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Effects of Paenibacillus polymyxa and soil nitrogen on lodgepole pine (Pinus contorta var. latifoliar (Dougl.) Engelm.) seedling growth Yang, Henry


Paenbacillus polymyxa strain P2b-2R has previously been shown to colonize lodgepole pine (Pinus contorta var. latifolia (Dougl.) Engelm.) internal tissues and stimulate seedling growth possibly through N₂ fixation. I evaluated the biochemical characteristics of P. polymyxa strain P2b-2R and the effects of soil nitrogen (N) concentrations on lodgepole pine seedling growth after inoculation with P2b-2R. The bacterium was capable of using major plant cell wall components including carboxymethyl cellulose, xylan and sodium polypectate in addition to 39 of 95 carbon sources from BioLog GP2 microplates. These results suggest that hydrolytic enzymes may facilitate endophytic colonization by P2b-2R. Bacterial colonization and lodgepole pine seedling growth responses (foliar ¹⁵N atom % excess, foliar N concentrations, total foliar N and root and shoot biomass) to inoculation with P2b-2R were assessed at varying soil N concentrations in a one-year greenhouse study. Surface sterilized pine seeds were sown in Ray Leach cone-tainers containing an autoclaved sand-montmorillonite clay mixture and inoculated with log 6 colony forming units (cfu) P2b-2R. Non-inoculated controls received phosphate-buffered saline. Seedlings were then subjected to monthly applications of a nutrient solution containing one of four soil N concentrations as Ca(NO₃)₂ (5% ¹⁵N label): 0.0029, 0.029, 0.29 and 2.9 mmol L-¹, referred to as ‘very low’, ‘low’, ‘medium’ and ‘high’ soil N treatments, respectively. Rhizospheric and endophytic population sizes of P2b-2R, foliar ¹⁵N atom % excess and foliar N concentrations were not significantly affected by P2b-2R during the experiment. Inoculation with P2b-2R resulted in seedling biomass across the four soil N treatments four months after sowing. However, seedling growth inhibition was transient as it was not detectable four months later and by month twelve, inoculated seedlings from the very low soil N treatment had accumulated 56.3 and 46.4% more root and shoot biomass, respectively, than controls. Seedlings from the medium soil N treatment responded similarly to bacterial inoculation, but no biomass enhancement was observed at low and high soil N concentrations. My results suggest that pine biomass stimulation by P2b-2R may depend on soil N concentrations and that P2b-2R can enhance biomass accumulation of pine seedlings without providing significant amounts of fixed N.

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