Epsilon Open Archive

Abstract

In many areas of the world undesirable dispersal of organic pesticides into untargeted environments has occurred, and is continuing, raising serious concerns about their potential impact on the environment and human health. The main agents ultimately responsible for the degradation of these substances are microorganisms, so it is important to understand their effects on microbial metabolism and the complex interactive effects of soil parameters, microbial activities, pesticide availability and pesticide degradation dynamics. The studies presented in this thesis elucidate some such interactions involving three pesticides: 2,6-dichlorobenzamide, 2,6-dichlorobenzonitrile and 2,4-dichlorophenol. The respiration kinetics of soil microorganisms in the presence and absence of pesticides were measured and used to derive information on catabolic and anabolic components of the microbial responses. Furthermore, the effects of charge density and pH of surfaces of soil particles were also investigated by manipulating soil solution pH, the pH of organic matter surfaces and the mineral composition (and hence surface charge density) of test soils. The pH and charge density of particle surfaces were found to strongly influence soil microbial processes and the fate and behaviour of pesticides in soils. In addition, the effects of sorption on the availability and degradability of glyphosate, one of the most commonly used pesticides in the world, were examined using respiration measurements in combination with attenuated total reflectance-Fourier transform infra-red spectroscopy analysis. Raising the pH of an acidic soil with high organic matter content was found to reduce the toxic effects of 2,4-dichlorophenol on anabolic microbial processes, but the effects of its toxicity towards catabolic processes were less pronounced. 2,6-dichlorobenzamide had greater negative effects on microbial metabolic processes at neutral than at acid pH. Increasing the surface charge density and raising the surface pH of the soil organic matter reduced the negative effects of 2,6-dichlorobenzonitrile on microorganisms. Soil surface charge density and pH therefore interactively influence the effects of the pesticides on microbial metabolism. Further experiments showed that sorbed glyphosate can be used by the microorganisms as a source of C, N and possibly P.