Differential effects of the herbicides bensulfuron and cinosulfuron on soil microorganisms

Bensulfuron toxicity on soil microbes was evaluated by the methods used in a previous study on cinosulfuron; the effects of the two sulfonylureas were compared. Cinosulfuron and bensulfuron, at the normal field application ra te and 100 times higher, had no effect on the total number of bacteria and nitrifiers, or on the respiration activity in the soil, but they did decrea se the nitrification activity. In vitro toxicity tests carried out on repre sentative soil microbial strains using bensulfuron at 50 mg l(-1) showed so me inhibition of three of the 17 bacterial strains and strong inhibition of almost all the 12 fungal strains; cinosulfuron had had no effect on any of these strains in the previous study. It is concluded that, compared with c inosulfuron, bensulfuron is potentially more toxic on soil heterotrophic mi croorganisms, but only at very high concentrations that are nearly impossib le to reach with the usual agricultural use of the herbicides. However, aut otrophic nitrifiers were more sensitive to both sulfonylureas than the othe r microorganisms.