Nitrogen Transfer from Four Nitrogen-Fixer Associations to Plants and Soils

Nitrogen (N) fixation is the main source of ‘new’ N for N-limited ecosystems like subarctic and arctic tundra. This crucial ecosystem function is performed by a wide range of N₂ fixer (diazotroph) associations that could differ fundamentally in their timing and amount of N release to the soil. To assess the importance of different associative N₂ fixers for ecosystem N cycling, we tracked ¹⁵N-N₂ into four N₂-fixer associations (with a legume, lichen, free-living, moss) and into soil, microbial biomass and non-diazotroph-associated plants 3 days and 5 weeks after in situ labelling. In addition, we tracked ¹³C from ¹³CO₂ labelling to assess if N and C fixation are linked. Three days after labelling, half of the fixed ¹⁵N was recovered in the legume soils, indicating a fast release of fixed N₂. Within 5 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. Carbon and N₂ fixation were linked in the lichen shortly after labelling, in free-living N₂ fixers 5 weeks after labelling, and in the moss at both sampling times. The four investigated N₂-fixer associations released fixed N₂ at different rates into the soil, and non-diazotroph-associated plants have no access to ‘new’ N within several weeks after N₂ fixation. Although legumes and free-living N₂ fixers are immediate sources of ‘new’ N for N-limited tundra ecosystems, lichens and especially mosses, do not contribute to increase the N pool via N₂ fixation in the short term.