Tundra moss transplants reveal host species-specific response of associated N2 fixation rates to environmental change

Background/Question/Methods

Rapid warming in Arctic and boreal ecosystems is expected to alter moss biomass and diversity. Process rates of moss associated N2 fixation, the largest source of novel nitrogen (N) to a nutrient-limited environment, are also expected to change directly with the environment and indirectly through changes in host moss assemblages. Understanding the interaction between moss identity and environmental change in determining rates of N2 fixation can improve future N input predictions in vulnerable regions which store vast amounts of C. We measured vascular and non-vascular community composition and moss species-specific rates of N2 fixation in a reciprocal tundra transplant experiment between Healy (63°N, 149°W) and Toolik Lake (68°N,149°W), Alaska, sites which have similar moist acidic tussock plant communities but differ by over 5°C in mean annual temperature. We hypothesized that transplanted mosses would not differ in N2fixation rates from their home environment within a moss species one year after transplantation due to the key role of host identity in determining associated N2 fixation rates.

Results/Conclusions

We found that the magnitude of response to transplantation varied greatly among mosses. While Hylocomium splendens showed no change associated with transplantation, A. turgidum varied moderately by location and P. schreberihad a strong increase in N2 fixation rates associated with movement to a colder environment. Overall, N2fixation rates were lower in our warmer site both for native and transplanted mosses. We saw no change in vascular or non-vascular species composition with transplantation, though both communities were distinct between home locations. We conclude that moss-associated N2 fixation responses to climate changes are host-dependent. Previous experiments have frequently seen a decline in moss biomass and moss diversity over time in response to warming. The host-specific responses of N2 fixation to changing temperature could precede or contribute to these observed declines.