Investigation of organophosphorus pesticide effects on soil bacteria
The toxicity of OP insecticides in aqueous solution and soil pore water was assessed using the lux-marked biosensors E. coli HB101 pUCD607 and P. fluorescens 10586r pUCD607. The effect of OP insecticides on soil microbial processes (respiration and nitrification) and the soil microbial community (viable counts and fatty acid analysis of whole soil extract) was investigated. The relative impact of a formulated insecticide (biomalathion) and the corresponding active ingredient (malathion) was assessed. The lux-biosensors successfully reported on the toxicity of OP insecticides in aqueous bioassays. P. fluorescens was the most sensitive biosensor and biomalathion was the most toxic of the OP insecticides tested. OP insecticides generally had stimulatory effects on soil microbial processes. It was not possible to predict effects on soil microbial processes on the basis of lux-biosensor response, due to the difficulties in interpreting the microbial process tests. For example, it is not clear whether a stimulatory effect should be classed as a toxic impact. Plate counts of OP spiked soil showed an increase in the bacterial population density of both total heterotrophic bacteria and Pseudomonas. However, fatty acid analysis of whole soil extracts failed to detect any effect of OP insecticide treatment on the soil microbial community. This may be because the OP insecticides are having a selective effect on the culturable fraction of the soil microbial community. This thesis attempted to assess the impact of an agrochemical on the soil microbial community using an array of techniques. Both the traditional and modern techniques provided useful information about the effect of OP insecticides on the soil microbial community. Changes due to insecticide treatment were evident. However, further research is needed to assess whether these changes will have a long term impact on soil quality and agricultural sustainability.