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

Abstract

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.