Degradation of Fenamiphos Sulfoxide and Fenamiphos Sulfone in Soil with a History of Continuous Applications of Fenamiphos

Mineralization rates of fenamiphos sulfoxide (FSO), total-toxic-residue [TTR, FSO+fenamiphos sulfone (FSO2)] disappearance rates for FSO, disappearance rates of FSO2, and metabolites in surface and subsurface soil samples collected from a turfgrass site (fairway) were determined. This site had been treated with fenamiphos annually or biannually for 20 years, and enhanced degradation of fenamiphos TTR was observed. Both the mineralization of FSO and the disappearance of FSO TTR, as well as the disappearance of FSO2 in soil samples collected from the fairway were much more rapid than in soil samples collected from a nearby site (rough) that had no previous history of fenamiphos application. Both FSO and FSO2 were degraded more rapidly in surface soil samples than in subsurface samples. The degradation pathway of FSO in the fairway soil samples was different from the rough samples. Hydrolysis to FSO phenol (FSO-OH) was the initial route of degradation of FSO in the fairway samples, whereas hydrolysis to FSO-OH and oxidation to FSO2 were the initial routes of degradation in the rough samples.