Degradation of isoproturon, propyzamide and alachlor in soil with constant and variable incubation conditions

Isoproturon, propyzamide and alachlor were incubated in a sandy clay loam soil either with constant conditions (10 °C or 20 °C with soil moisture at 10% or 20% by weight) or with variable temperature at constant soil moisture, variable soil moisture at constant temperature, or with both temperature and soil moisture varying over time.Rates of degradation in the constant condition incubations approximated to ®rst-order kinetics in all treatments, and the ®rstorder half-lives derived from the data were used to parameterize a simple mathematical model that simulated degradation when temperature and/or moisture varied on a daily basis.Calculations from the model gave excellent agreement with observed dissipation patterns of isoproturon, gave acceptable predictions of the behaviour of alachlor but were not precise with propyzamide.When the soil was exposed to wetting and drying cycles, propyzamide degradation was initially slower than that predicted, but the rate of loss increased after about 30±35 days, indicating microbial adaptation to degrade the compound under these experimental conditions.The implications of the results for simulation of pesticide degradation in ®eld soils are discussed.