Degradation of Herbicides Atrazine and Bentazone Applied Alone and in Combination in Soils1

The application of a mixture of bentazone (3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H)-one-2,2-dioxide) and atrazine (6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine) is a practical approach to enhance the herbicidal effect. Laboratory incubation experiments were performed to study the degradation of bentazone and atrazine applied in combination and individually in maize rhizosphere and non-rhizosphere soils. After a lag phase, the degradation of each individual herbicide in the non-autoclaved soil could be adequately described using a first-order kinetic equation. During a 30-d incubation, in the autoclaved rhizosphere soil, bentazone and atrazine did not noticeably degrade, but in the non-autoclaved soil, they rapidly degraded in both non-rhizosphere and rhizosphere soils with half-lives of 19.9 and 20.2 d for bentazone and 29.1 and 25.7 d for atrazine, respectively. The rhizosphere effect significantly enhanced the degradation of atrazine, but had no significant effect on bentazone. These results indicated that biological degradation accounted for the degradation of both herbicides in the soil. When compared with the degradation of the herbicide applied alone, the degradation rates of the herbicides applied in combination in the soils were lower and the lag phase increased. With the addition of a surfactant, Tween-20, a reduced lag phase of degradation was observed for both herbicides applied in combination. The degradation rate of bentazone accelerated, whereas that of atrazine remained nearly unchanged. Thus, when these two herbicides were used simultaneously, their persistence in the soil was generally prolonged, and the environmental contamination potential increased.