Nitrogen (N) added through atmospheric deposition or as fertilizer to boreal and temperate forests reduces both soil decomposer activity (heterotrophic respiration) and the activity of roots and mycorrhizal fungi (autotrophic respiration). However, these negative effects have been found in studies that applied relatively high levels of N, whereas the responses to ambient atmospheric N deposition rates are still not clear. Here, we compared an unfertilized control boreal forest with a fertilized forest (100 kg N ha−1 yr−1) and a forest subject to N-deposition rates comparable to those in Central Europe (20 kg N ha−1 yr−1) to investigate the effects of N addition rate on different components of forest floor respiration and the production of ectomycorrhizal fungal sporocarps. Soil collars were used to partition heterotrophic (Rh) and autotrophic (Ra) respiration, which was further separated into respiration by tree roots (Rtr) and mycorrhizal hyphae (Rm). Total forest floor respiration was twice as high in the low N plot compared to the control, whereas there were no differences between the control and high N plot. There were no differences in Rh respiration among plots. The enhanced forest floor respiration in the low N plot was, therefore, the result of increased Ra respiration, with an increase in Rtr respiration, and a doubling of Rm respiration. The latter was corroborated by a slightly greater ectomycorrhizal (EM) fungal sporocarp production in the low N plot as compared to the control plot. In contrast, EM fungal sporocarp production was nearly eliminated, and Rm respiration severely reduced, in the high N plot, which resulted in significantly lower Ra respiration. We thus found a nonlinear response of the Ra components to N addition rate, which calls for further studies of the quantitative relations among N addition rate, plant photosynthesis and carbon allocation, and the function of EM fungi.