Some physicochemical and environmental factors affecting transformation rates and sorption of the herbicide metamitron in soil

In addition to the molecular structure of a pesticide, environmental conditions may influence its persistence through their effect on the growth and activity of pesticide-degrading micro-organisms. As a result, transformation rates may decrease rapidly when a compound is leached into subsoil. Metamitron sorption isotherms were determined and incubation series were set up for a sandy loam soil, simulating single and combination effects that occur during transport of metamitron into subsoils. K-OC values increased with increasing depth from 185 to 700 litre kg(-1). A combination of conditions that are unfavourable for microbial activity, such as low temperature (5 degrees C), low concentrations (0.5 mg kg(-1)) and a large sorbed fraction (K-OC = 700) resulted in half-lives of over one year. Oxygen inhibition decreased the transformation rate of metamitron from 0.058 to 0.019 day(-1). In order of significance, the transformation of metamitron appears to be a function of temperature, oxygen availability and sorption to organic carbon. Increasing doses did not change transformation rates significantly, although different transformation pathways were observed.