Long-term effects of inoculating lodgepole pine seedlings with plant growth-promoting bacteria originating from a disturbed gravel mining ecosystem

Abstract

Gravel mining is prevalent in forest landscapes across Canada, typically resulting in complete loss of vegetation and topsoil. Despite such extreme disturbance, lodgepole pine (Pinus contorta Dougl. var. latifolia Engelm.) trees are thriving at unreclaimed gravel pits located in central-interior British Columbia, possibly owing to, at least in part, the association of pine trees with their endophytic bacteria. To test this possibility, several bacterial strains were previously isolated from pine trees growing at these pits; of the strains isolated, 14 were identified as effective nitrogen fixers. In this study, we evaluated the inoculation effect of these 14 strains on lodgepole pine growth under nitrogen-poor conditions. Eighteen months after sowing and inoculation, each strain had colonized the rhizosphere and internal tissues of pine seedlings and had significantly enhanced their length (24%–65%) and biomass (100%–300%). Notably, three Pseudomonas strains increased pine seedling length by 1.6-fold and biomass by 4-fold. Most strains also demonstrated substantial potential to promote plant growth via phosphorus solubilization, siderophore production, 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase activity, indole-3-acetic acid production, lytic enzyme activity, and catalase activity. Our results suggest that such nitrogen-fixing bacteria could be sustaining pine growth on bare gravel, indicating a possible ecological association that may explain natural tree regeneration in such a disturbed ecosystem.