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

Abstract

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 (r2 = 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).