FATE AND BEHAVIOR OF TRIASULFURON, METSULFURON-METHYL, AND CHLORSULFURON IN THE AUSTRALIAN SOIL ENVIRONMENT - A REVIEW

The sulfonylurea herbicides comprise a group of compounds designed to control broad-leaved weeds and some grasses in a variety of crops. The herbicides have become popular because of their low application rates (10-40 g/ha), law mammalian toxicity, and unprecedented herbicidal ac tivity. We present a review of the fate and behaviour of these herbici des in soils with particular reference to alkaline soils of Australia. The review shows that the low application rates of sulfonylurea herbi cides continue to present an analytical challenge, although in recent years a number of new methods capable of detecting them at very low co ncentrations have been developed. A range of analytical methods is ava ilable, including high performance liquid chromatography, gas chromato graphy, immunoassay, and bioassay. However, analytical sensitivity req uired to detect trace levels of these herbicides continues to pose pro blems in routine detection of herbicide residues in soils. The review reveals that there are no reports of studies of the behaviour of sulfo nylureas in soils with pH >8.2. This is of particular significance to Australian conditions because a number of Australian soils are even mo re alkaline, and the pH(water) in subsoils can be as high as 10.2. Sor ption of sulfonylureas is pH-dependent and has a strong negative corre lation with pH. At pH >8.0 sorption is very low. In acid soils, howeve r, sorption of chlorsulfuron, metsulfuron-methyl, and triasulfuron is strongly influenced by the soil temperature, clay content, and, partic ularly, organic matter content. The principal modes of degradation of the herbicides are acid hydrolysis and microbial degradation with the latter being the only major pathway in alkaline soils. Hydrolysis of t he sulfonylureas is more rapid under acidic conditions (pH 4-7), and t he data suggest that hydrolysis is likely to be very slow in alkaline soils. Data from other countries suggest that the half-life of chlorsu lfuron increases exponentially with pH, and that it is also influenced by variations in the temperature and water content of the soil. Being acidic in nature, the herbicide molecules become anionic at high pH a nd can move to a considerable depth in the soil profile by leaching. M ovement of the sulfonylureas in soil is largely influenced by organic matter content and soil pH and the reviewed data show that sulfonylure as have substantial leaching potential in the sandy alkaline soils of Australia. This is likely to result in increased persistence in alkali ne subsoils lacking in organic matter and biological activity. Compute r models to predict the persistence and movement of the sulfonylureas are available; however, additional input parameters are required to pr edict accurately the behaviour of specific herbicides in alkaline soil s under Australian conditions. Since new herbicides with chemistry sim ilar to existing sulfonylureas are increasingly likely to be available for use, there is a need to develop comprehensive understanding of th eir fate, behaviour, and impact on Australian cropping and ecological systems.