parathion (O,O-diethyl O-p-nitrophenyl phosphorothioate) in a silty loam sierozem soil (Gilat, Israel) occurred primarily through microbial action. Parathion (labelled with ¹⁴C in the alkyl chain) was applied at levels of 10–160 μg dry soil^?1 to soil remoistened to 20% and incubated at 25°C for 8 days. Bacterial numbers increased to a maximum 4–5 days after application of parathion and the increase was proportional to the concentration of parathion added. The rate of hydrolysis of parathion per μg applied was independent of the concentration of parathion. A model developed to predict the relationship between parathion concentration, microbial numbers and hydrolysis kinetics was in general agreement with the data experimentally obtained. The course of decomposition of successive additions of parathion, determined experimentally and predicted by the model, was characterized by rapid hydrolysis of parathion and successive increases in bacterial numbers. A portion of the ¹⁴C applied in these experiments was strongly absorbed by the soil and was not used by the soil microorganisms during the incubation period tested.