Degradation of S-metolachlor in soil as affected by environmental factors

Abstract

S-Metolachlor is a promising alternative to metolachlor; however, there are concerns about its environmental fate following soil application. The present study was conducted to investigate the effects of several environmental factors on the degradation rate of S-metolachlor in soil. The rate of S-metolachlor degradation in soil under different conditions was determined by extracting and detecting residual levels over a 60 d incubation period. Degradation of S-metolachlor in the five tested soils followed the first-order kinetics, yielding the half-lives (T 1/2 ) varying from 26.3 to 40.1 d, which was significantly controlled by soil organic matter (OM) content. The degradation rate of S-metolachlor in 1﹟soil increased with increasing temperature. The first-order half-lives ranged from 23.7 d at 35 °C to 64.8 d at 10 °C. Soil moisture content influenced the rate of herbicide degradation in the following order: 80 > 60 > 40 > 20% for 1﹟soil. The degradation rate of S-metolachlor in sterilized soil was 3.3 times slower than in corresponding unsterilized soil, indicating that microbial degradation was the predominant contributors to dissipation of S-metolachlor in soil. The degradation rate of S-metolachlor decreased as the initial concentration increased. These findings might have practical implications for the environmental fate of S-metolachlor in the agricultural soil. Environmental factors, especially soil OM, temperature and moisture, should be considered in combination with the minimum effective application rate of S-metolachlor for achieving satisfactory weed-control efficacy, reducing herbicide runoff, and minimizing effects on environmental quality.