RHIZOSPHERE MICROBIAL POPULATIONS IN CONTAMINATED SOILS T. D. NICHOLS 1 , D. C. WOLF 1 , H. B. ROGERS 1 , C. A. BEYROUTY 1 and C. M. REYNOLDS 2 1 Department of Agronomy, University of Arkansas, Fayetteville, AR 72701, U.S.A.; 2 U.S. Army, Cold Regions Research Engineering Laboratory, Hanover, NH 03755, U.S.A. (Received 20 July, 1995; accepted 21 February, 1996) Abstract. Rhizosphere microbial populations may increase bioremediation of soil contaminated with organic chemicals. A growth chamber study was conducted to evaluate rhizosphere microbial popu- lations in contaminated and non-contaminated soil. Alfalfa (Medicago sativa L.) and alpine bluegrass (Poa alpina L.) were grown in soil containing a mixture of organic chemicals for 14 weeks. The equal millimolar mixture of hexadecane, (2,2-dimethylpropyl)benzene, cis-decahydronaphthalene (decalin), benzoic acid, phenanthrene, and pyrene was added at levels of 0 and 2000 mg/kg. Organic chemical degrader (OCD) populations were assessed by a Most-Probable-Number technique, and bacteria and fungi were enumerated by plate count methods. Different methods for expressing OCD rhizosphere populations were investigated to determine the effect it had on interpretation of the results. At 9 weeks, the OCD numbers were significantly higher in rhizosphere and contaminated soils than in bulk and non-contaminated soils, respectively. Alfalfa rhizosphere OCD levels were 4 10 7 /g for contaminated and 6 10 6 /g for non-contaminated soils. Bluegrass rhizosphere OCD levels were 1 10 7 /g and 1 10 6 /g in contaminated and non-contaminated soils, respectively. Selective enrich- ment of OCD populations was observed in contaminated rhizosphere soil. Higher numbers of OCD in contaminated rhizospheres suggest potential stimulation of bioremediation around plant roots. Key words: bioremediation, phytoremediation, polycyclic aromatic hydrocarbons, petroleum 1. Introduction For soils contaminated with organic chemicals, remediation techniques such as incineration and landfilling can be expensive (Riggle, 1995). An alternative tech- nology, known as bioremediation, involves the use of microorganisms to break- down contaminants (Alexander, 1994). Microorganisms can use the contaminants as energy and C sources while transforming them into carbon dioxide and water. Microorganisms can also transform certain organic compounds while not deriving energy or C in a process known as cometabolism (Perry, 1979). The area around plant roots, known as the rhizosphere, contains higher populations and greater diversity of microorganisms than soil with no plants (Anderson and Coats, 1995; Angle et al., 1996; Curl and Truelove, 1986). A cost-effective technique could be the use of plants to enhance microbial populations in the soil, which may stimulate degradation of organic chemicals (Anderson et al., 1993; Cunningham and Lee, 1995; Schwab and Banks, 1994). Published with the approval of the Director, Arkansas Agricultural Experiment Station, manu- script # 95079. Author for all correspondence. Water, Air, and Soil Pollution 95: 165–178, 1997. c 1997 Kluwer Academic Publishers. Printed in the Netherlands.