Temporal variation of nitrogen fixation in moss-cyanobacteria associations in the Mediterranean region: integrating experiments and process-based modelling

Biological nitrogen (N) fixation by cyanobacteria on mosses is a critical N source for pristine ecosystems, but studies have largely focused on northern ecosystems, such as boreal and arctic regions. The contribution of moss-associated N fixation in Mediterranean ecosystems remains largely neglected, despite the high abundance of mosses in Mediterranean forests and previous evidence of substantial N fixation activity by their associated cyanobacteria. Here, we combined high-frequency in situ measurements of N fixation in Mediterranean mosses from forest and open sites with a process-based model that incorporates moss-associated N fixation responses to key climatic drivers (light, temperature, humidity). This integrated approach allows us for the first time to simulate diurnal and seasonal dynamics of N fixation and to upscale these dynamics to estimate annual N fixation rates. Our results highlight substantial nocturnal N fixation and indicate that large-scale estimates of N fixation across space and time derived from limited, single-time-point field measurements may be associated with considerable uncertainty. The presented model provides a new framework for simulating N fixation by moss-associated cyanobacteria.