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

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 (N₂O). However, the microorganisms involved in the cycling of N₂O in this system remain largely uncharacterized. Since tundra soils are variable sources and sinks of N₂O, we aimed at investigating differences in community structure across different soil ecosystems in the tundra.

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 N₂O 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 N₂O sink potential and were dominated by members of the Alphaproteobacteria such as Bradyrhizobium and Reyranella. Fens, which had in general net-zero N₂O fluxes, had a high abundance of poorly characterized taxa affiliated with the Chloroflexota lineage Ellin6529 and the Acidobacteriota subdivision Gp23.

By coupling an in-depth characterization of microbial communities with in situ measurements of N₂O fluxes, our results suggest that the observed spatial patterns of N₂O fluxes in the tundra are related to differences in the composition of denitrifier communities.