Impact of radioactive contamination of soils on the diversity of micropopulation and the transformation of organic substances

Aim. To study the impact of different levels of radioactive contamination on the organic matter decomposition and the population development of microbial decomposers of organic matter in soil. Methods. Gamma-spectrometry for the determination of the relative activity of 137Cs and beta-spectrometry for the determination of the relative activity of 90Sr in order to choose the contamination range for the studies; Tea Bag Іndex (TBI), the standard glob- ally accepted method to determine the rate of organic matter decomposition; gas chromatography – to determine the impact of the investigated factors on the formation of the biomass of microorganisms by means of the СО2 production potential; classic microbiological methods, using elective media to estimate the population densities of culturable microbial decomposers. Results. The studies (from April to September 2021) comprised two soils with different radioactive contamination ranges: Range No. 1 in Narodychi district of Zhytomyr region (the village Khrystynivka, 3 sampling points) in the unconditional (obligatory) resettlement zone after the catastrophe in the Chornobyl nuclear power plant (ChNPP) (it has an absorbed dose rate gradient, evaluated for soil microorganisms, from 0.2 μGy/h (at sampling point Narodychi-1: 51,24076°N, 29,21497°E) to 1.57 μGy/h (at sampling point Naro- dychi-3: 51,23815°N, 29,22245°E)), located on the agricultural field, abandoned after the catastrophe and Range No. 2, located on the afforested area, directly bordering the territory of the so-called Red Forest in the ChNPP ex- clusion zone (4 sampling points – the first three points – natural ecosystems, and 4th – pyrogenically transformed territory after fires in 2020) with the absorbed dose rate gradient from 3.7 μGy/h (at sampling point ChEZ-1: 51,38595°N, 30,03035°E) to 84.0 μGy/h (at sampling point ChEZ-4: 51,38231°N, 30,03298°E). The dependence of the studied indicators on the gradient of soil contamination with radionuclides was demonstrated and these are the conditions that the difference is more than 400 times in the values of the ionizing radiation absorbed dose rates formed by radionuclides in soil. In Range No. 1 (on the fallow land), with the increase in the dose rate up from 0.2 to 1.57 μGy/h, there was statistically significant increase of mineralization coefficient and percentage of reduced weight of rooibos tea between the least and most contaminated sampling points), stimulation of the development of micromycetes (by 1.2–2.3 times), cellulose-decomposing bacteria (by 7.1–7.9 times), and ammonifying bacteria (by 1.8–6.3 times) due to a significant increasing the number of colony-forming units in points with higher radio- nuclide contamination and an increase of microbial biomass in soil more than twofold between the least and most contaminated sampling point of Range No. 1.In Range No. 2, in the ChNPP exclusion zone, characterized by low parameters of sod-podzolic soil fertility (humus content from 1.2 ± 0.2 to 0.9 ± 0.1 %, exchangeable acidity from 4.0 ± 0.1 to 5.0 ± 0.7), the number of micromycetes and ammonifying bacteria in soil was one-two orders below the indices of Range No. 1. Micromycetes dominated in the groups of cellulose-decomposing microorganisms. There was statistically significant increase of mineralization coefficient and percentage of reduced weight of rooibos tea with increasing of radionuclide contamination level for the first three plots: ChEZ-1, ChEZ-2 and ChEZ-3 of Range No. 2. Conclusions. In the fallow land of the unconditional (obligatory) resettlement zone, the increase of the radia- tion absorbed dose rate in soil by one order from 0.2 to 1.57 μGy/h does not inhibit the development of microorgan- isms in soil, rather it stimulates their activity and increases their numbers. On poor sod-podzolic soils in the exclu- sion zone of the Chernobyl nuclear power plant, the preponderance of development of micromycetes over bacterial cellulolytics is noticeable. The radioactive contamination affected the development of soil microorganisms and the biological processes in soils not only in the first years after the Chornobyl catastrophe, which has been described in the literature, but has had its influence for rather a long time (for over thirty years after the accident). Among the microorganisms of a saccharolytic mode of organic plant residuals decomposition, the micromycetes dominate.