Phytotoxicity and persistence of flucarbazone-sodium in soil

Flucarbazone-sodium, a new herbicide, exhibits high bioactivity at low concentrations. To elucidate potential carryover and crop injury, the behavior of flucarbazone in six Western Canadian soils was studied in the laboratory. A sensitive bioassay was developed for the detection of flucarbazone. Of five crops tested, oriental mustard showed the highest degree of root and shoot inhibition from the presence of flucarbazone in soil. Flucarbazone concentrations as low as 1 μμg kg⁻⁻¹ were detected by the mustard root inhibition method. This bioassay was used to examine phytotoxicity and persistence of flucarbazone. Phytotoxicity was related to soil organic carbon content. Concentrations corresponding to 50%% inhibition (I₅₀ values) were estimated after fitting the data to a log-logistic model. I₅₀ estimates ranged from 6.0 to 27.5 μμg kg⁻⁻¹ for soils containing 1.1 to 4.3%% organic carbon, respectively, and were correlated (R == 0.979) with percent organic carbon in the investigated soils. Persistence of flucarbazone was examined in soils incubated at 25 C and moisture content of 85%% field capacity (FC). Flucarbazone dissipation followed first-order kinetics in one soil, but a two-compartment model provided the best fit for dissipation in the other soils. Half-lives (t₀.₅), calculated from dissipation curves in each soil, ranged from 6 to 110 d. Half-lives were correlated (R == 0.776) with soil organic carbon. Flucarbazone dissipation was more rapid in soils containing less organic carbon. Flucarbazone was more persistent in drier soil; t₀.₅ was 11 d in soil at 85%% FC and was 25 d in soil at 50%% FC. Soil characteristics and environmental conditions will affect the degree of plant injury to sensitive crops the year after flucarbazone application.Nomenclature: Flucarbazone-sodium; oriental mustard, Brassica juncea L. ‘‘AC Vulcan’’.