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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 γ-irradiation, treated with [ 14 C]chlorpyrifos at 10 µg/ g, and incubated under standardized conditions (25 °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 e 7). In alkaline soils, however, hydrolytic rate constants varied greatly (0.004-0.063 day -1 ). 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 µg/g provided evidence that the hydrolytic reaction was inhibited at higher concentration (1000 µg/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.

Republication or reproduction of this report or its storage and/or dissemination by electronic means is permittedSynopsis: Pesticide use is an important component of agricultural and non-agricultural pest control in tropical areas. However, the fate of pesticides in tropical soils is not as well understood as that for soils from temperate regions. Tropical soils defy easy generalizations, but they are typically very old soils characterized by year-round uniformity of temperature regime. Although only a few studies have directly compared pesticide fate in tropical and temperate soils, there is no evidence that pesticides degrade more slowly under tropical conditions. Laboratory studies in which soils have been held under standardized conditions reveal that pesticide degradation rate and pathway are comparable between tropical and temperate soils. However, field investigations of tropical pesticide soil fate indicate that dissipation occurs more rapidly, in some cases much more rapidly, than for pesticides used under similar temperate conditions. The most prominent mechanisms for this acceleration in pesticide dissipation appear to be related to the effect of tropical climates, and would include increased volatility and enhanced chemical and microbial degradation rates on an annualized basis.

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