Indirect effects of climate change reduce nitrogen fixation in the Subarctic

Some cyanobacteria, or blue-green bacteria, can turn atmospheric nitrogen (N) into forms that can be taken up by plants, like ammonium, by a process called nitrogen fixation. As a result, these cyanobacteria establish symbiotic relationships with several plants such as mosses. Moss-cyanobacteria associations are a crucial source of N for northern ecosystems like boreal forests and arctic/subarctic tundra. However, climate change will interfere with moss-cyanobacterial N fixation through both direct and indirect means according to the findings of a new study by scientists from the University of Copenhagen.

- “To investigate how climate change could also indirectly affect cyanobacterial activity on mosses, we sampled a field experiment that was carried out in collaboration with researchers from Lund University for three years in a subarctic tundra site near Abisko, in Swedish Lapland”, says Dr. Danillo Alvarenga from Section of Terrestrial Ecology at the Department of Biology, University of Copenhagen.

The plots were subjected to annual additions of ammonium, dried birch litter or a combination of both. These additions intended to simulate increased N mineralization in soil and higher litter deposition, respectively, which are known effects of climate change. After three years, the mosses in these plots were sampled and analyzed by Danillo Alvarenga together with Professor Kathrin Rousk, Department of Biology.

- Danillo Alvarenga continues, “The analyses unveiled a significant link between the colonization of moss branches by cyanobacteria, N fixation rates and pH of the moss. While ammonium additions significantly acidified the plots, reducing both cyanobacterial colonization and their N fixation rates, birch litter buffered the acidification slightly in treatments receiving both ammonium and litter additions, but at the cost of a further decrease in cyanobacterial colonization and N fixation”.

These results were recently published in the journal Science of the Total Environment.

Previous studies have shown that higher temperatures resulting from climate change can either stimulate or lower moss-associated N fixation activity depending on factors like moisture or season. However, this is the first time that climate change was shown to affect the activities and abundance of N₂-fixing cyanobacteria associated with mosses through indirect means.

Vascular plants can readily retrieve N from soil once it becomes available and may therefore benefit from increased N mineralization, but non-vascular plants like mosses have a limited capacity of N uptake from soil, which makes decreases in their number of cyanobacterial symbionts and their activity concerning. Since mosses are the dominant ground cover in the arctic and subarctic tundra, this will also have large consequences for northern ecosystems.

- “We hope that the results of this research will contribute to a better understanding of the ways in which climate change will interfere with biogeochemical cycles in northern ecosystems and help in the development of new strategies for the mitigation of its negative effects”, ends Danillo Alvarenga.