Comparative study on microbial and chemical transformation of eleven sulfonylurea herbicides in soil

The transformation of 11 sulfonylurea herbicides in native and sterile soil was investigated in laboratory studies. Analyses of the compounds in nativ e soil were performed with a method using capillary electrophoresis. Recove ries were higher than 70 % with relative standard deviations lower than 15 %. The limit of determination was 2 mu g/kg soil. Analyses of the compounds in sterile soil were performed with high-performance liquid chromatography . Most sulfonylurea herbicides were mainly transformed by micro-organisms, al though chemical hydrolysis played a significant role. Tribenuron-methyl, ho wever, was purely transformed chemically and thifensulfuron-methyl only by microbial action. Transformation in soil followed first-order reaction kine tics in all cases. Distribution coefficients were low. Correlations with re action rates were not significant and could, therefore, not explain the dif ferences between reaction rates. Transformation of sulfonylurea herbicides by pure cultures of micro-organis ms could be measured only in the culture filtrate of Aspergillus niger. Thi s was not due to biotransformation, bur natural production of citric acid b y the fungus, resulting in decreasing pH followed by chemical hydrolysis of the compounds. Sulfonylureas were also not biotransformed by 72 other, dif ferent soil micro-organisms either from collections or isolated from soil.