Sorption and degradation of bentazon in conventional- and no-till Dundee soil

Herbicides applied postemergence, such as bentazon [3-(1-methyl-ethyl)-(1H)-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide], reach the soil surface due to incomplete interception by the plant canopy or foliar washoff. Thus, potential off-site transport is influenced by sorption equilibrium/kinetics and degradation. This study addressed the effects of tillage practice on the sorption and degradation of 14 C-labeled bentazon using Dundee silt loam soil (fine-silty, mixed, thermic, Aeric Ochraqualf) taken from the conventional-till (CT) and no-till (NT) treatments of a tillage experiment without bentazon exposure for more 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 sorption was low and the equilibrium distribution of solution and sorbed bentazon could be described using a linear model. Differences in soil organic matter between tillage treatments and with depth were not reflected in sorption behavior. However, degradation was more rapid in the CT surface soil than in the corresponding NT soil and more rapid in surface than subsurface soil. In all cases, degradation proceeded with little mineralization (<3%) over the 22-d duration of these experiments, however, the amount of unextractable 14 C (80 :20, CH 3 OH :0.01 M CaCl 2 extractant) increased with time. Analysis of the extractable fraction by HPLC gave no evidence of metabolites. Bentazon degradation was approximated using simple first-order kinetics, however, the data were best described using two-compartment models. Development of a bound 14 C fraction, despite limited bentazon sorption and no evidence for extractable metabolites, suggested that metabolic intermediates exhibit high sorption affinity.