- Nodulated roots of young red alder trees and soil samples were collected from a 5 year old clearcut on Mary's Peak, Oregon. Frankia was isolated from root nodules, andactinomycetes from nodules, roots, and soil. A total of 229 pure culture actinomycete isolates were recovered: 131 colonies from root surfaces, 52 colonies
from soil, and 46 colonies from nodule surfaces. Thirty morphologically different colonies were selected for the experiment, 10 from each source. Red alder seedlings
were grown in the greenhouse for 6 months. They were placed in Ray Leach tubes containing a 1:1 soil mixture of peat moss and vermiculite. Seedlings were inoculated
with a Frankia-actinomycete combination when they were 1 month old. N-free mineral solution was applied weekly. The study consisted of thirty-two treatments, thirty in
which seedlings were inoculated with Frankia plus one of thirty actinomycete isolates (10 each from nodule surfaces, root surfaces, and open soil), one inoculated with Frankia only, and one in which seedlings were
inoculated with neither Frankia nor an actinomycete. Each inoculation treatment was replicated on ten seedlings. Three variables were measured: acetylene reduction, oven-dry nodule weight, and total dry bioxnass of seedlings. Actinoinycete colonies varied significantly in their influence on nodule weight per seedling, acetylene reduced per gram nodule, and total seedling weight, but not in their effects on acetylene reduced per
seedling. Colonies isolated from nodule surfaces produced slightly greater nodule weights on seedlings than colonies isolated from roots, but colonies from the three sources (nodules, roots, and open soil) did not differ in their effects on the other measured variables. Compared to seedlings inoculated only with Frankia, five of the thirty actinomycete colonies that were tested
enhanced acetylene reduction per gram nodule; one colony reduced nodule weight, and none altered the amount of acetylene reduced per seedling. By far the major influence of actinoinycetes was to reduce seedling weight, 13 of 30 colonies producing seedlings that were smaller than those inoculated only with Frankia.