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Soil properties and functions are largely determined by the amount of available organic matter, which is most significant for the vital activity of microorganisms. The influence of the content of available soil organic matter on the composition and functioning of microbial community during the transformation of barley straw in the soil was studied. Samples from the upper 20-cm-thick layer of soddy-podzolic soil (Umbric Albic Retisols (Abruptic)) were incubated (T = 25°C, 60% TFC) for 4 months, and a model soil with a reduced content of available organic matter was obtained. Then, we performed a laboratory experiment with the application of barley straw into native and model soils. The dynamics (0, 7, 40, and 70 days) of soil respiration, microbial biomass (SIR), the number of microorganisms, and the taxonomic composition of the microbial community (16S-rRNA sequencing) were analyzed. The quality of soil organic matter was assessed from data on the contents of total organic carbon (Ctot) and its available (Cavail), water-soluble (Cwat), and labile (Clab) fractions. It was shown that the amount of Cwat and Cavail in the model soil decreased by half in the absence of significant differences in Ctot. The aggregate structure of the soil worsened, and the straw transformation proceeded 25% faster as compared with that in the native soil. The taxonomic composition of the soil bacterial community was influenced by the amount of available organic matter in the soil, the addition of straw, and the incubation period. A decrease in the content of available organic matter caused changes in the composition of the microbial community: the portions of the phyla Acidobacteria and Firmicutes decreased, while those of Actinobacteria, Bacteroidetes, Chloroflexi, Planctomycetes, and archaea increased. It was found that the main part (50% of the total) of the microbial community of the depleted soil is characterized by great diversity and oligotrophy. Indicators for comparing soils by the degree of oligotrophy of the microbial community are proposed on the basis of data on its taxonomic composition. [ABSTRACT FROM AUTHOR]

Significance of soil genesis for the composition and succession of bacterial communities during straw decomposition has been studied. Sterile grinded oat straw in nylon bags was placed in humus horizons of two soils (soddy-podzolic and chernozem; Albic Retisol (Abruptic, Ochric) and Haplic Chernozem (Pachic)) and incubated in the laboratory under constant hydrothermal conditions. On the basis of respiration dynamics (measured by absorption method) the stages of straw decomposition (early, middle and late) were specified. For each stage, the composition of bacterial communities was determined by high-performance sequencing of amplicon libraries of the variable region V4 of the 16S rRNA gene. It has been shown that carbon losses during decomposition of straw from the soddy-podzolic soil were higher than those from chernozem: 3913 ± 668 and 2981 ± 297 CO2 mg/m2 day, respectively. Phylogenetic analysis showed an increase in the diversity of the bacterial community in the process of straw decomposition: from 51 and 79 on the third day, up to 213 and 310 phylotypes from 17 phyla on the last day, for chernozem and soddy-podzolic soil, respectively. On the third day, Proteobacteria, Firmicutes, and Bacteroidetes were mainly present; on the 161st day, Actinobacteria were added. Soil had a significant impact on the taxonomic composition of straw communities. For the microbial community of straw formed on the basis of the microbiocenosis of soddy-podzolic soil the proportion of Proteobacteria was higher, but Bacteroidetes was lower, and the contribution of unidentified prokaryotes was more significant. Bacterial succession during straw decomposition had common features for both soils. The decomposition started with the quick consuming of easily available organic compounds of straw by copiotrophs, representatives of the Pseudomonas genus, the Stenotrophomonas genus from the family of Xanthomonaceae, and the Chitinophaga (Bacteroidia) genus. In addition to actinobacteria, the oligotrophs were represented by Firmicutes and part of Bacteroidetes. Throughout the entire observation period, probably due to the formation of available organic compounds during the decomposition of straw, the contribution of bacteria of the Rhizobiaceae family was significant. [ABSTRACT FROM AUTHOR]