Transformation of Microbial Complexes in Components of Soil Constructions of Different Origin (Soil, Peat, Sand) during Freezing-thawing Processes

In a model experiment, the transformation of microbial complexes of cultivated saprotrophic bacteria and yeasts during freezing-thawing was studied in various natural substrates that are used to create soil constructions for urban landscaping and for growing herbaceous plants. The number of saprotrophic bacteria and yeasts depended both on the type of substrate and on temperature changes during freezing-thawing. At the stage of freezing of peat and soil (arable horizon) to 0 and –5°C and at the subsequent stage of thawing to 0°C, a significant increase in yeast number was registered. The maximum number of yeasts in soil and peat was 5.1 log (CFU/g). In contrast to the number of yeasts, number of saprotrophic bacteria in soil and peat was characterized by a sharp decrease when the substrate temperature was negative and peaked at 19–22 and 10°C, respectively. The maximum bacterial number in soil and peat was 7.5 and 8.0 log (CFU/g), respectively. In sand, number of both saprotrophic bacteria and yeasts did not depend on the temperature and was 5.0 log (CFU/g) for bacteria and 3.4 log(CFU/g) for yeasts at all stages of the freezing-thawing cycles. In total, 15 saprotrophic bacterial species and 29 yeast species were isolated from different components of soil constructions. At the maximum temperature of freezing in the cycles (–5°C), three bacterial species with psychrophilic properties (Flavobacterium psychrophilum) and the ability to form endospores resistant to various adverse effects (Bacillus subtilis, B. megaterium) were isolated from soil and peat. Among the yeasts isolated from soil and peat at negative temperature and also having psychrophilic properties were Candida sake, Rhodotorula glutinis, Rh. mucilaginosa, and Solicoccozyma terricola. Bacteria with psychrophilic properties, F. psychrophilum and Pseudomonas fluorescens, as well as two species of bacilli, Bacillus subtilis and B. megaterium were revealed in sand at negative temperatures. Only one yeast species, Debaryomyces hansenii, capable of surviving stress conditions in the form of ascospores, was isolated from sand at –5°C. The effect of short-term temperature drops on the microbial communities’ number and diversity dynamics in soil constructions in a model experiment showed that specialized soil constructions were able to “tolerate” short-term temperature stress drops typical of the spring and autumn period, restoring the number of initial populations after cessation of the negative impacts. This also indicates development of the soil constructions in the process of functioning, rather than their rapid degradation.