SORPTION AND DEGRADATION OF BENTAZON IN CONVENTIONAL-TILL AND NO-TILLDUNDEE SOIL

Herbicides applied postemergence, such as bentazon [3-(1-methyl-ethyl) -( -ethyl)-(1H)-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide], reach th e soil surface due to incomplete interception by the plant canopy or f oliar washoff. Thus, potential off-site transport is influenced by sor ption equilibrium/kinetics and degradation. This study addressed the e ffects of tillage practice on the sorption and degradation of C-14-lab eled bentazon using Dundee silt loam soil (fine-silty, mixed, thermic, Aeric Ochraqualf) taken from the conventional-till (Cn and no-till (N T) treatments of a tillage experiment without bentazon exposure for mo re than 3 yr. Data were generated for surface and subsurface samples. The kinetics of bentazon sorption in CT surface soil were rapid, with apparent equilibrium achieved in 1 h. Furthermore, the extent of sorpt ion was low and the equilibrium distribution of solution and sorbed be ntazon could be described using a linear model. Differences in soil or ganic matter between tillage treatments and with depth were not reflec ted in sorption behavior. However, degradation was more rapid in the C T surface soil than in the corresponding NT soil and more rapid in sur face than subsurface soil. In all cases, degradation proceeded with li ttle mineralization (<3%) over the 22-d duration of these experiments, however, the amount of unextractable C-14 (80:20, CH3OH:0.01 M CaCl2 extractant) increased with time. Analysis of the extractable fraction by HPLC gave no evidence of metabolites. Bentazon degradation was appr oximated using simple first-order kinetics, however, the data were bes t described using two-compartment models. Development of a bound C-14 fraction, despite limited bentazon sorption and no evidence for extrac table metabolites, suggested that metabolic intermediates exhibit high sorption affinity.