Relative abundance of denitrifying and dinitrogen-fixing bacteria in layers of a forest soil

The populations of N-2-fixing and denitrifying bacteria in an acid forest s oil near Cologne were characterized by gene probing. The DNA isolated from the soil for this purpose was suitable For DNA-DNA hybridization using 0.4- 0.7-kb probes targeting denitrification enzymes, dinitrogenase reductase (n ifH) and eubacterial 16S rRNA. The densitometrical comparison of band inten sities obtained in these Southern hybridizations indicated that the highest number of total bacteria, of denitrifying and Nz-fixing microorganisms alw ays occurred in the upper (similar to 5 cm) soil layer. The concentration o f all these organisms decreased in parallel with the soil depth. The soil i nvestigated was rich in nitrate in all layers, and the availability of nitr ate apparently did not govern the distribution of denitrifying and N-2-fixi ng bacteria in this soil. Soil cores investigated in the laboratory formed N2O on addition of nitrate irrespective of the presence of C2H2. Hybridizat ion intensities, with a gene probe for the 16S rRNA, and MPN numbers were g enerally higher in soil samples taken from the roots of plants than in the bulk soil. There was no selective enrichment of denitrifying or N-2-fixing bacteria at the roots. The data obtained by hybridizing isolated soil DNA g enerally matched previous results obtained with culturable bacteria. (C) 20 01 Federation of European Microbiological Societies. Published by Elsevier Science B,V. All rights reserved.