Degradation and side effects of three sulfonylurea herbicides in soil

Soil degradation and side effects of triasulfuron [3-(6-methoxy-4-methyl-1,3,5-triazin-2-yl)-1-(2-(2-chloroethoxy) phenylsulfonyl)-urea], primisulfuron methyl [2-(4,6-bis(difluoromethoxy)-pyrimidin-2-ylcarbamoylsulfamoyl)benzoic acid], and rimsulfuron [1-(4,6-dimethoxypyrimidin-2-yl)-3-(3-ethylsulfonyl-2-pyridylsulfonyl)- urea] were investigated under laboratory conditions in a pH 6.5 sandy loam soil. In microbially active and sterile soil, the degradation of the three sulfonylureas followed first-order kinetics and was independent of the herbicide soil concentration up to 5 mg a.i. kg-1 of soil. In microbially active soil the mean half-lives of triasulfuron (26.2 d), primisulfuron methyl (30.2 d) and rimsulfuron (7.5 d) were, respectively, 4.0, 3.7, and 1.9 times lower than those in sterile soil, thus confirming the decisive contribution of biological processes in soil degradation of the three sulfonylureas. At the concentrations of 0.2 and 5 mg a.i. kg-1 of soil, no detrimental effects of the three sulfonylureas on soil respiration and dehydrogenase activity were detected. Indeed, soil amended with the three sulfonylureas at 5 mg a.i. kg-1 of soil showed a transient increase of both respiration and dehydrogenase activity. Results showed that triasulfuron, primisulfuron methyl, and rimsulfuron have probably no effects on soil microbial activities at the concentrations used in agricultural practice, which are approximately 1/10 of the lowest concentration tested in the present study.