Degradation of chlorsulfuron and triasulfuron in alkaline soils under laboratory conditions

The degradation of chlorsulfuron and triasulfuron was investigated in alkal ine soils (pH 7.1-9.4) spiked at 40 mu g a.i. kg(-1) under laboratory condi tions at 25 degrees C and a moisture content corresponding to 70% field cap acity (-33 kPa), using high-performance liquid chromatography, Degradation data for the two herbicides did not follow first-order kinetics, and observ ed DT50 values in surface soils ranged from 19 to 42 days and from 3 to 24 days for chlorsulfuron and triasulfuron respectively. Disappearance of both chlorsulfuron. and triasulfuron was faster in non-sterile than in sterile soil, demonstrating the importance of microbes in the breakdown process. Th e persistence of chlorsulfuron increased with increasing depth, which can b e attributed to the decline in the microbial populations down the profile. The DT50 value for chlorsulfuron at 30-40 cm depth was nearly four times hi gher than that in the tea-soil. The results obtained show that persistence of these herbicides in alkaline surface soils at 25 degrees C and at a mois ture content of 70% field capacity is similar to those reported in other Eu ropean and North American soils. The study shows that if these herbicides a re contained in surface soil layers, the risk of residue carry-over under s outhern Australian conditions is small. However, the rate of their degradat ion in alkaline subsoils is very slow, and under conditions conducive to le aching their prolonged persistence in the soil profile is possible.