Nitrogen fixation rates of ground-dwelling mosses and the influence of forest gaps in fir forest in western Sichuan

Symbiotic nitrogen fixation by moss-cyanobacteria associations is a critical process that contributes up to 50% of the nitrogen input in many ecosystems. However, how the nitrogen fixation rate of the moss-cyanobacteria association is affected by ecosystem dynamics remains unknown. This lack of clarity is a substantial impediment to effectively modeling ecosystem N budgets. Here, we investigated dynamic changes in the nitrogen fixation rates of understory mosses in a typical fir forest in the eastern Tibetan Plateau during the growing season and the influence of forest gaps on this process. The results showed that Actinothuidium hookeri and Hylocomium splendens exhibited average nitrogen fixation rates of 77.49 and 129.69 nmol·(g·d)⁻¹, respectively, during the growing season. Nitrogen fixation rates displayed significant seasonal variation, with peaks in May, July, and August, coinciding with peak plant growth periods. Water limitation in June resulted in minimal nitrogen fixation. The rates in October were also lower, reaching only one-seventh of those observed in July. Forest gaps increased the surface temperature, light availability, and water content of mosses. However, forest gaps had only a weak positive effect on the nitrogen fixation rate of mosses. In summary, these results provide a basis for understanding nitrogen cycling in ecosystems.