RHIZOSPHERE MICROBIAL-POPULATIONS IN CONTAMINATED SOILS

Rhizosphere microbial populations may increase bioremediation of soil contaminated with organic chemicals. A growth chamber study was conduc ted to evaluate rhizosphere microbial populations in contaminated and non-contaminated soil. Alfalfa (Medicago sativa L.) and alpine bluegra ss (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/k g. 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 interp retation of the results. At 9 weeks, the OCD numbers were significantl y higher in rhizosphere and contaminated soils than in bulk and non-co ntaminated soils, respectively. Alfalfa rhizosphere OCD levels were 4 x 10(7)/g for contaminated and 6 x 10(6)/g for non-contaminated soils. Bluegrass rhizosphere OCD levels were 1 x 10(7)/g and 1 x 10(6)/g in contaminated and non-contaminated soils, respectively. Selective enric hment of OCD populations was observed in contaminated rhizosphere soil . Higher numbers of OCD in contaminated rhizospheres suggest potential stimulation of bioremediation around plant roots.