15N foliar dilution of western red cedar in response to seed inoculation with diazotrophic Paenibacillus polymyxa

Diazotrophic bacteria previously isolated from internal tissues of naturally regenerating lodgepole pine (Pinus contorta var. latifolia (Dougl.) Engelm.) seedlings were tested for their ability to fix N in association with western red cedar (Thuja plicata Donn.). Surface-sterilized cedar seed was sown in glass tubes containing an autoclaved sand/montmorillonite clay mixture that contained a N-free nutrient solution labeled with 15N as 0.35 mM Ca(15NO3)2 (5 % 15N label). After sowing, seed was inoculated with one of three bacterial strains: Paenibacillus polymyxa P2b-2R, P. polymyxa P18b-2R, or Dyadobacter fermentans P19a-2R. At the end of the 27-week plant growth period, P2b-2R was the only strain detected in the cedar rhizosphere. No bacteria were found inside plant tissues. Cedar foliar N concentrations were significantly enhanced (26–33 %, P < 0.05) in response to inoculation with all three bacterial strains but 15N dilution (23 %, P < 0.05) was observed only in seedlings treated with strain P2b-2R. This strain also reduced seedling dry weight (27 %, P < 0.05). We observed similar trends in a second experiment with slight modifications to the protocol, but the magnitude of foliar 15N dilution was greater (56 %, P < 0.05). Based on our results, we conclude that cedar seedlings inoculated with strain P2b-2R derived 23 and 56 % of their foliar N from bacterial N fixation in two seedling growth experiments.