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.

苔藓相关的氮 (N ₂ ) 固定是原始生态系统中新 N 的主要输入之一，其特点是 N 可用性低。先前的研究表明，N ₂固定受到无机 N (IN) 输入的抑制，但苔藓中的 N ₂固定是否同样受到有机 N (ON) 的影响仍然未知。在这里，我们评估了北方森林中两种主要苔藓（Pleurozium schreberi和Sphagnum capillifolium ）中 N ₂的固定，以响应不同水平的 N，模拟现实（高达 4 kg N ha ^-1 年^-1）和极端 N 添加率原始生态系统（高达 20 kg N ha ^-1 年^-1) 的 IN（硝酸铵）和 ON（丙氨酸和尿素）。我们还评估了 N ₂固定是否可以从 N 添加中恢复。在现实情况下，增加P. schreberi中的 NH ₄ NO ₃添加量会抑制N ₂固定，但在S. capillifolium中则不会，丙氨酸和尿素刺激两种苔藓中的N _2固定。相反，在极端的 N 添加中，增加 N 输入抑制了苔藓物种和所有 N 形式的 N ₂固定。固氮对P. schreberi的 N 输入比S. capillifolium更敏感与极端 N 添加相比，在现实后的恢复阶段更高。这些结果表明，苔藓中的 N ₂固定对有机 N 输入的敏感性低于无机 N 输入，并强调了在估计 N 输入对生态系统功能（例如与苔藓相关的 N ₂固定）的影响时考虑不同 N 形式和物种特异性反应的重要性.