Evidence of endophytic diazotrophic bacteria in lodgepole pine and hybrid white spruce trees growing in soils with different nutrient statuses in the West Chilcotin region of British Columbia, Canada

The West Chilcotin is a remote region located in the Sub-Boreal Pine-Spruce biogeoclimatic zone of British Columbia, Canada where cold climate and low annual precipitation have resulted in relatively dry and weakly developed soils. Lodgepole pine is the most widely found tree species in this region followed by hybrid white spruce. Their ability to grow on such nutrient-poor soils raise questions regarding nitrogen (N) inputs to these forest stands. A rarely evaluated but possible source of N could be N-fixing bacteria living in the internal tissues of pine and spruce trees (known as 'endophytic diazotrophic bacteria'). To examine this possibility, we selected two sites with different soil moisture contents in a predominantly spruce stand and collected soil and plant samples from each site. Similarly, soil and plant samples were also collected from two sites with different soil moisture contents in a predominantly pine stand. Analyses of soil samples revealed that overall nutrient content of soils of low-moisture (LM) sites was significantly lower than high-moisture (HM) sites in both stands, particularly, available and mineralizable N in soil. We isolated 55 (LM: 27 and HM: 28) and 48 (LM: 20 and HM: 28) bacteria from internal tissues of pine and spruce trees, respectively on N-free media. N-fixing ability of these isolates was evaluated using the acetylene reduction assay and 18 isolates from spruce (LM: 10 and HM: 8) and 23 isolates from pine (LM: 13 and HM: 10) were tested positive in this assay. These endophytic diazotrophic bacteria were identified as mainly belonging to genera: Bacillus, Caballeronia, Paenibacillus, and Pseudomonas. These results indicate that pine and spruce trees growing on different sites in this region harbour naturally occurring N-fixing bacteria in their tissues, possibly to gain fixed N.