Pergamon 0045-6535(94)E0076-6 Chemosphere, Vol. 28, No. g, pp. 1551-1557, 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0045-6535/94 $7.00+0.00 ENHANCED DEGRADATION OF A MIXTURE OF THREE HERBICIDES IN THE RHIZOSPHERE OF A HERBICIDE-TOLERANT PLANT T. A. Anderson*, E. L. Kruger, and J. R. Coats Pesticide Toxicology Laboratory, Department of Entomology, Iowa State University, Ames, Iowa 50011-3140 (Received in Germany 11 August 1993; accepted 29 November 1993) ABSTRACT 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 x 10s) compared with the edaphosphere soil (3.5 x 104.) A degradation experiment that did not incorporate vegetation was carded 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) enhanced 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. *To whom correspondence should be sent. INTRODUCTION The use of microorganisms for remediating contaminated environments is currently an area of intense interest [1]. Recent successes utilizing the natural metabolic capabilities of bacteria and fungi to cleanup soil, sediment, and water have been instrumental in encouraging this interest [2,3,4]. In addition, the relative cost-effectiveness of bioremediation compared with other remediation technologies has spurred research to identify situations for which remediation using microorganisms can be appropriate. Although use of the term bioremediation is relatively new, actual biological 1551