Factors affecting the hydrolytic degradation of chlorpyrifos in soil

The abiotic hydrolysis of the organophosphorus insecticide chlorpyrifos was examined in 37 different soils, which were chosen to represent a wide variety of physicochemical characteristics (e.g., pH 3.8-8.5). Samples of soil were sterilized via gamma-irradiation, treated with [C-14]chlorpyrifos at 10 mu g/g, and incubated under standardized conditions (25 degrees C, field moisture capacity, darkness) for up to 4 months. Chlorpyrifos hydrolysis proceeded at a slow rate (<0.008 day(-1)) in acidic soils (pH less than or equal to 7). In alkaline soils, however, hydrolytic rate constants varied greatly (0.004-0.063 day-l). Corresponding hydrolytic half-lives for acidic and alkaline soils ranged from 92 to 341 and 11 to 200 days, respectively. Correlation analyses indicated that soil pH was the independent variable displaying the strongest association with hydrolytic rate constant (r = 0.55), but multiple regression models based on combinations of this parameter with other soil properties, including phosphatase enzyme activities, did not offer strongly predictive models for explaining the variability in kinetics observed (best fit r(2) = 0.59). Incubation of chlorpyrifos with both sterile and nonsterile soils revealed that although both microbial and hydrolytic mechanisms contributed to chlorpyrifos degradation in all soils, there were clearly soils in which hydrolysis constituted the major route of degradation. Chlorpyrifos hydrolysis was greatly accelerated under low moisture conditions, both in acidic and alkaline soils. Additional experiments in several soils that displayed rapid chlorpyrifos hydrolysis at 10 mu g/g provided evidence that the hydrolytic reaction was inhibited at higher concentration (1000 mu g/g). Results highlight the importance but also the complex nature of the hydrolytic breakdown of chlorpyrifos in soil. Under certain conditions (e.g., some alkaline soils, air-dry soils) hydrolysis may be the driving factor modulating chlorpyrifos persistence.