Rapid biodegradation of diuron and other phenylurea herbicides by a soil bacterium

Abstract

An investigation into intra-field spatial variability in rates of degradation of phenylurea herbicides revealed the sudden onset of rapid diuron degradation in one laboratory-incubated soil sample after 43 d. Subsequent applications of diuron to this soil were degraded very rapidly (DT 50<24 h). Through liquid enrichment culture, a bacterial isolate, that degraded diuron rapidly with no requirement for supplementary carbon and nitrogen sources, was obtained from this `enhanced' soil. Addition of this isolate at 9.3×10 6 cfu g −1 to soil containing `aged' diuron residues resulted in rapid diuron degradation in both pre-fumigated and non-fumigated soil. However, at an inoculum density of 9.3×10 3 cfu g −1, degradation only occurred in fumigated soil over the 16 d duration of the experiment. The isolate degraded a range of phenylureas in liquid culture, with rate of degradation in the order linuron>diuron>monolinuron≫metoxuron⋙isoproturon. However, the N′-monomethyl and demethylated derivatives of diuron were not degraded. Degradation of diuron and linuron resulted in accumulation of a single metabolite, which had the same retention time as 3,4-dichloroaniline. This, together with the lack of degradation of the monomethyl and demethylated derivatives of diuron, suggests that the molecules had been split directly at the carbonyl group, rather than undergoing step-wise demethylation. Isoproturon degradation by this isolate was greatly accelerated in the presence of additional carbon and nitrogen sources, resulting in the production of a metabolite with the same retention time as 4-isopropylaniline, again suggesting that degradation proceeded via carbonyl group cleavage.