The fixation of atmospheric nitrogen (N) is a major pathway for available N entering ecosystems. In N-limited boreal forests, a significant amount of N is fixed by cyanobacteria that live epiphytically on feather mosses. Despite the dominance of bryophytes in the boreal biome and their association with cyanobacteria, the role of feather mosses in boreal N cycling remains poorly understood. Further, evaluating abiotic controls on N2 fixation in the feather moss-cyanobacteria association is a challenge yet to be addressed. Therefore, the aims of this thesis were to identify the main factors that drive N2 fixation in the feather moss-cyanobacteria association, and to gain a deeper understanding of the feather moss’ role for the N cycle in boreal forests. The effects of natural and artificial N additions, N deprivation as well as the impact of drying and rewetting events on N2 fixation rates in the ubiquitous- and cyanobacteria-hosting feather moss Pleurozium schreberi (Brid.) Mitt. were analyzed. Further, the ability of P. schreberi to take up N from soil was tested, assessing another possible pathway through which the moss can utilize N. The cyanobacterial contribution to the decomposition resistance of the moss was evaluated; and finally, when and how the fixed N enters the soil to become available for plants and microbes was assessed. The results of my work demonstrated that N2 fixation is negatively affected by natural as well as artificial N inputs, but the inhibition of N2 fixation is dependent on the amount of added N and further, N2 fixation seems to be resilient to N deposition as well as to drying-rewetting events indicated by the increase of N2 fixation rates in the moss-cyanobacteria association upon removal of the stressors. P. schreberi was found to be able to take up organic and inorganic N from soil, but it accounted for a small fraction of total N demand. Cyanobacteria do not contribute to the moss’ resistance towards decomposition by soil bacteria. Feather mosses effectively retain the acquired N, thus representing a short-term N sink. By using lab as well as field-based approaches to identify factors that drive N2 fixation in the feather moss-cyanobacteria association, I gained a deeper understanding of a major component of the N cycle in boreal forests. The moss was shown to have the ability to use soil-N and to retain N over several months, however, the moss likely represents a long-term N source to the boreal system, due to its association with N2-fixing cyanobacteria. Nevertheless, assessing the transfer and exchange of N and carbon (C) between feather moss and cyanobacteria, and defining the interface of this, would improve our understanding of their role in ecosystem nutrient cycling and should be subject to future research.