Degradation of the sulfonylurea herbicides chlorsulfuron and triasulfuron in a high-organic-matter volcanic soil

The degradation rates of two sulfonylurea herbicides, chlorsulfuron and tri asulfuron, were determined at two application rates, 15 and 30 g a.i. ha(-1 ), in a sandy loam soil of volcanic origin under controlled environment and field conditions. Residues were measured using a modified gas chromatograp hic (gc) determination method. Both herbicides degraded rapidly in the acid ic soil (pH 5.7) with high organic matter levels (7.3% o.m,), generally acc ording to first-order rate kinetics. The respective half-lives ranged from 22 to 38 d for chlorsulfuron and from 31 to 44 d for triasulfuron under fiv e controlled temperature/soil moisture regimens, ranging from 10 to 30 degr ees C and between 40% and 80% maximum water-holding capacity. Half-lives in the field were considerably shorter (13 d for chlorsulfuron and 12-13 d fo r triasulfuron). The degradation rates of the herbicides were influenced mo re by soil temperature than by soil moisture content, Bioassays using white mustard (Sinapis alba L.) and forage sorghum [Sorghum bicolor (L.) Moench] were also used to determine the persistence of phytotoxic residues of both herbicides in the field, and the results showed that the effects of chlors ulfuron disappeared within 8 weeks. Triasulfuron residues disappeared withi n 9 and 14 weeks for the 15 and 30 g a.i. ha(-1) rates respectively.