Enhanced degradation of a mixture of three herbicides in the rhizosphere of a herbicide-tolerant plant

The rhizosphere of herbicide-tolerant plants may be an important component in biologically remediating pesticide-contaminated soils. A pesticide-contaminated site at an agrochemical dealership in Iowa was characterized, and soil from the site was brought to the laboratory for degradation experiments. Three major herbicides were identified in the soils by gas chromatography-atrazine, metolachlor, and trifluralin. Although concentrations of these chemicals were as high as 2 to 3 times field application rates, herbicide-tolerant plants were found growing in the contaminated soil. Initial numbers of microorganisms were determined in rhizosphere soil from Kochia sp. and in edaphosphere (nonvegetated) soil. The rhizosphere soil had an order of magnitude higher microbial numbers (4.2 × 10⁵) compared with the edaphosphere soil (3.5 × 10⁴.) A degradation experiment that did not incorporate vegetation was carried out by using sterile control soil, Kochia sp. rhizosphere soil, and edaphosphere soil spiked with a mixture of atrazine, metolachlor, and trifluralin at levels typical of point-source spills. Significantly (p ≤ 0.10) enchanced degradation was observed in the rhizosphere soil after 14-d incubations. Microorganisms in nonvegetated soil also showed the ability to degrade the three compounds, but not to the extent of the rhizosphere soil. Some abiotic degradation occurred for all three herbicides. The results of these preliminary experiments suggest that the rhizosphere of certain plant species may be important for facilitating microbial degradation of pesticide wastes in soils and beneficial for remediating pesticide-contaminated sites.