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 137 Cs and beta-spectrometry for the determination of the relative activity of 90 Sr in order to choose the contamination range for the studies; Tea Bag І ndex (TBI), the standard globally 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 СО 2 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


INTRODUCTION
The elimination of the consequences of extensive radiation accidents has provided mankind with rich experience of putting into practice both practical and fundamental knowledge about the radiation safety of society and environmental objects.Since the global scientifi c community got involved in solving the Chornobyl problems, in rather a short period of time (3-5 years), the radiation situation in the entire territory of Ukraine was taken under control (Baryakhtar et al, 1996), including the agricultural production under acceptable levels of its radionuclide contamination.After the accident on Fukushima-1 NPP (IAEA, 2015), these processes were handled much more effi ciently due to objective reasons, including the experience of Chornobyl (Steinhauser et al, 2013).The prevalence of practical needs in scientifi c research has narrowed down the volumes of conducting fundamental works in studying the impact of the radiation factor on biological objects in the impact zone of radiation accidents, thus, the achievements in this direction are much more modest (Prister et al, 2013).The study of the impact of penetrating radiation on some biota representatives, including soil microbiota, even in the ChNPP exclusion zone, is very limited to date (Gudkov, 2020).There is an ongoing scientifi c debate about the consequences of the extended impact of ionizing radiation rates on biodiversity (Beresford et al, 2020).
It is noteworthy that the radiation situation around the faulted radiation objects is unique from the standpoint of using the territories as a scientifi c range to conduct various scientifi c studies, including the conditions for the existence of soil microbial populations and to evaluate their activity depending on the levels of radionuclide contamination (RC) of soil.However, researches up till now were mainly directed at determining the potential of microorganisms in terms of their impact on the transformation of radioactive substances.For instance, considerable attention was paid to microorganisms isolated from uranium mines and other natural environments with increased content of radionuclides (Avery et al, 1999); there is a detailed description of different ways of physical and chemical transforma-tion of cesium and uranium compounds (including biosorption, biotransformation, biomineralization, and intracellular accumulation) under the impact of microorganisms (Lloyd et al, 2005).It was also shown that there is a possibility of both an increase and a decrease in the uptake of radionuclides into plants when some bacterial strains are used for pre-sowing inoculation of seeds of cultivated plants (Illienko et al, 2019;Illienko et al, 2020).A positive effect of seed inoculation is not limited to rhizosphere bacteria.For instance, Japanese researchers (Haidary et al, 2017) described that inoculation of soybean seeds with arbuscular fungi promoted the accumulation of 137 Cs by mycelium in the roots, which limited its relocation to other parts of the plant.
Studies, directed at determining the specifi cities of soil microbiota functioning under the effect of penetrating radiation, however, are scarce.Among them, noteworthy are publications on micromycetes of the scientifi c school of N.M.Zhdanova (Zhdanova et al, 1991;Zhdanova et al, 1991;Tugay et al, 2005).These authors studied the effect of higher doses of ionizing radiation on the metabolism of micromycetes and changes in their populations.They demonstrated the increase in the synthesis of melanin, radiotropism, and stimulation of micromycete development induced by ionizing radiation.Melanin-containing genera of fungi occupied a dominant position in the soil micromycete complexes of places with the highest levels of radionuclide contamination and were recognized by the authors as more radioresistant.
Romanovska et al (1996) conducted investigations in the 10-km area of the ChNPP zone in 1993-1995 and demonstrated a decrease in the number and diversity of soil prokaryotes compared to the control soils.In particular, the number of cellulose-decomposing, nitrifying, and sulfate-reducing bacteria decreased by one to three orders.
An attempt of a comprehensive evaluation of the total microbial activity in soil, contaminated with radionuclides due to the ChNPP catastrophe, was made by Kalashnikova et al (1996) The authors suggested that the processes of formation of 137 Cs forms available to plants are determined mainly by the microbiologi-in the literature, but has had its infl uence 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.
Key words: the Chornobyl NPP, radioactive contamination, microorganisms-decomposers of organic matter, vegetative material decomposition.DOI: https://doi.org/10.15407/agrisp9.03.003IMPACT OF RADIOACTIVE CONTAMINATION OF SOILS ON THE DIVERSITY OF MICROPOPULATION cal activity in the soil, while 90 Sr is determined by the physicochemical conditions of the medium.Kravchenko et al (1999) noted a considerable decrease in the number of soil bacteria in the soil, contaminated with radionuclides, which positively correlated with a reduced distance from the ChNPP.
