Soil-Water Partitioning Behavior of Oxyfluorfen Under California Rice Field Conditions

The herbicide oxyfluorfen [OXY; 2-chloro-1-(3-ethoxy-4-nitrophenoxy)-4-(trifluoromethyl)benzene] has recently surged in interest among rice farmers with the development of OXY-tolerant rice and its demonstrated effectiveness against problematic rice weeds in California. Not currently registered for use with rice, its fate in rice fields is poorly understood. Using a batch equilibrium method, we characterize the soil-water partitioning behavior of OXY under simulated California rice field conditions. Sorption data imply strong, primarily concentration-independent binding correlated with soil organic carbon (log[K-oc] 4.79-5.19; N 0.87-1.08) across all soil, temperature, and salinity treatments. Temperature significantly enhanced binding affinity for sorption and desorption processes (P < 0.01). Bound OXY was poorly desorbed (9.3 to 27.0% desorption) from rice soils and exhibited significant sorption hysteresis (HI > 0) in all treatments. These results indicate that OXY will predominantly remain in soil and will be resistant to release into water in California rice fields.