Moss-associated nitrogen (N 2 ) xation is one of the main inputs of new N in pristine ecosystems that receive low amounts of atmospheric N deposition. Previous studies have shown that N 2 xation is inhibited by inorganic N (IN) inputs, but if N 2 xation in mosses is similarly affected by organic N (ON) remains unknown. Here, we assessed N 2 xation in two dominant mosses in boreal forests (Pleurozium schreberi and Sphagnum capillifolium) in response to different levels of N, simulating realistic (up to 4 kg N ha −1 yr −1 ) and extreme N deposition rates in pristine ecosystems (up to 20 kg N ha −1 yr −1 ) of IN (NH 4 NO 3 ) and ON (alanine and urea). We also assessed if N 2 xation can recover from the N additions. In the realistic scenario, N 2 xation was inhibited by increasing NH 4 NO 3 additions in P. schreberi but not in S. capillifolium, and alanine and urea stimulated N 2 xation in both moss species. In contrast, in the extreme N additions, increasing N inputs inhibited N 2 xation in both moss species and all N forms.Nitrogen xation was more sensitive to N inputs in P. schreberi than in S. capillifolium and was higher in the recovery phase after the realistic compared to the extreme N additions. These results demonstrate that N 2 xation in mosses is less sensitive to organic than inorganic N inputs and highlight the importance of considering different N forms and species-speci c responses when estimating the impact of N inputs on ecosystem functions such as moss-associated N 2 xation.
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