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Degradation of Metolachlor in Tobacco Field Soil

The extensive use of metolachlor to control weeds in tobacco fields in China has aroused concern about its environmental fate. The aim of this study was to investigate the degradation and residue fate of metolachlor in tobacco field soil (silt loam) under laboratory and field conditions. In laboratory experiments, metolachlor in bulk soil exhibited fast degradation in a temperature range from 10 to 35°C and a soil moisture level of 20–80%, with half-lives (T1/2) from 66.7 to 28.8 days. The degradation rate of metolachlor decreased as the application dose increased. Owing to higher microbial populations and enzymatic activities, metolachlor rapidly dissipated in rhizosphere soil as compared to bulk soil. Field persistence of metolachlor was evaluated in the same soil during the tobacco (Nicotiana tabacum K326) growing season in 2012 and 2013. The dissipation of metolachlor followed the first-order kinetics and its T1/2 values were 11.7–13.5 days in soil and 9.0–9.6 days in green tobacco leaves, respectively. At harvest time, the residual levels of metolachlor in soil and green tobacco leaves were in the range of 0.626–1.623 and 0.083-0.481 mg kg−1, respectively. These findings might have practical implications for the fate of metolachlor residue in tobacco fields. Environmental factors, especially temperature and moisture, should be considered in combination with the appropriate application dose of metolachlor for achieving satisfactory weed-control efficacy, reducing runoff, and minimizing effects on environmental quality.