N2-fixation and growth promotion in cedar colonized by an endophytic strain of Paenibacillus polymyxa

Inoculation of western red cedar with Paenibacillus polymyxa P2b-2R, an endophytic diazotroph of a pine, was previously shown to result in biological nitrogen fixation (BNF) in seedlings grown under N-limited conditions, but biomass accumulation was reduced after a 9-month growth period. To determine if the seedling growth reduction was temporary, we inoculated cedar seed with strain P2b-2R and grew seedlings for up to 13 months in a N-limited soil mix containing 0.7 mM of available N labeled as Ca(¹⁵NO₃)₂. P2b-2R developed a persistent endophytic population comprising 10²–10⁶?cfu g^?1 plant tissue inside pine roots, stems, and needles. At the end of the growth period, P2b-2R had reduced ¹⁵N foliar N abundance by 36 % and increased shoot biomass by 46 % compared to controls. Our results indicate that inoculated seedlings derived 36 % of foliar N from the atmosphere and suggest that BNF by P. polymyxa can significantly enhance growth of cedar in a N-limited soil if seedlings are grown for a sufficient amount of time. These findings support the hypothesis that endophytic diazotrophs may facilitate regeneration and growth of western red cedar at N-poor sites.