Can naturally-occurring endophytic nitrogen-fixing bacteria of hybrid white spruce sustain boreal forest tree growth on extremely nutrient-poor soils?

The West Chilcotin region in British Columbia, Canada is located in the Sub-Boreal zone characterized by dry and weakly developed soils lacking essential plant nutrients, particularly nitrogen. Yet, hybrid white spruce (Picea glauca x engelmannii) trees thrive on such nitrogen-limited soils, raising a crucial question regarding their nitrogen sources. The presence of endophytic diazotrophic bacteria (nitrogen-fixing bacteria living inside the plant tissues) was reported in these spruce trees, previously. But, can these bacteria actually sustain tree growth on nitrogen-limited soils of this region? To answer this question, we tested six endophytic diazotrophic bacteria under nitrogen-poor conditions in a year-long greenhouse trial with their original host (i.e. hybrid white spruce). In a different year-long trial, we also tested these bacteria with another host native to the West Chilcotin region (i.e. lodgepole pine) to examine their interaction with a foreign host. Endophytic colonization, seedling growth promotion and the amount of nitrogen fixed in planta by each bacterium were examined. We found that each bacterium colonized one or more tissues of the spruce and pine seedlings (10²–10⁷ colonies per gram fresh tissue) and fixed significant (P < 0.05) amounts of nitrogen from the atmosphere (17–56% of the total plant nitrogen). In addition, bacteria-inoculated seedlings were considerably longer and accumulated significantly (P < 0.05) more biomass than non-inoculated controls after 12 months. Notably, the bacterium Caballeronia sordidicola LS-S2r fulfilled >50% of the nitrogen requirements of pine and spruce seedlings via nitrogen fixation, and enhanced seedling length and biomass by nearly 1.5-fold and 5-fold, respectively. Therefore, our results suggest that endophytic diazotrophic bacteria harboured by hybrid spruce trees can sustain their growth on nitrogen-limited soils via biological nitrogen fixation. These bacteria can also provide similar benefits to a foreign host - lodgepole pine, indicating their wide-range ecological applications in improving the N-supply of forest stands in this region and beyond. In particular, C. sordidicola LS-S2r holds strong potential to be possibly used as a biofertilizer in boreal forest stands, as an economical and eco-friendly alternative to chemical fertilizers.