Parameters Influencing Biotransformation Rates of Phenylurea Herbicides by Soil Microorganisms

The biotransformation of phenylurea herbicides and some of their most prominent degradates by different soil microorganisms was investigated. Most soil fungi, but only some soil bacteria, were able to reduce the herbicide concentration in the culture medium. Biotransformation rates of the compounds were different. Identification of the products demonstrated that N-demethylation was the most common initial biotransformation step. Cleavage of the urea bridge was only observed by bacteria. Sorption of the compounds to the mycelium or cells was different among the microorganisms investigated, but studies with14C-labeled compounds revealed that the herbicides were not bound to any organic material such as cell walls or membranes. Mineralization of the phenyl ring did not occur. None of the microorganisms were able to use the herbicides as the sole carbon source, indicating that biotransformation was a cometabolic process. In some cases, the different biotransformation rates correlated with the lipophilicity of the compounds. Herbicides with high octanol-water partition coefficients were then transformed faster than compounds with a high water solubility. Some microorganisms, however, showed rapid biotransformation of certain, but different, herbicides. This indicates either additional biotransformation pathways or substrate specificities.