Nitrogen fixation in Arctic lichens and mosses: A survey across circumpolar subzones

Nitrogen bioavailability frequently constrains primary production in the Arctic with tundra communities vulnerable to ecological and metabolic disruption from climate variability. Diazotrophs associated with lichens and mosses are the primary source of new nitrogen (N) in the Arctic. We made 526 laboratory measurements of biological nitrogen fixation (BNF) in 272 lichens and 254 bryophytes representing 23 and 39 genera, respectively. These samples were collected from 49 tundra sites across the Arctic. We found 65 % of lichen and 44 % of bryophyte genera analyzed fixed N. We also identified potentially new cryptogam-diazotroph relationships in the lichen genera Asahinea, Nephromopsis and Thamnolia and the bryophyte genera Dicranoweisia and Amphidium. We found that while over 95 % of individual bryophyte samples fixed N within three months of storage, this dropped to less than 15 % after one year at room temperature. Individual lichen samples maintained a stable ~45 % fixation rate over nearly two years of frozen storage. Our experiments highlight the complexity in establishing robust BNF measurements required for model simulations.