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 N 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 N fixers for ecosystem N cycling, we tracked N-N into four N-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 C from CO labelling to assess if N and C fixation are linked. Three days after labelling, half of the fixed N was recovered in the legume soils, indicating a fast release of fixed N. Within 5 weeks, the free-living N fixers released two-thirds of the fixed N into the soil, whereas the lichen and moss retained the fixed N. Carbon and N fixation were linked in the lichen shortly after labelling, in free-living N fixers 5 weeks after labelling, and in the moss at both sampling times. The four investigated N-fixer associations released fixed N at different rates into the soil, and non-diazotroph-associated plants have no access to ‘new’ N within several weeks after N fixation. Although legumes and free-living N 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 N fixation in the short term.