High-N 2 -fixing activities of Frankia populations in root nodules on Alnus glutinosa improve growth performance of the host plant. Therefore, the establishment of active, nodule-forming populations of Frankia in soil is desirable. In this study, we inoculated Frankia strains of Alnus host infection groups I, IIIa, and IV into soil already harboring indigenous populations of infection groups (IIIa, IIIb, and IV). Then we amended parts of the inoculated soil with leaf litter of A. glutinosa and kept these parts of soil without host plants for several weeks until they were spiked with

The competitive ability for nodulation of Alnus glutinosa (L.) Gaertn. plants by Frankia strains inoculated into soil with indigenous Frankia populations was studied at two matric potentials representing "dry" (-0.016 MPa) and "wet" (-0.001 MPa) conditions. In pots kept at a matric potential of -0.001 MPa, nitrate concentrations decreased within 3 weeks more than 10-fold to an average of approx. 200 µmol·(g soil dry wt.)-1. After 4 months, nitrate concentrations in these pots were 16 ± 16 and 277 ± 328 µmol·(g soil dry wt.)-1 (mean ± SD) for non-inoculated and inoculated soils, respectively. At a matric potential of -0.016 MPa, nitrate concentrations for non-inoculated and inoculated soils were 687 ± 491 and 1796 ± 1746 µmol·(g soil dry wt.)-1, respectively. Inoculated plants always grew better than their non-inoculated counterparts. The largest plants were found on inoculated soil at a matric potential of -0.001 MPa, whereas the smallest plants were found on non-inoculated soil at the same matric potential. At a matric potential of -0.016 MPa, plants grown on non-inoculated soil were not as tall as those grown on inoculated soil and were slightly chlorotic, indicating that the high level of nitrate in the soil was not providing optimal plant growth conditions. The number of nodule lobes formed on plants was not significantly different among treatments, though size and weight of lobes differed. Nodules from plants grown on inoculated soils always harbored vesicle-producing Frankia populations, while nodules from plants grown on non-inoculated soils harbored only Frankia with distorted vesicles or no Frankia at all. All strains in nodules from plants grown on non-inoculated soil were of Alnus host infection group IIIa. Nodules from plants grown on soil inoculated with strains ArI3 (group IIIa), Ag45/Mut15 (group IV), and AgB1.9 (group I) were also infected by Frankia strain Ag45/Mut15. These results indicate that by inoculation, Frankia populations could be established under conditions that did not favour vesicle formation in root nodules formed by the indigenous Frankia population. Inoculation even in soils with high nitrogen content might therefore be an appropriate strategy to enhance plant growth.Key words: competition, fluorescent oligonucleotide probes, inoculation, in situ hybridization, matric potential, nitrate, rRNA.

In recent years, molecular approaches have increasingly supplemented nodulation‐dependent detection methods for studying Frankia populations in nature. The new methods are revealing much about the genetic diversity and distribution of Frankia, as well as refining and expanding knowledge about endophyte‐host specificities. PCR‐based approaches have been used to unravel the phylogenetic relationships of isolates, as well as of uncultured endophytes in root nodules of many actinorhizal plants from which no isolates have been obtained. A comparative sequence analysis of PCR‐amplified 16S ribosomal DNA led to the emendation of the family Frankiaceae to contain only the genus Frankia with four main subdivisions: (i) a large group mainly comprising Frankia alni and other typical nitrogen‐fixing strains belonging to the Alnus and the Casuarina host infection groups, respectively, (ii) uncultured endophytes of Dryas, Coriaria and Datisca species, (iii) strains of the Elaeagnus host infection group and (iv) atypical non‐nitrogen‐fixing strains. A considerable diversity among both cultured Frankia strains and uncultured endophytes in nodules was indicated using RFLP analyses of PCR‐amplified fragments of the 16S rRNA gene, the glutamine synthetase II (glnII) gene, the intergenic spacer of the 16S‐23S rRNA operon or the intergenic spacer between the nitrogenase nifH and nifD (nifH‐D) or the nifD and nifK (nifD‐K) genes. The growing database of discriminative target sequences for frankiae is increasingly exploited for studies on the distribution of specific Frankia populations in the environment using PCR or in situ hybridization. Until recently, most studies have focused on the analysis of Frankia populations in root nodules, the natural locale of enrichment for this organism. These populations, however, represent only the fraction of physiologically active, infecting frankiae in soils rather than the total Frankia population. Future approaches to studies of Frankia populations should therefore incorporate the many opportunities for more than just phylogenetic analyses, the description of diversity and studies of Frankia populations in nodules. The molecular approaches open the door to more sophisticated studies of environmental influences on the dynamics of indigenous or introduced Frankia populations in plants and soil. These studies may lead to advancements in the management of actinorhizal plants and Frankia, provided specific Frankia populations can be attributed with silviculturally beneficial features. Such features include persistence and the growth in soil, competition with less efficient Frankia populations for nodule formation, prompt and efficient nodule formation and an ultimately superior nitrogen‐fixing capacity.