ENHANCED DEGRADATION OF A MIXTURE OF 3 HERBICIDES IN THE RHIZOSPHERE OF A HERBICIDE-TOLERANT PLANT

The rhizosphere of herbicide-tolerant plants may be an important compo nent in biologically remediating pesticide-contaminated soils. A pesti cide-contaminated site at an agrochemical dealership in Iowa was chara cterized, and soil from the site was brought to the laboratory for deg radation experiments. Three major herbicides were identified in the so ils by gas chromatography-atrazine, metolachlor, and trifluralin. Alth ough concentrations of these chemicals were as high as 2 to 3 times fi eld application rates, herbicide-tolerant plants were found growing in the contaminated soil. initial numbers of microorganisms were determi ned in rhizosphere soil from Kochia sp. and in edaphosphere (nonvegeta ted) soil. The rhizosphere soil had an order of magnitude higher micro bial numbers (4.2 x 10(5)) compared with the edaphosphere soil (3.5 x 10(4).) A degradation experiment that did not incorporate vegetation w as carried out by using sterile control soil, Kochia sp. rhizosphere s oil, and edaphosphere soil spiked with a mixture of atrazine, metolach lor, and trifluralin at levels typical of point-source spills. Signifi cantly (p less-than-or-equal-to 0.10) enhanced degradation was observe d in the rhizosphere soil after 14-d incubations. Microorganisms in no nvegetated soil also showed the ability to degrade the three compounds , but not to the extent of the rhizosphere soil. Some abiotic degradat ion occurred for all three herbicides. The results of these preliminar y experiments suggest that the rhizosphere of certain plant species ma y be important for facilitating microbial degradation of pesticide was tes in soils and beneficial for remediating pesticide-contaminated sit es.