Fluometuron Sorption and Degradation in Cores of Silt Loam Soil from Different Tillage and Cover Crop Systems

Fluometuron [N,N-dimethyl- N'-[3-(trifluoromethyl)phenyl] urea], a herbicide used on cotton (Gossypium hirsutum), is fairly mobile in soil. This study quantified effects of tillage (conventional-till [CT] and no-till [NT]) and cover crop (native vegetation, hairy vetch [ Vicia villosa] and wheat [Triticum aestivum]) on fluometuron sorption and degradation in intact cores (10-cm diam. by approximately 7.5 cm long) of Gigger (fine-silty, mixed, thermic Typic Fragiudalfs) soil. Batch sorption was well described by Freundlich isotherms. Sorption generally increased with soil organic C and was greater in NT, than in CT, 0- to 3-cm soil. No-till soil had more earthworms and arthropods. suggesting greater physical heterogeneity and potential physical nonequilibrium during transport. Tracer elution from slightly unsaturated (-0.1 bar) cores did not show preferential flow. First-order degradation rate constants were obtained by fitting a convective-dispersive/ diffusive transport model to effluent fluometuron concentrations from seven simulated rains. Degradation was faster in NT, than in CT, native soil (k d s = 0.09 and 0.04 d -1 ). Tillage did not affect degradation in the vetch (k d s = 0.07 d -1 ) or wheat (k d s = 0.8 d -1 ) soils. Degradation in CT and NT vetch cores was faster than in an earlier batch study. To the extent intact cores better represent field conditions than homogeneous soil, degradation in cores may more accurately reflect degradation in the field.