Competition of two inoculated Frankia strains in root nodulation of Alnus glutinosa seedlings and associated Frankia-strain growth in rhizospheric and non-rhizospheric soils.

Aims: Research objectives were to determine nodule-forming capacities and competition for nodule-formation of Frankia strains ArI3 and Ag45/Mut15, which were assessed 90 days after planting of host seedlings in containers within soil microcosms. Methods: Both strains were inoculated in a '4 × 4' factorial design with all combinations (n = 16) of cell numbers of the two strains (0, 102, 103, or 104 cells g−1 of soil) determined by qPCR. Results: In all cases, inoculation significantly increased plant height compared to non-inoculated controls. Root nodules were formed on all inoculated plants with strain Ag45/Mut15 alone forming few nodules, for all inoculum amounts whiles strain ArI3 alone formed about ten-times more nodules at the lowest inoculum amount than Ag45/Mut15, with notable increases in nodule numbers for each ten-fold increase in inoculum. Except for nodules formed after inoculation of strain Ag45/Mut15 alone, nodules in all combinations harbored strain ArI3 only. Both strains were detected in rhizosphere soils for all three inoculum amounts. In contrast to nodules and rhizosphere soil, however, strain ArI3 was not detected in any bulk soil samples after three months of incubation, while strain Ag45/Mut15 was detected with densities about ten-fold higher than the original inoculum. Conclusions: These analyses demonstrate that strain ArI3 is initially more infective and competitive for root nodule formation than strain Ag45/Mut15. Strain Ag45/Mut15, however, survived and increased in cell numbers in bulk soil after 90 days whereas strain ArI3 disappeared indicating a reversal of competitive advantage between these two strains in the soil matrix over time. [ABSTRACT FROM AUTHOR]