Fate and behaviour 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), low mammalian toxicity, and unprecedented herbicidal activity. We present a review of the fate and behaviour of these herbicides in soils with particular reference to alkaline soils of Australia. The review shows that the low application rates of sulfonylurea herbicides continue to present an analytical challenge, although in recent years a number of new methods capable of detecting them at very low concentrations have been developed. A range of analytical methods is available, including high performance liquid chromatography, gas chromatography, immunoassay, and bioassay. However, analytical sensitivity required to detect trace levels of these herbicides continues to pose problems in routine detection of herbicide residues in soils. The review reveals that there are no reports of studies of the behaviour of sulfonylureas in soils with pH >8·2. This is of particular significance to Australian conditions because a number of Australian soils are even more alkaline, and the pH(water) in subsoils can be as high as 10· 2. Sorption of sulfonylureas is pH-dependent and has a strong negative correlation with pH. At pH >8·0 sorption is very low. In acid soils, however, sorption of chlorsulfuron, metsulfuron-methyl, and triasulfuron is strongly influenced by the soil temperature, clay content, and, particularly, 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 the sulfonylureas is more rapid under acidic conditions (pH 4{7), and the data suggest that hydrolysis is likely to be very slow in alkaline soils. Data from other countries suggest that the half-life of chlorsulfur on 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 and can move to a considerable depth in the soil profile by leaching. Movement of the sulfonylureas in soil is largely influenced by organic matter content and soil pH and the reviewed data show that sulfonylureas have substantial leaching potential in the sandy alkaline soils of Australia. This is likely to result in increased persistence in alkaline subsoils lacking in organic matter and biological activity. Computer models to predict the persistence and movement of the sulfonylureas are available; however, additional input parameters are required to predict accurately the behaviour of specific herbicides in alkaline soils under Australian conditions. Since new herbicides with chemistry similar to existing sulfonylureas are increasingly likely to be available for use, there is a need to develop comprehensive understanding of their fate, behaviour, and impact on Australian cropping and ecological systems.