Short-term fate of nitrogen fixed by moss-cyanobacteria associations under different rainfall regimes

Nitrogen (N) fixation by moss-cyanobacteria associations has been recognized as an important N input pathway in many ecosystems from arctic tundra to tropical forests. However, the transfer of fixed N₂ from mosses to the soil as well as the effects of rainfall frequency and volume on this N transfer has hardly been studied – even though mosses can leach nutrients upon rewetting. In this study, we investigated the transfer of fixed N₂ by moss-cyanobacteria associations in one month under four watering regimes with a combination of high and low volume and frequency. For this, we used two morphologically similar moss species collected from ecosystems with different climate and N availability (subarctic - Hylocomium splendens; and tropical - Thuidium delicatulum). Acetylene reduction assays were conducted as a measure of N₂ fixation rates in mosses, and ¹⁵N-N₂ tracing was used to follow the fixed N₂ from moss to the underlying substrate. Nitrogen fixation rates were higher in T. delicatulum than in H. splendens, but rainfall volume and frequency did not show strong effects on N₂ fixation rates. Nonetheless, the extent of N leached from mosses was more sensitive to an increase in rainfall volume than to an increase in frequency, and more N was lost from T. delicatulum under high volume precipitation than from H. splendens. Both total nitrogen and ¹⁵N enrichment results demonstrate that the fixed N₂ was mostly stored in moss tissues with less than 1 % leached to the substrate. Our results show that both moss species retain almost all fixed N₂ within their tissues under small rainfall disturbances within one month, while increased N availability under higher precipitation volume renders some moss species an important N source for the soil.