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