| The effects of acetanilide herbicide mefenacet [2-(l,3-benzothiazol-2-yloxy) -N-methylacetanilide] on soil microbial communities, soil respiration and soil enzymes were studied by using paddy soil samples with a short-term treatment in this thesis. Biological community assessments included culturable bacteria (traditional technigues), soil enzyme activities, changes in community structure (denaturing gradient gel electrophoresis (DGGE) analysis) and substrate utilization potential of microbial communities (a colour development assay). The responses of SOD, CAT and ATPase in Sphingbobaceterium multivorum, Escherichia. coli and Bacillus subtilis to mefenacet were also observed in this work. Moreover, several mefenacet-degrading bacteria were isolated and identified, and metabolic pathway of mefenacet was inferred. The results will be valueable to build up alert index systems in mefenacet-polluted paddy soil, environmental quality evaluation and bioremediation of mefenacet-polluted paddy soil. The main results are reported as follows:1. It was found that the influences of mefenacet varied on culturable microorganism populations in paddy rice soil based on the culture-dependent counting methods. The results showed that aerobic bacteria were stimulated, however, the numbers of fungi and actinomycetes were inhibited after application of mefenacet. Anaerobic fermentative bacteria, hydrogen-producing acetogenic bacteria and methane-producing bacteria were stimulated by mefenacet at lower concentrations while depressed at higher concentrations. The number of denitrifying bacteria increased significantly for 4 weeks after application of mefenacet. Mefenacet also stimulated the number of anaerobic nitrogen-fixing bacteria at the 2nd week of mefenacet application but inhibited finally. Soil respiration was enhanced at the beginning but reduced slightly afterwards. The activity of dehydrogenase and phosphorylase was increased, however, urease was strongly depressed and theactivity of proteinase took on the slight inhibition within the whole period of experiment. The activity of catalase was little affected by mefenacet. According to the t-test, Significant differences (P<0.05) existed in the activities of urease among the treated soils with various concentrations of mefenacet, therefore, the change of urease activity can be suggested as an index to indicate the contamination degree of paddy soil by mefenacet.2. DGGE was applied to analyses 16S rDNA of prokaryotes from the unpolluted and mefenacet-polluted soils. The degree of similarity between the 16S rDNA profiles of the communities was quantified by numerically analyzing the DGGE band patterns. Similarity dendrograms showed that the microbial community structures of the mefenacet-treated and nontreated soils were different. But mefenacet had no significantly toxic to the soil microbial populations and microbial diversity of polluted soil did not decrease remarkably. These demonstrated that mefenacet with 0.133ug g-1 dry soil had no significant effect to the structure of microbial communities in paddy soil. These results were coincident to those conclusions based on the traditional techniques. The supplement of Sphingobacterium multivorum Yl, a bacterium efficiently degrading the mefenacet, might decrease the negative influence of mefenacet, consequently, stimulated the other species of bacteria to grow and increased the diversity of the microbial community in the mefenacet-polluted paddy soil. This work is a new attempt to apply the Sphingobacterium multivorum Yl to remediate the mefenacet-polluted environment. The numerical analysis of DGGE patterns provides an analytical tool to study the diversity of microbial communities of mefenacet-treated soils. DGGE technique is a valuable method to detect changes of the microbial community from the herbicide-polluted soil.3. The Biolog technique was used to further understand the microbial ecology of the paddy soil and to validate the hypothesis that its substrate utilization patterns would change afte...
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