MICROBIAL RESPONSES TO LARGE CONCENTRATIONS OF HERBICIDES IN SOIL

To determine the effects of herbicide spills on microbial activity, soil was treated with excessive amounts (10,000 mg/kg) of formulated alachlor alone or as a mixture with formulated atrazine, metolachlor, and trifluralin. Soil was also treated with the same combinations of herbicides at a rate of 10 mg/kg, which is typical of normal farming practices. Simulated spills initially inhibited bacteria, but after 7 d, bacterial numbers had recovered to levels similar to those in untreated controls. Fungal populations were drastically reduced after 1 d and became undetectable after 7 and 21 d of incubation in the mixed herbicide and alachlor-only treatments, respectively. Dehydrogenase activites were severely depressed after 1 d in both high-level herbicide treatments and did not recover after 125 d. Esterase activities were depressed in soils receiving the mixture of herbicides but were generally unaffected in the alachlor-only treatment. Normally applied concentrations did not have any pronounced effect on bacterial and fungal populations or on the activities of soil dehydrogenase and esterase. At 10 mg/kg, all the herbicides degraded by 80 to 90% within one year; there was no difference between degradation of alachlor applied alone or as a herbicide mixture. With high concentrations of herbicides, degradation was negligible after one year of incubation.