DNA-PROBING FOR GENES-CODING FOR DENITRIFICATION, N-2-FIXATION AND NITRIFICATION IN BACTERIA ISOLATED FROM DIFFERENT SOILS

Bacteria isolated from different layers of four soils of the Cologne a rea were analyzed for denitrifying, nitrifying and N-2-fixing isolates by colony hybridization using gene probes. In the soils tested, the p ercentage of denitrifying bacteria among the total population isolated was 3-8% (in one case exceptionally 15%) and thus small. Denitrifying bacteria were particularly enriched in the upper layer (depth similar to 5 cm) and were present only in low amounts at 25 cm depth in two g leysol soils. Nitrate apparently did not determine the distribution of denitrifying bacteria in these soils. The potential denitrification a ctivity of different soil layers coincided with the distribution patte rn of isolates assessed by DNA-probing. The total number of bacteria a nd of denitrifying isolates was considerably higher in or at the roots of plants than in the bulk, root-free soil adjacent to the plants. Th e percentage of the isolated aerobic Nz-fixing bacteria varied between 0-3%, and these bacteria could be isolated mainly from the upper 5 cm layer. A small portion of the isolates hybridized with the probe codi ng for part of one subunit of ammonia monooxygenase from Nitrosomonas europaea. The investigation showed that DNA-probing can provide useful information about the relative distribution of denitrifying and N-2-f ixing bacteria in different soils and their layers.