Biomass increase and associative nitrogen fixation of mycorrhizal Pinus contorta seedlings inoculated with a plant growth promoting Bacillus strain

One-month-old Pinus contorta Dougl. seedlings were inoculated with mycorrhizal fungi (Wilcoxina mikolae isolate R947), plant growth promoting bacteria capable of fixing nitrogen (Bacillus strain L6), or coinoculated with a combination of Wilcoxina and Bacillus. Seedlings were grown with 0.35 mM Ca( 15 NO 3 ) 2 (5% 15 N atom excess) as the only nitrogen source. Fungal inoculation alone resulted in the synthesis of ectendomycorrhizae but decreased shoot biomass. Bacterial inoculation alone did not affect seedling biomass or foliar nitrogen content. Coinoculation of seedlings with Wilcoxina and Bacillus resulted in a similar degree of ectendomycorrhizal infection compared with fungal inoculation alone, but shoot biomass was greater than that of seedlings receiving the fungus alone. Root biomass and stem height were not altered by coinoculation. Total foliar nitrogen was lower in seedlings inoculated with Wilcoxina, and coinoculation with Bacillus did not reverse this trend. Based on the 15 N to 14 N ratio of foliage, associative nitrogen fixation by Bacillus contributed 4% (P < 0.05) of seedling foliar nitrogen whether or not seedlings were mycorrhizal. However, coinoculated seedlings had lower foliar nitrogen (percentage basis) compared with uninoculated controls. These results suggest that growth of mycorrhizal pine can be stimulated by inoculating with beneficial bacteria. Growth promotion does not result from increased formation of ectendomycorrhizae and may, only in part, result from associative nitrogen fixation.