Effects of Paenibacillus polymyxa inoculation and different soil nitrogen treatments on lodgepole pine seedling growth

An endophytic diazotroph, Paenibacillus polymyxa P2b-2R, has been shown to colonize internal tissues of lodgepole pine (Pinus contorta var. latifolia Engelm. ex S. Watson) and stimulate seedling growth possibly through nitrogen (N) fixation in an N-limited environment. We evaluated the effects of different soil N concentrations on lodgepole pine seedling growth after inoculation with P2b-2R in a year-long greenhouse study. Seedlings were subjected to monthly application 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 µmol·L^-1, referred to as very low, low, medium, and high soil N treatments, respectively. Foliar ¹⁵N atom percent excess and foliar N content values of P2b-2R inoculated seedlings were not significantly different from controls at any N treatment. Similarly, rhizospheric and endophytic population of P2b-2R did not vary significantly across N treatments. Inoculation with P2b-2R resulted in seedling growth inhibition during early stages of the experiment; however, by month 12, inoculated seedlings from the very low soil N treatment had accumulated 56.3% and 46.4% more root and shoot biomass than controls, respectively. These results suggest that pine biomass stimulation by P2b-2R may depend on soil N concentrations and that such growth stimulation may occur without providing significant amounts of fixed N. (author)