Influences Of Herbicide Quinclorac On Microecosystem In Flooded Paddy Soils And Characterization Of Quinclorac-degrading Bacteria

In this thesis, the effects of herbicide quinclorac (3,7-dichloro-8-quinoline-carboxylic) on soil micro ecosystem were studied in paddy soil samples with a short-term treatment of quinclorac at different concentrations. Biological community assessments including culturable bacteria (plate counts), soil enzyme activities, changes in community structure (denaturing gradient gel electrophoresis (DGGE) analysis) and substrate utilization potential of microbial communities (a colour development assay) were used. The activity response of the antioxidant enzymes superoxide dismutase, catalase, ATP enzyme activities of Escherichia coli (G), Bacillus subtilis (G+) and Burkholderia cepacia WZ1 (G") following exposure to quinclorac was also investigated. Moreover, several bacteria degrading quinclorac were isolated, identified and phylogenetically analyzed. The metabolic pathway of quinclorac by strain WZ1 was detected and inferred. The results will be valuable to build up alert index systems in quinclorac-contaminated paddy soil, environmental quality evaluation and bioremediation of quinclorac-polluted paddy soil. The main results of this study are as follows:1. The influences of quinclorac on the cultural microorganisms in paddy soil showed that the bacteria differed markedly in their response to quinclorac. The concentration of quinclorac applied is an important factor affecting populations of various microorganisms, except those characteristics of quinclorac itself. The results obtained demonstrated that although there were significant differences in the aerobic heterotrophic bacteria, AFB and DNB between soils supplemented with quinclorac and non-quinclorac at the early stage after application, but none of which were persistent. The numbers of fungi and DNB were increased in soil samples treated by lower than concentration of 1.33g g-1 dried soil, while the CFUs of fungi and HPAB were lower in soil samples treated by higher than 1.33g-g-1 dried soil. The population of actinomycetesdeclined in negative proportion to the concentrations of quinclorac applied after 4 days, however, application of quinclorac greatly stimulated the growth of AFB and NFB. The MB numbers in the samples with lower concentrations of quinclorac were nearly equal to that in the control at the end of experiments.The various enzymes differed markedly in their response to quinclorac. Quinclorac inhibited the activities of proteinase, hydrogen peroxidase, phosphorylase, and urease. The higher the concentration of quinclorac applied, the more significant the inhibition to these observed activities with a longer time required to recover to the level of the control. However, the inhibition to the activities of proteinase, phosphorylase and urease in soils supplemented with quinclorac were no persistent as opposed to that in soils without quinclorac. Dehydrogenase activity was also sensitive to quinclorac. It declined in three soil samples with concentrations of quinclorac higher than 1 ug g soil to less than 20% of that in the control. However, the highest dehydrogenase activity (up to 3.28-fold) was detected in soils with 2g g soil quinclorac on the 25th day after treatment. Quinclorac had a relatively mild effect on saccharase activity at the concentrations used in this experiment. Soil respiration was stimulated when soil was added at normal field concentrations of quinclorac but inhibited at higher concentrations. Therefore, quinclorac is still relatively safe to the soil ecosystem as applied at a normal concentration (0.67 ug g dried soil) but may have short-term and slight effects on different bacteria and soil enzymes at higher concentrations.2. DGGE method was applied to determine the relative genetic complexity of microbial communities in flooded paddy soil treated with quinclorac. The results obtained showed a significant effect on the development of bacterial populations in soil contaminated with herbicides at the early time. In general, the populations of the same soil sample differed unobviously with increasing incubation time in th...