Direct and indirect effects of warming on moss abundance and associated nitrogen fixation in subarctic ecosystems

Purpose In many northern ecosystems, mosses account for an important component of the plant community. Further, mosses host nitrogen (N-2)-fixing bacteria that have been shown to contribute a large part of 'new' N in pristine ecosystems. This study aimed to assess direct and indirect effects of warming on moss-associated N-2 fixation. Methods We assessed how N-2 fixation activity at ecosystem- and at moss species-level is affected by direct (warming) and indirect (vegetation shift) climate change using two long-term field manipulation experiments in a subarctic birch forest combined with mesocosm studies from a close-by heath. We linked the abundance of ground covering mosses to N-2 fixation activity and assessed the temperature sensitivity of N-2 fixation in two common feather mosses, Hylocomium splendens and Pleurozium schreberi. Results Our study shows a steep decline in moss ground cover as well as in associated N-2 fixation in response to 10 years of manipulated field-warming, whereas a shift in dominant shrub species did not impact moss cover or N-2 fixation activity. Interestingly, the moss H. splendens was more sensitive than P. schreberi to warming both in terms of cover and N-2 fixation activity. Conclusion Our study shows that warming affects moss-associated N-2 fixation activity negatively - both indirectly via a decline in moss cover and directly by reducing N-2 fixation activity. This suggests lower N input via moss-associated N-2 fixation in a future climate, which will consequently affect nutrient turnover and ecosystem productivity.

Automated summary

The study found that long-term warming negatively affects moss ground cover and associated nitrogen-fixation activities, while a shift in dominant shrub species did not impact moss cover or nitrogen-fixation activity. Interestingly, it was observed that H. splendens was more sensitive to warming compared to P. schreberi in terms of both cover and nitrogen-fixation activity.