Fate of the herbicide bentazon in rice plant-grown lysimeters over four consecutive cultivation years

When the herbicide benzene‐ring‐14C‐bentazon was applied onto two 0.25 m2 lysimeters by the conventional method(1.6 kg a.i./ha) in Korea and rice plants were grown over four consecutive years, 3.16(lysimeter I) and 17.27%(lysimeter II) of the originally applied radioactivities leached in total up to the 167th week after the application. The 14C activities translocated into rice straw in the first year ranged from 1.4 to 2.2% of the originally applied amount, reducing remarkably in the second year, and those into brown rice grain were below the MRL of 0.1 ppm bentazon set by FAO/VHO. The 14C activities remaining in the 0∼10, 10∼20, 20∼30, and 30∼40 cm soil layer were approximately 14.07, 16.45, 27.53, and 21.28%(lysimeter I) and 4.37, 10.06, 10.45, and 9.34%(lysimeter II), of the originally applied 14C, respectively, after 2 years, amounting to a total of 79.33 and 34.22%, respectively. After 4 years the residual radiocarbon in the 0—60 cm soil layer was reduced, indicating that 26.60(lysimeter I) and 24.38%(lysimeter II) of the originally applied benzene‐ring‐14C still persisted in the soil. The total 14C activities found in leachates, mineralization to 14CO2, rice plants, and soil layers suggested that more than half the original benzene‐ring‐14C of the bentazon could be lost probably through mineralization to 14CO2 The amounts of the 14CO2 evolved from the soil surfaces of the lysimeters up to the 20th week and the microbial activities in the lysimeter soils strongly suggested that mineralization to 14CO2 was the principal factor for the loss. The translocation of the applied benzene‐ring‐14C‐bentazon into rice shoots reached the maximum concentration after 3 weeks.