Moss-cyanobacteria associations as biogenic sources of nitrogen in boreal forest ecosystems

The biological fixation of atmospheric nitrogen (N) is a major pathway for available N entering ecosystems. In N-limited boreal forests, a significant amount of N ₂ is fixed by cyanobacteria living in association with mosses, contributing up to 50% to the total N input. In this review, we synthesize reports on the drivers of N ₂ fixation in feather moss-cyanobacteria associations to gain a deeper understanding of their role for ecosystem-N-cycling. Nitrogen fixation in moss-cyanobacteria associations is inhibited by N inputs and therefore, significant fixation occurs only in low N-deposition areas. While it has been shown that artificial N additions in the laboratory as well as in the field inhibit N ₂ fixation in moss-cyanobacteria associations, the type, as well as the amounts of N that enters the system, affect N ₂ fixation differently. Another major driver of N ₂ fixation is the moisture status of the cyanobacteria-hosting moss, wherein moist conditions promote N ₂ fixation. Mosses experience large fluctuations in their hydrological status, undergoing significant natural drying and rewetting cycles over the course of only a few hours, especially in summer, which likely compromises the N input to the system via N ₂ fixation. Perhaps the most central question, however, that remains unanswered is the fate of the fixed N ₂ in mosses. The cyanobacteria are likely to leak N, but whether this N is transferred to the soil and if so, at which rates and timescales, is unknown. Despite our increasing understanding of the drivers of N ₂ fixation, the role moss-cyanobacteria associations play in ecosystem-N-cycling remains unresolved. Further, the relationship mosses and cyanobacteria share is unknown to date and warrants further investigation.