Enhanced degradation of atrazine in soils with a history of repeated use.

Residual herbicides are the principal means for achieving good weed control in maize crops in New Zealand. The most frequently used herbicides belong to the triazine and chloroacetamide groups. In recent years many maize farmers have noticed a shorter period of effective weed control from these compounds. As New Zealand was the first country to note enhanced degradation of EPTC + antidote some 30 years ago, we investigated if a similar phenomenon exists for atrazine, the most commonly used compound in maize. Its degradation rates were measured in seven soils, six with a long history of atrazine use and one that had never been used for cropping. One half of each soil was sterilised by autoclaving and samples fortified with atrazine were incubated at 10, 20 or 30°C for 1–35 days and analysed afterwards through HPLC. In general, microbial degradation appeared to be the major pathway and the rate of breakdown was highest in the two soils that had a very long history of atrazine use. The degradation in sterilised soils followed 1st order kinetics, however, the decay rate in unsterilised soils was up to five times faster and could not be explained by simple exponential decay.