Interactions of tillage and soil depth on fluometuron degradation in a Dundee silt loam soil

Fluometuron (N,N-dimethyl-N′-[3-(trifluoromethyl)phenyl]urea) is the most widely used soil applied herbicide in cotton (Gossypium hirsutum) production in the Southeastern United States. Thus, there is a need to understand the persistence of this herbicide under various crop management systems in regards to both herbicide efficiency and environmental fate. The effects of long-term no-tillage practices on fluometuron degradation were studied in a Dundee silt loam under laboratory conditions. Soil was collected at four depths (0–2, 2–5, 5–10, and 10–25 cm) from field plots following 11 years of continuous no-tillage (NT) or conventional tillage (CT). Organic carbon content, microbial biomass, and fluorescein diacetate (FDA)-hydrolytic activity were 48, 106, and 127% greater, respectively, in the upper 2 cm of NT soil compared to CT soil. Microbial biomass and FDA-hydrolytic activity were 39–66 and 34–99% greater in the CT compared to NT soil, respectively, in the 2–5 and 5–10 cm depths. Fluometuron degradation was assessed under laboratory conditions using 14C-ring labeled fluometuron. Fluometuron was degraded more rapidly at depths between 0 and 10 cm of the CT soil relative to the NT soil. Fluometuron degradation was described as first-order (k=0.0754, 0.0608, 0.0273, and 0.0074 per day in the 0–2, 2–5, 5–10, and 10–25 cm depths of CT soils compared to 0.035, 0.0368, 0.0145, and 0.0138 per day in respective depths of NT soil). Although the surface 0–2 cm of the NT soil had greater microbial activity and biomass, higher fluometuron sorption in the surface 0–2 cm NT soil compared to CT (Freundlich coefficient [Kf] NT=7.12 versus CT=1.88) reduced fluometuron solution concentration in NT soil and may have impeded degradation. Interactions between soil biological activity and fluometuron soil solution concentrations, mediated by sorption, may determine the potential for fluometuron degradation in soils under long-term NT practices.