A comparison of the fate of diflufenican in agricultural sandy soil and gravel used in urban areas

Diflufenican is used in both agricultural and urban areas to control weeds. However, in Europe pesticides are regulated using agricultural soil data only. Urban soils where the top layer is replaced by gravel (e.g. driveways, outdoor tiled areas) can evidently differ from agricultural soils in many biotic and physical properties. In the present study, we compared the degradation, mineralization, sorption and aging of diflufenican between an agricultural sandy soil to a gravel used in urban areas. Both diflufenican and its two main aerobic metabolites were investigated. Diflufenican and the metabolites degraded slower in gravel than in agricultural soil. One of the metabolites, 2-[3-(Trifluoromethyl)phenoxy]nicotinic acid (AE B107137 as identified by EFSA; further abbreviated as AE-B), was formed from the incubation of diflufenican in both soil and gravel, however, showing different formation patterns in the two materials: No accumulation of AE-B was determined in the soil, whereas in gravel, an accumulation of AE-B was determined over the full study period of 150 days. After 150 days, approximately 10% of the applied diflufenican was mineralised in the soil (cumulative), while it was not mineralised in the gravel. Diflufenican showed much stronger sorption to the soil than to the gravel, while the sorption of the metabolites was weaker than diflufenican in both soil and gravel. Within the experimental period, the influence of aging on the fate of diflufenican in soil and gravel is limited (<0.9 and <1.4%, respectively) when compared to the amount of compound still present in the soil. Overall, the results imply shortcomings in the risk assessment procedures requested for the registration of pesticides for urban areas.