Influence of actinomycetes on Frankia infection, nitrogenase activity and seedling growth of red alder

The influence of actinomycetes isolated from different sources was examined in respect to acetylene reduction, nodule weight and total biomass of red alder (Alnus rubra Bong.) seedlings. Soil samples and nodulated roots of young red alder were collected from a 5 yr old clearcut on Marys Peak, Ore. Nitrogen-fixing Frankia was isolated from root nodules, and other actinomycetes from nodule surfaces, roots and soil. Ten morphologically-different actinomycetes from each of the three sources were selected for the experiment. Red alder seedlings were greenhouse-grown in a 1:1 soil mixture of peat moss and vermiculite. After 1 month, seedlings were either inoculated with Frankia and an actinomycete isolate from one of the 30 colonies or inoculated with Frankia only. A control group was not inoculated. Five months after inoculation, seedlings inoculated with isolates from nodule surfaces had slightly greater nodule weight than seedlings inoculated with isolates from roots, but the source of isolate did not appear to influence other measured variables. However, the influence of the different isolates on C₂H₂ reduction g nodule^-1, nodule weight seedling^-1 and total seedling weight varied significantly. Acetylene reduction seedling^-1 did not differ. Seedling capacity for N₂ fixation was not associated with reduced seedling growth. With 5 of the 30 isolates, seedlings had greater C₂H₂ reduction g nodule^-1 than those inoculated only with Frankia, and with one isolate, they had lower nodule weight. By far the major influence of aetinomycetes was reduced seedling weight. Of 30 Frankia-actinomycete mixtures, 13 were associated with seedlings that were significantly smaller than seedlings inoculated with Frankia alone.