Variation in nitrogen fixation among populations ofFrankia sp. andCeanothus sp. in actinorhizal association

Summary Wildland shrub improvement is needed for sound range and disturbed land revegetation practice. The possibility of selecting superior N₂-fixingFrankia-Ceanothus spp. actinorhizal associations was examined. Greenhouse tests were used to expose various soil-borne microsymbiont andCeanothus sp. population accessions in reciprocal combination. The acetylene reduction rate was used as a measure of N₂-fixation capacity. There was no significant interaction between host and microsymbiont regardless of source for all variables measured. The acetylene reduction rate, nodule number and mass, plant biomass, and root: shoot ratio were significantly different among soil sources. The acetylene reduction rate was not significantly different amongCeanothus sp. accessions. Neither was it strongly correlated with other variables. It was concluded that the N₂-fixation rate is more a function ofFrankia sp. than the hostCeanothus sp. in actinorhizal associations. It appears possible to select soil sources with superior N₂-fixing microsymbiont populations.The use of trade or firm names in this paper is for reader information and does not imply endorsement by the U.S. Department of Agriculture of any product or service.This article was produced by government employees and is in the public domain and not subject to copyright. It may be freely reprinted with customary crediting of the source.