Composition of diazotrophic bacterial assemblages in bean-planted soil compared to unplanted soil

The effect of common bean (Phaseolus vulgaris L.) on the composition of nitrogen fixing bacterial assemblages in soil was studied by comparing planted and unplanted soil. The community composition was studied by terminal restriction fragment length polymorphism (T-RFLP) of the nitrogenase reductase gene (nifH). Principal component analysis (PCA) of T-RFLP profiles showed the separation of profiles from planted and unplanted soil. Terminal restriction fragments (T-RFs) corresponding to rhizobial bacteria were identified preferentially in planted soil; however most nifH T-RFs in soil could not be assigned to T-RFs simulated from a database of known diazotrophs. To specifically study rhizobial bacteria in the soil and nodules, PCR products from the alpha subunit of the nitrogenase enzyme (nifD) were analyzed by denaturing gradient gel electrophoresis (DGGE). DGGE results showed the specific stimulation of the rhizobial microsymbionts in planted soil.