Diazotrophic bacteria in root-free soil and in the root zone of pine (Pinus sylvestris L.) and oak (Quercus robur L.)

Studies on the occurrence, identity and potential nitrogenase activity of d iazotrophic bacteria in soil and the root zone of Scots pine (Pinus sylvest ris L.) and common oak (Quercus robur L,) were carried out. Diazotrophic ba cteria were more numerous in soil and the root zone of oak than in those of pine. Most of the bacteria isolated from the roots of both trees belonged to the genera Pseudomonas and Bacillus. The majority of these bacteria had nitrogenase activity (tested by the acetylene reduction assay method) withi n the range from 4 to 20 nmoles C2H4 per culture per hour. No clear relatio nships between the taxonomic identity and/or source of isolation of the bac teria studied and their nitrogenase activity were found. Biotin, p-aminoben zoic acid (PABA) and yeast extract, applied both separately and in combinat ion, significantly (at least several-fold) stimulated the activity of N-2 f ixation in three selected (nitrogenase highly active) bacterial strains, as compared to a control without growth factors. (C) 1999 Elsevier Science B. V. All rights reserved.