Degradation of chlorsulfuron and triasulfuron in alkaline soils under laboratory conditions

The degradation of chlorsulfuron and triasulfuron was investigated in alkaline soils (pH 7.1–9.4) spiked at 40 μg a.i. kg^–1 under laboratory conditions at 25 °C and a moisture content corresponding to 70% field capacity (–33 kPa), using high-performance liquid chromatography. Degradation data for the two herbicides did not follow first-order kinetics, and observed 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. The persistence of chlorsulfuron increased with increasing depth, which can be 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 higher than that in the top-soil. The results obtained show that persistence of these herbicides in alkaline surface soils at 25 °C and at a moisture content of 70% field capacity is similar to those reported in other European and North American soils. The study shows that if these herbicides are contained in surface soil layers, the risk of residue carry-over under southern Australian conditions is small. However, the rate of their degradation in alkaline subsoils is very slow, and under conditions conducive to leaching their prolonged persistence in the soil profile is possible.