Climate Change Impacts the Structure and Nitrogen‐Fixing Activities of Subarctic Feather Moss Microbiomes Across a Precipitation Gradient

ABSTRACT Associations between feather mosses and cyanobacteria are crucial sources of new biologically available nitrogen (N) in arctic and subarctic ecosystems. The physiology of both mosses and cyanobacteria is strongly influenced by environmental factors such as temperature and moisture, which directly affect N 2 fixation rates. These associations may be threatened by climate change, since it leads to warmer and drier conditions in polar regions. In this study, we investigated the N 2 ‐fixing microbial communities associated with two common feather mosses across a precipitation gradient in the subarctic tundra, followed by a temperature and moisture experiment. Using acetylene reduction assays, nifH gene sequencing and qPCR, we evaluated how shifts in temperature and moisture influence nitrogenase activity and N 2 ‐fixing community structure. Our results showed that N 2 fixation was highest in sites with greater precipitation and increased with both temperature and moisture. Cyanobacteria dominated N 2 ‐fixing communities, but currently unclassified bacteria also seemed to play a significant role, particularly at higher temperatures. The number of cyanobacterial nifH copies tended to remain stable or decrease with temperature, while the relative abundance of unclassified bacteria increased. These findings suggest that the N 2 ‐fixing activity, abundance, and diversity of cyanobacteria associated with feather mosses in the subarctic will decline under warmer and drier conditions, potentially leading to a shift in the composition of feather moss‐associated microbial communities in a warmer Arctic, with potential consequences for N input into the ecosystem.