THE ASSOCIATION OF FREE-LIVING NITROGEN-FIXING BACTERIA WITH THE ROOTS OF HIGH ARCTIC GRAMINOIDS

Samples of graminoid roots and adjacent soil collected from tundra and semidesert sites in the Canadian High Arctic were introduced to a nit rogen-free, nutrient medium formulated to culture N-2-fixing microorga nisms. Some enrichment cultures of root and soil material showed acety lene reduction activity, suggesting the presence of N-2-fixing microor ganisms. These microorganisms were found to be associated with the roo ts of 10 graminoid species (six grasses, two sedges, two rushes). Bact eria isolated from enrichment cultures included members of the genera Clostridium, Desulfovibrio, Klebsiella, and Azospirillum. Neither nitr ogenase activity nor the presence of N-2-fixing microorganisms was det ected in soil or root enrichments of early-season (early July) samples ; however, these were observed in enrichments of samples collected lat e in the growing season (late July to early August). The occurrence of Azospirillum species in arctic soils has not been previously reported , nor has the association of high arctic plants with free-living N-2-f ixing bacteria. The implications of rhizospheric interactions between N-2-fixing bacteria and arctic plants which grow in nitrogen-deficient soils are discussed.