Isolation and identification of endophytic diazotrophs from lodgepole pine trees growing at unreclaimed gravel mining pits in central interior British Columbia, Canada

摘要

Unreclaimed gravel mining pits located in the central interior of British Columbia have very limited soil nitrogen (N) levels due to gravelly textured soils with weak profile development, no organic forest floor, and low atmospheric N inputs through precipitation. However, lodgepole pine (Pinus contorta var. latifolia Engelm. ex S. Watson) trees can be found growing well at these pits with tissue N content and growth rate unaffected by extremely low soil N levels, indicating that pine trees are able to meet their N requirements from an unknown source. We hypothesized that biological N fixation by bacteria living in the tree tissues (known as endophytic diazotrophs) could be a potential N source for these trees. To test this hypothesis, we isolated 77 potential endophytic diazotrophs from needle, stem, and root tissues of pine trees on N-free culture medium. Of these, 32 bacteria showed positive N-fixing ability when tested for nitrogenase enzyme activity using the acetylene reduction assay. These endophytic N-fixing bacteria were identified as mainly belonging to the genera Pseudomonas, Bacillus, Paenibacillus, and Rhizobium, which are well known for their plant-beneficial traits including N fixation. Therefore, it can be concluded that pine trees growing at these severely N-limited gravel pits naturally harbour endophytic diazotrophs, which could be involved in sustaining their vigorous growth, possibly through biological N fixation.