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No description available.

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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.

In soils of high matric potential, low oxygen conditions often develop that favour disease development by many soil-borne plant pathogens. The introduction of a third party, or biocontrol agent, to suppress disease development would require that the agent remains metabolically active under such conditions. In the rhizosphere, plant roots not only supply carbon as an electron donor but cause a localised lowering of oxygen concentrations, conditions favourable for nitrate respiration. The effect of the addition of nitrate on the activity of antagonistic strains of Bacillus subtilis, Pseudomonas fluorescens and P. corrugata was studied in vitro on agar plates, but no significant (P 0.05) quantitative effect was observed . A sealed plate method, using aerobic, anoxic, and anaerobic conditions with 0, 1, 2.5, 5, 10 and 100mM nitrate concentrations was investigated using the B. subtilis strains. This assay tested the activity of antifungal volatiles (AFV) produced by the bacteria. The results indicated that nitrate led to an increased AFV production and/or activity against fungal pathogens under anoxic conditions with nitrate at or above 10 mM. To investigate root colonisation and the establishment of biocontrol colonies in the rhizosphere, lux marking of the biocontrol bacterial strains was undertaken. The transformed bioluminescent B. subtilis strains lost the ability to antagonise the test fungi on agar plates. This loss of antagonism appeared to be due to luciferase utilising metabolites involved in antibiosis and producing a low, but significantly different (P ?0.05) from background and parental strains level of luminescence. The effects of nitrate on a soil based biocontrol system were studied in greenhouse trials. Unfortunately, disease failed to develop, and the effects of the addition of nitrate could not be assessed. The potential involvement of nitrate in maintaining certain biocontrol aspects under conditions that favour pathogen attack seems likely from in vitro based studies. The removal of the antagonistic phenotype, from lux marked B. subtilis strains, raised questions as to the suitability of luciferase for use in this system and highlighted the need for careful monitoring and screening of genetically modified organisms.

Atrazine has been one of the most successful herbicides used both in agriculture and in urban situations. Its use has now been banned in U.K. agriculture. Atrazine applied to agricultural soils has been shown to leach down the profile with residues remaining in the soil up to 9 years after initial application. Residues are frequently found in sub-soils and aquifers world wide. In this study, systems were developed that enabled sampling and incubation of intact sub-soils cores that contained a sub-soil atmosphere. The sub-soil represents the last zone of significant potential degradation of xenobiotics, such as atrazine, as leaching into deeper soils and ground waters occurs. Conditions prevailing in these soils are different in terms of soil atmosphere, structure and activities of the soil microbial community. Laboratory studies were carried out which investigated the rate of atrazine mineralisation in intact sub-soil cores. Soil cores were aerated with either laboratory air or CO2-rich air generated through soil microbial activity from within a sealed sub-soil air reservoir. Results showed that atrazine mineralisation occurred at significantly higher rates in sub-soil cores aerated with sub-soil air compared to the rates in sub-soil cores aerated with laboratory air. Studies comparing mineralisation rates in intact sub-soil cores, incubated under sub-soil air or labroatory air, and soil biometers, containing sieved, mixed sub-soil, showed that higher rates of atrazine mineralisation occurred in the biometer studies than occurred in intact sub-soil cores. Similar studies using intact top-soil cores showed higher rates of mineralisation. Investigations carried out using intact sub-soil cores amended with a range of glucose concentrations, showed that no difference occurred in glucose mineralisation rates between soil cores aerated under sub-soil air and under laboratory air.