Exposure to nitrogen does not eliminate N₂ fixation in the feather moss Pleurozium schreberi (Brid.) Mitt

Background and aims: The feather moss Pleurozium schreberi (Brid.) Mitt. is colonized by cyanobacteria, which fix substantial amounts of atmospheric nitrogen (N) in pristine and N-poor ecosystems. Cyanobacterial N₂ fixation is inhibited by N deposition. However, the threshold of N input that leads to the inhibition of N₂ fixation has not been adequately investigated. Further, the ability of N₂ fixation to recover in mosses from high N deposition areas has not been studied to date. Methods: We conducted two laboratory studies in which we (1) applied a range of concentrations of N as NH₄NO₃ to mosses from low N-deposition areas, and (2) we deprived mosses from a high N-deposition area of N to test their ability to recover N₂ fixation. Results: Higher addition rates (up to 10 kg N ha^-1) did not systematically inhibit N₂ fixation in P. schreberi. Conversely, upon weeks of N deprivation of mosses from a high N environment, N₂ fixation rates increased. Conclusions: The threshold of total N deposition above which N₂ fixation in P. schreberi is inhibited is likely to be > 10 kg N ha^-1. Further, cyanobacteria are able to recover from high N inputs and are able to fix atmospheric N₂ after a period of N deprivation.