The evaluation of pesticide-mineralising microorganisms to clean-up contaminated soils was studied with the widely applied and easily detectable compound atrazine, which is rapidly mineralised by several microorganisms including the Pseudomonas sp. strain Yaya 6. The rate of atrazine removal was proportional to the water content of the soil and the amount of bacteria added to the soil. In soil slurry, 6 mg atrazine kg soil-1 was eliminated within 1 day after application of 0.3 g dry weight inoculant biomass kg soil-1 and within 5 days when 0.003 g kg soil-1 was used. In partially saturated soil (60% of the maximal water-holding capacity) 15 mg atrazine kg soil-1 was used. In unsaturated soil, about 60% [U-ring-14C] atrazine was converted to 14CO2 within 14 days. Atrazine was very efficiently removed by the inoculant biomass, not only in soil that was freshly contaminated but also in soil aged with atrazine for up to 260 days. The bacteria exposed to atrazine in unsaturated sterile soil were still active after starvation period of 240 days: 15 mg newly added atrazine kg soil-1 was eliminated within 5 days.

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The herbicidal effect of atrazine on sensitive plant species was studied in soils inoculated with bacteria capable of mineralizing atrazine. Nasturtium officinale and Solanum nigrum plants died within 15 days after sowing in soil containing atrazine incorporated at 4 mg/kg. Normal growth of N. officinale was obtained when 5 mg/kg atrazine-mineralizing bacteria was mixed into the soil containing atrazine, prior to seeding. Atrazine concentrations in soil declined by 90% within 5 days as a result of the atrazine degradation by the bacteria added to the soil. Normal growth of S. nigrum plants was observed in soils receiving only 0.1 mg/kg atrazine-degrading bacteria. The efficacy of atrazine in the presence of atrazine-degrading bacteria was also tested on N. officinale and Agrostis tenuis in greenhouse trials under simulated field conditions using a commercial atrazine formulation and the herbicide sprayed onto soil in open containers. Here, too, the microbial herbicide breakdown was rapid and the effect of the herbicide on the indicator plants was drastically diminished when the soil was kept under conditions favorable for the atrazine-degrading bacteria.

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