TRANSFORMATION AND DEGRADATION OF FENAMIPHOS NEMATICIDE AND ITS METABOLITES IN SOILS

Fenamiphos is an important nematicide-insecticide and commonly used in horticultural crops and turfs in Australia. We studied the transforma tion/degradation of fenamiphos under controlled conditions, in surface and subsurface soils from the Swan Coastal Plain of Western Australia . In the sandy surface soil, fenamiphos (F-en) was rapidly oxidised to its sulfoxide (FenSO) analogue. Further oxidation of FenSO to sulfone (FenSO2), however, was found to be very slow, resulting in an accumul ation of FenSO. Little accumulation of FenSO2 occurred during the stud y period (139 days). The time taken for 50% loss of the total residue of fenamiphos (F-en+FenSO+FenSO2) was found to be approximately 50 day s in the surface soil and about 140 days in the subsurface soil. Simul ations using the LEACHM model showed that the rate of transformation o f F-en to FenSO in the surface soil (k(1) = 0.5/day) was 100 times fas ter than that of FenSO to FenSO2 (k(2) = 0.005/day). In the subsurface soil, the difference between the 2 oxidation steps was much smaller ( 4-fold). The conversion of F-en to FenSO was much faster in the surfac e soil (k(1) = 0.5/day) than the subsurface soil (k(1) = 0.02/day). Th e observed differences in transformation behaviour of F-en between the surface and subsurface layers of soil appear to be associated with th e differences in the microbial biomass and the organic matter contents of the soils. The slower transformation of F-en in the subsurface soi ls can have major implications on its potential for groundwater contam ination in vulnerable areas, such as the Swan Coastal Plain of Western Australia.