Comprehensive ecological role of beneficial bacteria in sustaining the growth of boreal forest trees on extremely nutrient-poor soils

Background/Question/Methods: The West Chilcotin region in British Columbia, Canada is located in a Sub-Boreal zone, where low annual precipitation because of the strong rain-shadow of the Coast Mountains have resulted in parched and weakly-developed soils. Hybrid white spruce (Picea glauca x engelmannii) is one of the most common tree species growing in this region. Soils examined from spruce forest stands revealed that they have poor physico-chemical health and limited plant available nutrients, especially nitrogen. Despite the severely nitrogen-limited soils, the continued, vigorous growth of spruce trees is a major conundrum. A rarely evaluated but possible nitrogen-input could be due to the activity of nitrogen-fixing bacteria living inside the tree tissues.

Results/Conclusions: Testing this hypothesis, we isolated 48 potential nitrogen-fixing bacterial strains from spruce tree tissues. We tested six strains that showed highest in vitro nitrogen-fixing ability in a yearlong greenhouse study with their original host (hybrid white spruce) and another host native to this region (lodgepole pine – Pinus contorta var. latifolia). All strains colonized the tissues of both spruce and pine trees and performed similarly well in both hosts, contributing 17-56% of host’s foliar nitrogen from the atmosphere after one year and significantly enhancing seedling length and biomass. We also observed that these strains possess several other plant-growth-promoting abilities such as inorganic and organic phosphate solubilization, indole-3-acetic acid (IAA) production, 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity, siderophore production and lytic enzyme activity. Notably, Caballeronia sordidicola LS-S2r fulfilled more that >50% of nitrogen requirements of spruce and pine trees in one year, enhanced seedling length and biomass by 1.5-fold and 5-fold, respectively, and showed significant potential in the aforementioned plant-growth-promoting mechanisms. Therefore, the ability of such mutli-faceted bacteria to form beneficial ecological associations with pine and spruce trees could explain the sustained growth of trees on extremely nutrient-limited soils of the West Chilcotin region. Furthermore, their effectiveness with a foreign host indicates the lack of plant x microbe specificity, raising the possibility of their utilization as potential biofertilizers to regenerate trees in disturbed and nutrient-poor ecosystems.