While studying the microbiota state in soils, contaminated with radionuclides, it is relevant to have an experimental determination of the changes in the state of populations of cellulose-decomposing soil microorganisms as one of the main microbiota groups, ensuring the initial links of trophic biological chains, including the formation of soil fertility (Singh et al, 2017).
Taking the above into consideration, our study was aimed at investigating the specifi cities of development and functional activity of microbiota, responsible for decomposition of organic plant residuals in soils of Ukrainian Polissia, contaminated with radionuclides.

MATERIALS AND METHODS
Prior to the study, we searched plots/fi elds with experimental ranges which would meet the following requirements: a) a considerable gradient by the index of soil contamination with radionuclides in a relatively small territory; b) maximal homogeneity in terms of soil-climatic conditions and landscape.
During screening (October-November 2020), several land plots were selected in the territories which according to the Law of Ukraine belongs to unconditional (obligatory) resettlement zone and the ChNPP exclusion zone.On these selected plots, we conducted an additional sampling of soil to determine the current level of RC, and measurements of the γ-radiation ambient equivalent dose rate (RKS-01 "STORA-TU", Ukraine, "ECOTEST", http://ecotest.ua/).
Composite soil samples were collected using the envelope method.Soil samplings were collected using a special cylindrical sampler (d = 37 mm) digging to the 10-cm layer depth in fi ve points per one sampling point with 1-2 m distance between those: four in the corners and one in the center of a sampling point.The samples were subsequently combined to form a composite.The soil composites were dried (at 105 °C), sifted through sieve (cells d = 1 mm), carefully homogenized and 1000 cm 3 volume of soil was taken for 137 Cs (14 soil composites were analyzed), 100 cm 3 volume -for 90 Sr activity concentration measurements (7 soil composites were analyzed).
To determine the rate of organic matter decomposition by microbiota, we used the Tea Bag Index (TBI) method, which is a standard for global practice (Keuskamp et al, 2013).The essence of the method lies in using two kinds of ТМ Lipton tea bagsgreen tea (EAN8714100770542) and rooibos tea (EAN8722700188438), as standardized vegetative material.In this case, the obtained results may be compared with similar data, obtained for various ecosystems in different regions in the world.
The tea bags were weighed on electronic scales, marked, placed into soil at the depth of 8 cm, and left for 90 days.After this exposure, the tea bags were removed, cleaned from soil residues, dried in the cabinet drier at 70 ºС for 48 h and weighed again.The data obtained were used to evaluate the parameters of the so-called TBI-index: decomposition coeffi cient (k) and stabilization coeffi cient (S).The coeffi cients were determined using formulas of Keuskamp et al, 2013.Totally we used 84 bags of green tea and 84 bags of rooibos tea.
Two ranges of contamination were chosen for further research.Range No. 1 is located on the border with the exclusion zone (Narodychi district of Zhytomyr region -a zone of obligatory (unconditional) resettlement).Within Range No. 1, three points of vegetative material layouts (tea) and sod-podzolic soil samples were chosen; they had different levels of RC (Narodychi 1, Narodychi 2, Narodychi 3 -see  IMPACT OF RADIOACTIVE CONTAMINATION OF SOILS ON THE DIVERSITY OF MICROPOPULATION Kyiv region).Attention was focused on the area of the so-called «Red Forest», because it is the most radionuclide contaminated territory of the exclusion zone.The territory of the «Red Forest» is not quite suitable for carrying out the tasks set by the research, as at the time, it had a powerful human impact (collecting and burying the trees which perished in the fi rst weeks after the catastrophe, taking off and burying the upper soil layer, covering the surface with sand, etc.).Therefore, we selected an area, bordering the Red Forest territory which might meet the set criteria.Certainly, the places, selected in Range No. 2, were characterized by a much higher level of radionuclide contamination compared to those in Range No. 1, see Table 1 and Figure.In each point, the layout of vegetative material (tea bags) was done in six repeats for each kind of tea (green and rooibos according to Keuskamp et al, 2013).
