Response of the bacteria and fungi of two soils to the sulfonylurea herbicide cinosulfuron

Changes in aerobic bacteria and autotrophic nitrifier numbers, and in respi ration and nitrification in two soils treated with cinosulfuron at 42 (fiel d rate) and 4200 mug/kg were studied after 1 and 4 weeks of incubation unde r laboratory conditions. Only nitrification at 1 week was slightly inhibite d by the cinosulfuron treatment, even at the field rate. In vitro toxicity tests carried out in agar media on representative aerobic bacteria, fungi a nd Azotobacter strains isolated from the two soils, as well as on nine coll ection soil bacteria, showed that only a very high cinosulfuron concentrati on (100 mg/l) can have negative effects on the growth of a limited number o f soil heterotrophic microorganisms, under conditions similar to those of s oil environment. The absence of three branched-chain amino acids increased bacterial sensitivity, thus showing the importance of the chemical conditio ns and suggesting acetolactate synthase enzyme blockage as the toxicity mec hanism. It is concluded that cinosulfuron has a negative effect on only a f ew aspects of the microbial community in soil ecosystems, even at concentra tions higher that those currently in use.