Fate and behaviour of flupyrsulfuron-methyl in soil and aquatic systems

Abstract

The environmental fate of [ 14 C]flupyrsulfuron-methyl, a sulfonylurea herbicide, was investigated in soil and aquatic systems. The major degradative pathways in both systems were contraction of the sulfonylurea bridge followed by intramolecular rearrangement (at pH > 7) or sulfonylurea bridge hydrolysis (at pH < 7). Hydrolysis was a first-order reaction and was pH- and temperature-dependent. Flupyrsulfuron-methyl was degraded rapidly at 25°C in pH 5, 7 and 9 sterile buffers with half-lives of 44, 12 and 0.42 days, respectively. At pH 7 and 9, sulfonyl bridge contraction and rearrangement was the major degradative mechanism; at pH 5 the sulfonylurea bridge was also hydrolysed. Unique photodegradation products were formed at pH 5 and pH 7 but, in general, hydrolysis was faster than photolysis at all three pH values. Aerobic aquatic metabolism involved biphasic degradation of the herbicide (DT 50 3-6 days), degradation being faster in the aerobic aquatic systems than in sterile buffers. Degradation in aerobic soils was rapid, both in the laboratory (DT 50 8-26 days) and in the field (DT 50 6-11 days, DT 90 35-123 days). In laboratory studies the rate of degradation in soil reduced with decreasing temperature (rate at 10°C half that at 20°C) but was unaffected by soil water content (50% vs 70% maximum water holding capacity). The compound was degraded in flooded anaerobic soils (DT 50 33 days). Flupyrsulfuron-methyl was weakly absorbed to soils, there being a linear relationship between adsorption and soil organic carbon content. Following application of [ 14 C]flupyrsulfuron-methyl to bare field soil the radioactivity moved little, with very little radioactivity found in soil below 60 cm from the surface.