Global nitrogen fixation patterns in moss-cyanobacteria associations

Nitrogen (N) fixation by cyanobacteria on mosses is a critical N source, particularly in moss-abundant and pristine ecosystems such as boreal forests, where it is estimated to contribute over 50% of total ecosystem N input. However, the upscaling of these field estimates in N fixation carries considerable uncertainty because they rely on point sampling of a limited number of species, which fails to capture the potential large spatial and temporal variation in N fixation. As a result, the global spatial pattern of moss-associated N fixation remains poorly understood, limiting the assessment of its relative importance at the global scale. Additionally, modeling global N fixation rates is constrained by the lack of a comprehensive understanding of how moss-associated N fixation relates to the full range of key abiotic drivers across diverse climate zones.

To address these uncertainties, we measured the response of N fixation rates in 2-3 dominant moss species across five climate zones (arctic, boreal, temperate, mediterranean, and tropical) to a full range of key abiotic drivers (i.e., water content, surface temperature, and incident light intensity). By identifying the key parameters of each response curve, we integrated the N fixation process into the process-based model LiBry. We then applied this extended LiBry model to simulate the global pattern of N fixation and assess its relative importance across climate zones.

Our results reveal that different species within the same climate zones exhibit similar response curves to light, water, and temperature, with comparable optimum values for temperature, light, and water content. Moreover, the optimum temperature (~27 °C) and water content (~100%) remain consistent not only across species but also across climate zones, regardless of variations in local temperature and humidity conditions, while the optimum light intensity varied among climate zones. However, moss-associated N fixation rates were higher in boreal, tundra, and mediterranean habitats compared to tropical lowlands and temperate regions. These results indicate that the responses of N fixation to temperature, light, and water content show little species specificity, and further, optimum temperature and water content were unaffected by sample origin (climate).

Our study provides the first global assessment of N fixation in moss-cyanobacteria associations, highlighting variation across climate zones. Our results also stress the need for ecosystem models to incorporate moss-associated N fixation as an essential component of total ecosystem N input, especially in unpolluted systems such as boreal and mediterranean forests and arctic tundra.