Across-Habitat Comparison of Diazotroph Activity in the Subarctic
2015
Rousk, Kathrin | Sorensen, Pernille L. | Lett, Signe | Michelsen, Anders
Nitrogen (N) fixation by N₂-fixing bacteria (diazotrophs) is the primary N input to pristine ecosystems like boreal forests and subarctic and arctic tundra. However, the contribution by the various diazotrophs to habitat N₂fixation remains unclear. We present results from in situ assessments of N₂fixation of five diazotroph associations (with a legume, lichen, feather moss, Sphagnum moss and free-living) incorporating the ground cover of the associations in five typical habitats in the subarctic (wet and dry heath, polygon-heath, birch forest, mire). Further, we assessed the importance of soil and air temperature, as well as moisture conditions for N₂fixation. Across the growing season, the legume had the highest total as well as the highest fraction of N₂fixation rates at habitat level in the heaths (>85 % of habitat N₂fixation), whereas the free-living diazotrophs had the highest N₂fixation 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 N₂fixation in any of the habitats despite its high ground cover. Moisture content seemed to be a major driver of N₂fixation in the lichen, feather moss and free-living diazotrophs. Our results show that the range of N₂fixers found in pristine habitats contribute differently to habitat N₂fixation and that ground cover of the associates does not necessarily mirror contribution.
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