Nitrogen (N) fixation in N-limited ecosystems: Unraveling the importance and fate of fixed N2

Background/Question/Methods

Nitrogen (N) fixation performed by N₂ fixing bacteria (diazotrophs) is the primary N input to pristine ecosystems like boreal forests and subarctic and arctic tundra. In these ecosystems, a wide range of N₂ fixers are present, where they could contribute fundamentally to increase the ecosystem N pool. However, the contribution by the various diazotrophs to habitat N₂ fixation remains unclear. Another unknown is the fate of the fixed N₂ and with that, the role of different N₂ fixers as N sources for the ecosystem.

We assessed the contribution by five diazotroph associations (with a legume, lichen, feather moss, Sphagnum-moss, and free-living) to habitat N₂ fixation in five typical habitats in the Subarctic (wet and dry heath, polygon-heath, birch forest, mire) using the acetylene reduction assay in situ. To further assess the importance of different N₂ fixers for ecosystem N cycling, we tracked ¹⁵N₂ into four N₂ fixer associations (with a legume, lichen, free-living, moss) and into soil, microbial biomass and non-fixing plants three days and five weeks after in situ labelling in subarctic tundra.

Results/Conclusions

Across the growing season, the legume had the highest total as well as the highest fraction of N₂ fixation rates at habitat level in the heaths (>85% of habitat N₂ fixation), whereas the free-living diazotrophs had the highest N₂ fixation rates in the polygon heath (56%), the lichen in the birch forest (87%) and Sphagnum in the mire (100%). The feather moss did not contribute more than 15% to habitat N₂ fixation in any of the habitats despite its high ground cover. Shortly after labelling, half of the fixed ¹⁵N was recovered in the legume soil, indicating a fast release from legume to the soil, including the soil microbes. Within five weeks, the free-living N₂ fixers released two-thirds of the fixed ¹⁵N into the soil, whereas the lichen and moss retained the fixed ¹⁵N. Our results highlight the fundamental differences between N₂ fixers as N sources due to different rates of N release and their contribution to habitat N₂ fixation, which should be considered in ecosystem N budgets for N-limited ecosystems like subarctic tundra.