Nonuniform temperature and light relationships of moss-associated nitrogen fixation across climates

Nitrogen (N) fixation by moss-associated cyanobacteria is a key N source in terrestrial ecosystems and thereby a critical limiting factor for ecosystem productivity. However, the responses of this process to variation in key abiotic factors are unknown. In particular, the lack of knowledge on how these responses vary across climates limits our ability to estimate and predict the global importance of this key ecosystem function. We established N fixation response curves in dominant mosses across five ecosystems with different climates, from arctic to tropical forests, to light (0-1500 μmol m^-2 s^-1), moisture (0-100% moss moisture), and temperature (< 10-40°C), and derived key physiological traits from these. Relationships between N fixation and both light and temperature varied across climatically contrasting ecosystems, with light optima of c. 500-800 μmol m^-2 s^-1 and temperature optima of c. 28-33°C. By contrast, the amount of water required to sustain N fixation was similar across ecosystems, regardless of local humidity. Our results highlight that microbial functional responses are not uniform across climates. The here quantified relationships between N fixation and key abiotic factors across climatically contrasting ecosystems provide the foundation for integrating moss-associated N fixation into vegetation models.