Nitrogen fixation associated with two cohabiting moss species expresses different patterns under Cu and Zn contamination

Nitrogen (N(2)) fixation by moss-associated cyanobacteria is an important N source in pristine ecosystems. Previous studies have shown that moss-associated N(2) fixation is sensitive to anthropogenic N pollution. However, we still lack understanding of the effects of other factors derived from anthropogenic sources, such as heavy metal pollution on N(2) fixation. To test this, we collected two dominant mosses (Pleurozium schreberi and Spaghnum palustre) from a temperate bog in Denmark and assessed their N(2) fixation responses to simulated heavy metal pollution by adding 5 levels (plus a control) of copper (Cu, 0–0.05 mg g dw(−1)) and zinc (Zn, 0–0.1 mg g dw(−1)). Metal concentrations in both mosses increased linearly with Cu and Zn addition, but N(2) fixation activity associated with S. palustre was to a greater extent negatively affected by both Cu and Zn additions than that associated with P. schreberi. Copper additions even promoted N(2) fixation in P. schreberi. Hence, the heavy metal sensitivity of N(2)-fixing cyanobacteria is dependent on the host moss-species, and the vulnerability of ecosystems towards heavy metal pollution could vary depending on the dominant moss species. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s11356-023-28404-0.