graphy, capillary electrophoresis and mass spectrometry.
The test chemical, [ring-U-14 C]paraquat (speciยฎc activity, 2.0 GBq mmol ร1 ) was supplied by the Jealott's Hill radiochemistry department. The radiochemical was diluted with non-radiolabelled paraquat (99.7% pure) prior to use.
3 Results
Paraquat was extensively metabolised with the rapid production of [ 14 C]carbon dioxide. Typical mineralisation (to CO 2 ) values were around 50% for both soil extracts and typical [ 14 C]carbon dioxide evolution plots are shown in Fig 2.
Chromatographic analysis of the residual radiochemical in solution at the end of each incubation showed almost identical metabolite proยฎles between the different micro-organisms. A major metabolite, comprising b85% of the radioactivity remaining in the incubated solution, together with a minor metabolite (5%), and a metabolite which was incorporated into the degrading microbial cultures (10%), were characterised. The major metabolite was identiยฎed as oxalic acid (6), and no paraquat remained in the solutions.
4 DISCUSSION AND CONCLUSIONS
This work has shown that bioavailable paraquat can be rapidly and completely degraded by micro-organisms present in soil. Complete degradation of paraquat takes less than two or three weeks, indicating that the half-life of bioavailable paraquat is considerably shorter than this. The metabolism was so fast that only small fragments of naturally occurring acids, and carbon dioxide as the ultimate mineralisation product, were seen.
These laboratory results correlate well with longterm ยฎeld trial data, taking into consideration the bioavailability of paraquat in the soil environment. 7