Structure-toxicity relationship of chloroacetanilide herbicides: relative impact on soil microorganisms.

The research was carried out to ascertain the effect of three chloroacetanilide herbicides, alachlor, butachlor and pretilachlor on soil microbial biomass growth and activity. Laboratory experiments were performed in a silty clay loam soil to relate changes of soil enzymatic activity to the herbicide persistence under laboratory condition up to 42 days at three application rates. The results showed that all the three herbicides caused enhancement of dehydrogenase activity. Higher concentrations of herbicide resulted in enhancement of the enzymatic activity. In addition, a similar trend was observed in β-glucosidase and acid phosphatase activity, although urease activity decreased upon incubation for 42 days as compared with initial soil incubation values. Based on the extent of impact for dehydrogenase activity in soil, the order was pretilachlor>alachlor>butachlor; whereas in case of urease activity, the order changed to pretilachlor>butachlor>alachlor. The soil half-lives of alachlor, butachlor and pretilachlor respectively, were 9.3, 12.7 and 7.3 days, which could be accounted for in terms of their respective chemical structures, as well as variable adsorption, degradation, differential effects of the agents on soil microbes. Soil management practices and the differing physicochemical properties of the herbicides may contribute to their rates of decay in soil.