The Degradation and Bioactivity of Metolachlor in the Soil

In bioassays, rice (Oryza sativa L.) recovery from metolachlor (2- chloro - N- (2- ethyl-6-methylphenyl)-N- (2- methoxy- 1-methylethyl)acetamide) injury tended to be slower in flooded rice, but was not significantly different from the recovery rate in a nonflooded rice. In soils treated with 1 ppm (w/w) metolachlor and incubated in constant- temperature chambers, the half-life of metolachlor was shorter at 40 C than at 30 C. The degradation rate of metol- achlor was not significantly correlated with declining moisture potentials in the range of -30 to -80 kPa. The CO2 evolu- tion from metolachlor- treated soil was negatively corre- lated with incubation time and positively correlated to declining moisture levels. In a field study, metolachlor, as determined by bioassay, was mobile in a Taloka silt loam soil profile. After being incorporated to 7.5 cm, it became evenly distributed in the top 15 cm of the soil profile within 18 days. Metolachlor adsorption was positively correlated with clay and organic carbon content. Additional index words. Bioassay, rice, leaching, microbial activity.