Too much of a good thing? Inorganic nitrogen (N) inhibits moss-associated N2 fixation but organic N can promote it

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.