The places of sampling in Range No. 1 are characterized by rather a considerable gradient of RC (Table 1) and their soil characteristics are close in terms of agrochemical indices.The points of Range No. 2 also have close agrochemical soil parameters within the range (Table 2) and sod-podzolic soil type.The homogeneity of climate conditions separately for Range No. 1 and Range No. 2 is ensured by the geographic proximity of the experimental plot locations (maximum of several hundred meters between some plots, see Figure ).The radionuclides soil activity concentration in the points selected for research differs many times (more than 7 times for 137 Cs activity concentration and 10 times for 90 Sr activity concentration on Range No. 1, more than 19 times for 137 Cs activity concentration and more than 42 times for 90 Sr activity concentration on Range No. 2), and it is logical to assume that the exposure absorbed dose of soil microbiota will also differ many times (7.85 times on Range No. 1 and 22.7 times on Range No. 2).
Table 1 shows that for Range No. 1, the index of γ-radiation ambient equivalent dose rate ranged from 0.127 ± 0.01 to 0.737 ± 0.04 μSv/h.For Range No. 2 in the ChNPP exclusion zone, this index ranged from 1.7 ± 0.04 to 34.8 ± 0.5 μSv/h.In terms of this parameter, the difference for the entire experiment was over 270 times.
There were also considerable differences in the level of soil contamination with radioactive isotopes.For Range No. 1, the values of the 137 Cs activity concentration in soil were within the range of 0.6 ± 0.04 to 4.6 ± 0.1 kBq/kg.The same index for Range No. 2 in the ChNPP exclusion zone was in the range of 10.4 ± ± 0.2 to 203.8 ± 4.1 kBq/kg.The difference between the values of 137 Cs activity concentration in point Narodychi-1 and ChEZ-4 was over 300 times.
The contamination with 90 Sr was also considerably different within each separate range (Table 1).The sodpodzolic soil of Range No. 1 had the 90 Sr activity concentration 0.03 ± 0.004 kBq/kg in the point of Narodychi-1 and 0.3 ± 0.01 kBq/kg in the point Narodychi-3.The soil of Range No. 2 at ChEZ-1 -0.8 ± 0.1 kBq/kg and at point ChEZ-4 -34.0 ± 0.3 kBq/kg, respectively.The difference between the 90 Sr activity concentration values for points Narodychi-1 and ChEZ-4 was over 1,000 times.Microbiota of the investigated soils, developing under this level of RC, will have been exposed to considerable ionizing radiation doses which was confi rmed with estimations of absorbed dose rate (Table 1).According to our evaluations, the absorbed dose rate (due to the ionizing radiation of 137 Cs and 90 Sr), received by microorganisms in Narodychi-1 and ChEZ-4, was 0.2 and 84 μGy/h, respectively, which differed more than 420 times.This gradient in radiological characteristics of soil should allow for the evaluation of the consequences of the impact of the radiologic situation on the population structure and development and activity of microorganisms.
Soil samples for microbiological research were selected close to the layouts of tea bags in April, July, and September of 2021.The number of microorganisms of some ecologic-trophic groups was studied: representatives of saccharolytic (micromycetes, cellulose-decomposing bacteria) and peptolytic (ammonifying microorganisms) modes of vegetative decomposition of plant residuals.Determination the total number of micromycetes was done using Chapek's medium, cellulosedecomposing bacteria -on a liquid mineral medium with cellulose (strips of fi lter paper) as the only carbon source, ammonifi ers -on meat-peptone agar (Gerhardt, 1981;Volkogon et al, 2010).
The total microbial biomass in the soil was determined gas chromatographically based on the maximum production of CO 2 (substrate-induced respiration method (SIRM).The essence of the method lies in evaluating the amount of active microbial biomass under the introduction of glucose solution into the investigated soil sample to achieve the maximal release of СО 2 and subsequent evaluations using empirical coeffi cients to convert the volume of carbon dioxide into the microbial biomass (Anderson, and Domsch, 1973;Bailey et al, 2008).
The statistical processing of experimental data was conducted using disperse analysis (ANOVA) and software packages of Microsoft Offi ce Excel 2010 and STATISTICA 10.While determining the number of cellulose-decomposing bacteria with the liquid medium of Imshenetsky and Solntseva, we used most probable number method (Ferguson and Ihrie, 2018).

RESULTS
The results of the study of cellulose-decomposing activity of a sod podzolic soil microbiota on Range No. 1 are presented in Table 3.The estimated coeffi cients S and k for tea samples of the fi rst layout, placed into soil for 90 days, under the lowest activity of radioactive isotopes in soil are 0.442 ± 0.056 and 0.0073 ± 0.0018, under the medium activity -0.407 ±  As for the decomposition of a more stable substrate -rooibos, noteworthy is a signifi cant increase in the intensity of vegetative material decomposition in points Narodychi-2 and Narodychi-3 compared to the point, characterized by the lowest RC level.
