Dependence of Soil Bacterial Community on the Amount of Available Organic Matter during Straw Decomposition

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 degrees 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 (C-tot) and its available (C-avail), water-soluble (C-wat), and labile (C-lab) fractions. It was shown that the amount of C-wat and C-avail in the model soil decreased by half in the absence of significant differences in C-tot. 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.