Vast areas of land in the forest-steppe of West Siberia are occupied by birch forests, the most common ecosystems there. However, currently, little is known about the microbiome composition in the underlying soil, especially along a sequence of soil genetic horizons. The study aimed at inventorying microbiome in genetic horizons of a typical Phaeozem under undisturbed birch forest in West Siberia. Bacteria and fungi were studied using 16S rRNA genes’ and ITS2 amplicon sequencing with Illumina MiSeq. Proteobacteria and Acidobacteria together accounted for two-thirds of the operational taxonomic units (OTUs) numbers and half of the sequences in each genetic horizon. Acidobacteria predominated in eluvial environments, whereas Proteobacteria, preferred topsoil. The fungal sequences were dominated by Ascomycota and Basidiomycota phyla. Basidiomycota was the most abundant in the topsoil, whereas Ascomycota increased down the soil profile. Thelephoraceae family was the most abundant in the A horizon, whereas the Pyronemataceae family dominants in the AEl horizon, ultimately prevailing in the subsoil. We conclude that soil genetic horizons shape distinct microbiomes, therefore soil horizontation should be accounted for while studying undisturbed soils. This study, representing the first description of bacterio- and mycobiomes in genetic horizons of the Phaeozem profile, provides a reference for future research.

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Background. The aim of the study was to assess the composition and structure of bacterial assemblages by estimating 16S rRNA gene sequences diversity in a young Technosol, developing on a revegetating fly ash dump. Location and place of the study. Two soils were chosen for the study in the environs of the thermoelectric power station No.5 in Novosibirsk region: 1) nine years old Technosol (54°59' NL, 83°03' EL), developing on the spontaneously revegetating fly ash dump, and 2) Phaeozem under undisturbed white birch forest (55°00' NL, 83°04' EL). Soils were sampled from the 0-5 cm layer as three individual replicates each. Materials and methods. The total DNA extracted from soil samples was used as a matrix for PCR amplification using V-3-V4 primers for 16S rRNA genes. The obtained amplicons were sequenced on Illumina MiSeq platform in the Genomics Core Facility of the Institute of Chemical Biology and Fundamental Medicine SB RAS (Novosibirsk, Russia). Descriptive statistics and principal components analysis were used to analyze and present the data. Results. Overall 4887 different operational taxonomic units (OTUs) were found at the 97% similarity level; the OTUs were ascribed to 306 genera, 212 families, 123 orders, 84 classes and 32 phyla of bacteria domain. At the phylum level bacterial assemblages in both soils were dominated by Proteobacteria, Acidobacteria and Actinobacteria, showing rather similar outline of the phyla relative abundance, confirming the direction of pedogenesis towards Phaeozem. Higher abundance of Chloroflexi a physiologically diverse phyla with aerobic and anaerobic thermophiles, anoxigenic phototrophs and anaerobes, able to respire organic halides, in the young Technosol complies with harsh edaphic conditions there. Although the relative abundance of Rhizobiales, Actinomycetales and unclassified Acidobacteria_Gp6 and Acidobacteria_Gp3 was found to be 1.5–2.4 times higher in the undisturbed Phaeozem, they also were predominating in the Technosol samples. The α-biodiversity indices, pertaining mostly to species/OTUs richness, i.e. Chao-1, OTUs number, Margalef, Fisher’s alpha, were higher in Phaeozem, whereas evenness and equitability were higher in Technosol. Both soils demonstrated similar Shanno indices, which were rather high (6.3). Conclusions. Nine years of spontaneous revegetation on the terminated fly ash dump of the thermoelectric power station resulted in the development of the Technosol, containing quite rich and diverse bacterial assemblage, which was rather close in structure at the phylum taxonomic level to the adjacent undisturbed Phaeozem. Overall the early stages of pedogenesis were dominated by specific bacterial assemblage (Chloroflexi, Xanthomonadales, Geobacter, Aciditerrimonas, Iamiaceae and some others), performing weathering of the pedogenic substrate, i.e. fly ash. The present of nitrificators (Nitrospira), denitrificators денитрификаторов (Reyranella) together with diazotrophs (Bradyrhizobium, Rhizobiales) dominance evidences the establishment of the entire network of nitrogen transformation process already at the early stages of revegetation of the fly ash dump. Large percentage (17%) of Bacteria, the information about which ribosomal gene sequences is most likely absent in the respective data bases necessitates more detailed research into the soil microbiome on fly ash dumps.