Purpose

In many northern ecosystems, mosses account for an important component of the plant community. Further, mosses host nitrogen (N₂)-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 N2 fixation.

Methods

We assessed how N₂ 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 N₂ fixation activity and assessed the temperature sensitivity of N₂ 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 N₂ fixation in response to 10 years of manipulated field-warming, whereas a shift in dominant shrub species did not impact moss cover or N₂ fixation activity. Interestingly, the moss H. splendens was more sensitive than P. schreberi to warming both in terms of cover and N₂ fixation activity.

Conclusion

Our study shows that warming affects moss-associated N₂ fixation activity negatively - both indirectly via a decline in moss cover and directly by reducing N₂ fixation activity. This suggests lower N input via moss-associated N₂ fixation in a future climate, which will consequently affect nutrient turnover and ecosystem productivity.