Presence and activity of nitrogen-fixing bacteria in Scots pine needles in a boreal forest: a nitrogen-addition experiment.

This article investigates the presence and activity of endophytic nitrogen-fixing bacteria inside 1-year-old needles of Scots pine (Pinus sylvestris L.) in a Scandinavian boreal forest. Using immunodetection of the nitrogenase enzyme subunit NifH and acetylene-reduction assays (ARA), the study confirmed that these bacteria are present and actively fix nitrogen, with no significant difference in activity between nitrogen-fertilized and control plots. Thirteen bacterial isolates belonging to the genera Bacillus, Variovorax, Novosphingobium, Sphingomonas, Microbacterium, and Priestia were identified via 16S rRNA and whole-genome sequencing, and ten demonstrated nitrogen-fixation ability in vitro. Although the estimated nitrogen fixed by these bacteria (~20 g N ha⁻¹ year⁻¹) is low relative to Scots pine nitrogen demand, the findings suggest that endophytic nitrogen fixation may contribute to the long-term nitrogen budget of nitrogen-limited boreal forests.