The Association of Free-Living Nitrogen-Fixing Bacteria with the Roots of High Arctic Graminoids

Abstract

Samples of graminoid roots and adjacent soil collected from tundra and semidesert sites in the Canadian High Arctic were introduced to a nitrogen-free, nutrient medium formulated to culture N2-fixing microorganisms. Some enrichment cultures of root and soil material showed acetylene reduction activity, suggesting the presence of N2-fixing microorganisms. These microorganisms were found to be associated with the roots of 10 graminoid species (six grasses, two sedges, two rushes). Bacteria isolated from enrichment cultures included members of the genera Clostridium, Desulfovibrio, Klebsiella, and Azospirillum. Neither nitrogenase activity nor the presence of N2-fixing microorganisms was detected in soil or root enrichments of early-season (early July) samples; however, these were observed in enrichments of samples collected late 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 N2fixing bacteria. The implications of rhizospheric interactions between N2-fixing bacteria and arctic plants which grow in nitrogen-deficient soils are discussed.