Leaching and degradation of triasulfuron, metsulfuron-methyl, and chlorsulfuron in alkaline soil profiles under field conditions

A field study was conducted on an alkaline cropping soil (plot size 4 m by 1 m), representative of cereal growing regions in southern Australia, to examine the degradation and leaching behaviour of triasulfuron, metsulfuron-methyl, and chlorsulfuron (150 g a.i./ha), along with a non-reactive tracer (KBr, 50 kg/ha), under 2 water treatments ('rainfall' and 'rainfall + irrigation'). The results showed that the distributions of water content, Br-, and the 3 herbicides were influenced by the amount of rainfall/irrigation received, and the biological activity as well as the physico-chemical properties of the soil within the profile. The largest concentrations of Br- were detected in the 10-20 cm depth under both treatments. No additional peak of Br- was observed in the plots which received only rainfall; however, there was another peak at 100 cm depth after a cumulative rainfall and irrigation of 158 mm. The spatial distribution of Br- in the plots was attributed to preferential flow through cracks or the presence of 'mobile' and 'immobile' water, which gave rise to bimodal distribution in the profile. Metsulfuron-methyl showed the greatest mobility, followed by chlorsulfuron and triasulfuron under both the treatments, with metsulfuron-methyl being detected at 120 cm depth under rainfall + irrigation. A bimodal distribution was also observed for the 3 herbicides, but the location of peaks was dependent on the solute. Rates of degradation of the herbicides could be described reasonably well with first-order reaction kinetics (r(2) = 0.72-0.91). The half-lives for triasulfuron, metsulfuron-methyl, and chlorsulfuron were 44, 45, and 32 days, respectively, under rainfall (69 mm), and 46, 59, and 48 days under rainfall + irrigation (158 mm).