Abstract Background In contrast to earlier assumptions, there is now mounting evidence for the role of tundra soils as important sources of the greenhouse gas nitrous oxide (N 2 O). However, the microorganisms involved in the cycling of N 2 O in this system remain largely uncharacterized. Since tundra soils are variable sources and sinks of N 2 O, we aimed at investigating differences in community structure across different soil ecosystems in the tundra. Results We analysed 1.4 Tb of metagenomic data from soils in northern Finland covering a range of ecosystems from dry upland soils to water-logged fens and obtained 796 manually binned and curated metagenome-assembled genomes (MAGs). We then searched for MAGs harbouring genes involved in denitrification, an important process driving N 2 O emissions. Communities of potential denitrifiers were dominated by microorganisms with truncated denitrification pathways (i.e., lacking one or more denitrification genes) and differed across soil ecosystems. Upland soils showed a strong N 2 O sink potential and were dominated by members of the Alphaproteobacteria such as Bradyrhizobium and Reyranella . Fens, which had in general net-zero N 2 O fluxes, had a high abundance of poorly characterized taxa affiliated with the Chloroflexota lineage Ellin6529 and the Acidobacteriota subdivision Gp23. Conclusions By coupling an in-depth characterization of microbial communities with in situ measurements of N 2 O fluxes, our results suggest that the observed spatial patterns of N 2 O fluxes in the tundra are related to differences in the composition of denitrifier communities.

In-depth characterization of denitrifier communities across different soil ecosystems in the tundra