Year-Round Variation in Bryophyte-Associated Nitrogen Fixation in the Arctic

In northern biomes, growth is nitrogen (N) limited, but bryophytes are abundant. These bryophytes often host N 2 -fixing microorganisms (diazotrophs) that play a crucial role in the N cycle of these ecosystems. Despite their importance, how the bryophyte-associated N 2 -fixation varies across species and seasons (summer, autumn, winter, and spring) remains poorly understood. We measured N 2 -fixation rates for 10 bryophyte species in situ throughout the entire year in the Arctic with additional incubations to verify the method. We measured positive N 2 -fixation during most of the year, except for the coldest period (February). The species growing in the wettest conditions ( Sphagnum spp.) had the highest N 2 -fixation rates in summer, while bryophytes in drier conditions peaked in N 2 -fixation rates in spring and autumn. The seasonal variation in N 2 -fixation activity was pronounced, but similar patterns were found among different species. This study reveals that bryophyte-associated N 2 -fixation in northern ecosystems is larger than previously assumed, as it occurs over a more extended part of the year than previously inferred. Furthermore, the importance of bryophyte-associated diazotrophs cannot be quantified without considering both the diversity of bryophytes and their variation in N 2 -fixing seasonal activity patterns. Both future changes in climatic conditions and biodiversity of bryophytes can thus have large implications for the N cycle in arctic regions.