Effects of repeated metolachlor applications on its persistence in field soil and degradation kinetics in mixed microbial cultures

The persistence of metolachlor, a soil-applied herbicide, was studied under field conditions involving repeated herbicide applications. The test field received four applications of metolachlor over an 8-month period, which in cluded two cropping seasons. There was a trend for more rapid rates of degr adation with increasing numbers of previous treatments, with fifty percent dissipation time (DT50) of metolachlor declining from 18 days in the first spray to 2.5 days in the fourth spray. An effort was made to isolate the mi crobial population which had become acclimated to the herbicide from this f ield soil. A fungal community isolated from this soil showed the capacity t o degrade up to 99.6% of the metolachlor within a span of 20 days. The bact erial community isolated could also degrade up to 81.5% of the metolachlor. Hence, this study clearly indicated that repeated applications of metolach lor to soil resulted in the generation of an adapted microbial population w ith an enhanced ability to degrade the applied herbicide.