Factors influencing the enhanced microbial degradation of pesticides in soil

A recent concern in the widespread use of soil-applied pesticides is the phenomenon of enhanced degradation by soil microorganisms. This study investigated selected factors influencing the enhanced microbial degradation of pesticides. Because hydrolysis is a significant initial step in the degradation of pesticides, the role of hydrolysis metabolites in the induction of enhanced degradation was studied. Salicylic acid, p-nitrophenol, and 2,4-dichlorophenol conditioned the soil for enhanced degradation of their parent compounds, isofenphos, parathion, and 2,4-dichlorophenoxyacetic acid, respectively. Repeated applications of 3,5,6-trichloro-2-pyridinol resulted in increased persistence of its parent compound, chlorpyrifos. Pretreatment of soil with hydrolysis metabolites of carbofuran, diazinon, fonofos, and 2,4,5-T did not have any effect on the degradation of their respective parent compounds;The properties of pesticide metabolites such as toxicity and availability to microorganisms may influence their potential to condition soils for the enhanced degradation of their parent pesticides. Mobility studies using a soil thin-layer chromatography technique indicated that for a pesticide to be susceptible to enhanced degradation, it need not necessarily be very mobile. Because some metabolites are capable of inducing or inhibiting the enhanced degradation process the mobility/availability of hydrolysis metabolites is of practical significance. Soil pH significantly affected mobility of most of the chemicals studied, with increased mobility in soils with higher pH\u27s. Water solubility, by itself, was not a good predictor of mobility and structural similarity does not seem to be an indicator of mobility. The Microtox system was employed to determine the toxicity of pesticides and their metabolites to bacteria. Hydrolysis metabolites of pesticides susceptible to enhanced degradation (isofenphos, diazinon, and carbofuran) recorded higher EC[subscript]50 values, indicating their low toxicity to bacteria;The effect of manuring on the degradation of pesticides was investigated. Manuring for seven years resulted in increased persistence of pesticides or their pesticidal metabolites. This study found no evidence that amending soil with manure encourages the development of enhanced microbial degradation of pesticides