COS 25-5 - Rapid assimilation of biologically fixed nitrogen by vascular plant species in a boreal forest

Background/Question/Methods

Vascular plants in boreal forests compete for a small pool of available nitrogen (N), much of which originates from bryophyte-diazotroph N₂-fixation. However, Earth system models currently do not account for bryophyte-diazotroph N₂-fixation in boreal forests, based on assumptions that fixed N is locked in bryophyte tissue and becomes available to plants over very long time scales. Nevertheless, previous work has speculated that recently fixed N can be pilfered by mycorrhizal fungi or leached into the soil, therefore providing a direct pathway for vascular plant uptake of recently fixed N, but evidence for this proposed pathway is limited. To test for evidence of recently fixed N uptake, we applied a ¹⁵N₂ pulse-labeling approach to boreal mesocosms containing spruce (Picea abies) seedlings growing in 2 × 2 factorial combinations of feathermoss (Hylocomium splendens) and bilberry (Vaccinium myrtillus) presence or removal, with all other plants removed. We incubated the mesocosms with ¹⁵N₂ for one week, and then quantified foliar ¹⁵N enrichment of spruce and bilberry. We hypothesized that spruce seedlings would acquire recently fixed N when feathermoss was present, and that seedlings grown without feathermoss would acquire a negligible amount of recently fixed N from free-living or endophytic N₂-fixing diazotrophs.

Results/Conclusions

Results revealed that spruce seedlings acquired recently fixed N in all treatments, but, contrary to our hypothesis, seedlings grown without feathermoss acquired 2.4- to 3.0-fold more ¹⁵N than seedlings grown with feathermoss, suggesting a substantial contribution from free-living or endophytic N₂-fixing diazotrophs. Moreover, bilberry acquired an average of 2.5- to 3.0-fold more ¹⁵N than co-occurring spruce, regardless of feathermoss presence. We also found that the absence of feathermoss reduced spruce total foliar N by ~28.5%, which in turn was somewhat negatively correlated with ¹⁵N uptake. These findings suggest that vascular plants in boreal forests can quickly acquire N that has been recently fixed by free-living, endophytic, or feathermoss-associated N₂-fixing diazotrophs, a phenomenon that has been overlooked in boreal N cycling components of Earth system models.