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Among 15 soils with different cropping practices, seven which had an history of repeated atrazine applications showed accelerated degradation of this herbicide. By contrast, grassland or agricultural soils with no recorded atrazine application, at least for the last three years, had a low degradation potential. No direct relation was found between the rate of atrazine mineralisation and the size of the microbial biomass. In adapted soils, the amounts of extractable residues were lowered and the very high percentages of radioactivity from [ring-14 C]atrazine recovered as [14 C]carbon dioxide demonstrated that N-dealkylation and deamidation were the only processes for micro-organisms to derive carbon and energy for heterotrophic growth. Kinetics of microbial 14 C accumulation revealed that atrazine ring carbon could be incorporated by direct oxidative condensation with structural components of the bacterial or fungal cell whereas side-chain carbon was preferentially used for biosynthesis of new protoplasmic cell material, as con®rmed by the high turnover rate of radiolabelled microbial components. From the determination of the Michaelis±Menten parameters, V m and K m in the presence of different selective biocides, it was possible to conclude that fungi were probably less active in atrazine degradation than bacteria and that over years the microbial atrazine-degrading community showed an increased ef®ciency.

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