The distribution and breakdown of paraquat in the soil

Abstract

The microbial degredation of paraquat dichloride by Lipomyces starkeyi was investigated by both, spectrophotometric determination and c14-labelled carbon dioxide emission. 90% of the paraquat, when used in liquid culture as the sole nitrogen source was decomposed in 72 hours. When incorporated as an unessential component of the medium, paraquat took longer to be broken down (7-11 days) depending on the time allowed for Lipomyces starkeyi establisment before addition of herbicide. The breakdown of paraquat, in liquid cultures containing a range of soils varying in organic/inorganic matter ratio, was investigated. Breakdown was seen to occur within the first 96 hours, after the commencement of incubation, in cultures containing high organic matter soils. Adsorption isotherms showed that the high organic matter soils had a greater total adsorption of paraquat in solution concentrations in excess of 250 ppm. A high organic matter soil was divided by ultra-sonic disintegration into it's organic and inorganic components. Paraquat breakdown was seen to occur in cultures containing the former but not the latter. The passage of c14-labelled paraquat from the organic components through a dialysis membrane was observed and demonstrated to be due largely to the presence of inorganic material outside the membrane. A system was proposed, whereby the decomposition of paraquat in the soil by Lipomyces starkeyi occured during the stage when the herbicide was weakly adsorbed onto the organic soil components. After a period of time (less then 96 hours in one instance) readsorption onto the inorganic soil component rendered the paraquat unavailable to microbial degradation.