Moss-associated nitrogen (N₂) fixation is one of the main inputs of new N in pristine ecosystems that are characterized by low N availability. Previous studies have shown that N₂ fixation is inhibited by inorganic N (IN) inputs, but if N₂ fixation in mosses is similarly affected by organic N (ON) remains unknown. Here, we assessed N₂ fixation 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 year⁻¹) and extreme N addition rates in pristine ecosystems (up to 20 kg N ha⁻¹ year⁻¹) of IN (ammonium nitrate) and ON (alanine and urea). We also assessed if N₂ fixation can recover from the N additions. In the realistic scenario, N₂ fixation was inhibited by increasing NH₄NO₃ additions in P. schreberi but not in S. capillifolium, and alanine and urea stimulated N₂ fixation in both moss species. In contrast, in the extreme N additions, increasing N inputs inhibited N₂ fixation in both moss species and all N forms. Nitrogen fixation 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 N₂ fixation in mosses is less sensitive to organic than inorganic N inputs and highlight the importance of considering different N forms and species-specific responses when estimating the impact of N inputs on ecosystem functions such as moss-associated N₂ fixation.