Effects of Sulfonyl Urea Herbicides on Soil Microbial Community.

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The sulfonylurea herbicides were discovered in 1975 and immediately set a new standard for chemical weed control. The herbicides include their exceptionally low application rates (often a factor of 10-100 times lower than conventional herbicides), favorable environmental and toxicological properties, and compatibility with the trend towards post emergence weed control. Bensulfuron methyl was the first sulfonylurea herbicide commercialized for use in rice. In this study, side-effects of the chemical on microflora were assessed according to the recommendations stated above.In the early 1970's regulatory authorities in Europe were considering the introduction of the requirement for tests of pesticides effects on soil microflora. Several workshops in Europe published recommendations for the testing. Also in Japan, a draft recommendation for the testing of side-effects of pesticides on soil microflora was proposed by Yamamoto et al.in 1993.As part of our evaluation of the proposed method, we studied the effects of bensulfuron methyl on 'ammonification', 'nitrification' and 'microbial biomass' using the proposed test method. The herbicide, applied at the recommended rate to paddy field soil, has no significant effect on 'ammonification and 'microbialbiomass'. The chemical, however, appears to have a temporary adverse effect on 'nitrification', and a complete recovery was evident at 28 days.For the evaluation of the data derived by those method, annual fluctuation of the microbial activities under field condition were investigated. The fluctuation were much more than those by the pesticide treatment.