Sebastian Petters, Milan Varsadiya, Patrick Liebmann, Jörg Schnecker, Georg Guggenberger, Jiří Bárta, Tim Urich

• Large amounts of terrestrial organic carbon (OC) are stored in Arctic permafrost-affected soils. Through processes of cryoturbation and solifluction, the subsoils can contain subducted topsoil material, which largely contribute to the large OC storage in these soils. While the bacterial, archaeal, and fungal communities in such soils have been studied to some degree, information about protists and meso- and macrofauna is scarce, although these groups might substantially contribute to OC processing, through e.g., food web interactions. Different organic and mineral horizons, including subducted topsoil material, of Arctic soils were investigated using a metatranscriptomics three-domain community profiling approach. Soil horizons were compared in regards to their total microbial community composition including all three domains of life. Furthermore, abundances of different pro- and eukaryotic micropredators were examined and a variety of functional groups involved in the carbon (C) and the nitrogen (N) cycle were analyzed in relation to specific taxonomic groups and abiotic soil parameters. Our study showed that RNA yields positively correlated with the OC content of the horizon and that the composition of the microbial community in subducted topsoil material rather matched that of mineral subsoils instead of organic top horizons. Horizon-resolved profiling revealed heterogeneity in the associated microbiomes and showed major differences in microbiomes of topsoil and subducted topsoil. The abundance of protist and nematode micropredators decreased in subducted topsoil, while predatory myxobacteria remained remarkably constant and comprised high proportions of the total communities in all horizons. Correlations analysis between functional guilds and biotic and abiotic parameters suggest a major impact of predatory myxobacteria on carbon and nitrogen cycles of subducted topsoils. The study adds urgently needed information about the total biota structure in permafrost soils and first insights into the associated soil microbial food webs.