Long-term fate of the herbicide mefenacet in a rice-grown lysimeter over a period of 6 consecutive years

To show the long-term fate of the herbicide mefenacet in rice paddies, [aniline-14C]mefenacet was applied to a lysimeter of loamy soil with a depth of 1 m, and rice plants were cultivated for 6 consecutive years according to the conventional methods in Korea. Mineralization of [14C]mefenacet to 14CO2 and volatilization from the soil surface were 12.01 and 0.02 %, respectively, of the originally applied amount during the first 23 weeks following application. Throughout the 6-year period, the total proportion of 14C-radioactivity that leached through the lysimeter soil was 0.778 % of the original radioactivity. The total 14C-radioactivity absorbed and translocated by rice plants throughout the 6 years was 2.46 % of the applied 14C. Measurement of the 14C-radioactivity distributed in each part of the rice plant (the straw, the ears without rice grain, the chaff, and the brown rice grain) indicated that the amount of 14C in straw was 25.87 times higher than that in brown rice grain over the 6 years. The 14C-radioactivity remaining in the soil layer after 6 years was 44.58 % of that applied, 91.45 % of which was distributed in the upper 0–20-cm layer. These results strongly indicate that mefenacet moved downward very slowly and more than half of the herbicide applied was released into the air, mainly through mineralization to CO2 in soil during the experimental period.