The coeffi cient of vegetative material mineralization on Range No. 2 got higher with an increase in RC in ChEZ 1, ChEZ 2, and ChEZ 3 (0.0035; 0.0074 and 0.0097, respectively), but in the area with the highest indices of ionizing radiation the coeffi cient decreased down to 0.0033 (Table 4).The rate of green tea decomposition was high and substantially comparable to the values obtained for Narodychi-3 on Range No. 1.The intensity of rooibos decomposition was somewhat lower.No statistical difference between the variants was found, but the lowest intensity of rooibos decomposition was observed at the highest level of soil RC.
The results of evaluating the indices under the second layout of the vegetative material on both ranges are considerably confi rmed by the data, obtained before.For instance, the increase in the mineralization coeffi cient and rooibos decomposition was noted for Range No. 1 in the point with the highest level of soil contamination (Table 5).
As for the range, located in the ChNPP exclusion zone (Table 6) there was a noteworthy tendency towards the increase in the mineralization coeffi cient (k) under the increase in the ionizing radiation dose from points ChEZ 1 to ChEZ 3. A place of the highest radiation level was characterized by a decrease in the index.Similar to the study of the fi rst layout of vegetative material into soil, we noted a gradual increase in the intensity of tea decomposition in the fi rst three points (ChEZ 1, ChEZ 2 and ChEZ 3) and some decrease in the indices for the highest RC level (ChEZ.4).
While determining the total microbial biomass of soil, some changes in indices were also noted depending on the level of contamination with radionuclides (Table 7).For instance, low levels of RC promoted the activation of the development of microorganisms.Within Range No. 1 the accumulation of the microbial biomass was the lowest under weak contamination (Narodychi 1) and the highest -under increased contamination (Narodychi 3).Analysis of soil in Range   No. 2 demonstrated much lower indices, especially in the point of the highest contamination.
The determination of the number of micromycetes in the investigated soil samples also demonstrated similar changes in indices depending on RC level (Table 8).For instance, the number of fungi in the soil of Range No. 1, as a rule, got higher with an increase in the contamination level (the exception is found in the study results for specifi c periods of sampling, for instance, in April 2021).However, in all the periods of the study, the lowest number of micromycetes among the investigated variants was noted in the sod-podzolic soil of Range No. 2. Yet the number of microscopic fungi was several times lower than the corresponding index for the soil of Range No. 1 depending on the places of sampling.
The feature of development depending on levels of RC noted for micromycetes was also remarkable for cellulose-decomposing bacteria, but their number was very low (Table 9).In early spring and autumn, on Range No. 2 these bacteria were not found by traditional microbiological methods at all, which may demonstrate their insignifi cant involvement in the processes of decomposing vegetative residues under the investigated conditions.
While investigating the number of microorganismsammonifi ers, it was found that the development of this group of soil microbiota representatives was activated along with the increase in RC level on Range No. 1 and at the same time, its development was inhibited in sodpodzolic soil of Range No. 2, especially in the place of the highest contamination level (Table 10).Thus, the peptolytic mode of organic residue decomposition (ammonifi cation) was generally in agreement with the specifi cities of the course of the saccharolytic mode (the development of such biodecomposers as micromycetes, fi rst and foremost).

DISCUSSION
The accumulation of microorganisms in soil has a decisive signifi cance for the support of the functions of the aboveground ecosystems due to their role in the circulation, maintenance, and release of the main nutrients (McKenney et al, 2018).The decomposition of the vegetative plant residuals is a key process, determining the rate of biota performance in the ecosystem (Singh et al, 2017).Therefore, the ability to decompose cellulose is considered one of the most important indices of the total activity of soil microorganisms.The intensity of cellulose decomposition by microorganisms in soil depends on many ecologic factors (humid-ity rate, temperature, acidity, etc.).The accumulation of microorganisms is also a sensitive indicator of environmental stress and refl ects even small changes in the geochemical composition of their environment due to anthropogenic activity (Bano et al, 2018;Guillot et al, 2019;Hallsworth, 2019;Xiao et al, 2019).So, the ionizing radiation may also have some (not completely determined yet) impact on the activity of soil microorganisms (Kalashnikova et al, 1996;Romanovskaya et al, 1996;McNamara et al, 2003;Ragon et al, 2011;Chapon et al., 2012;Gu et al., 2014;Mousseau et al, 2014;Bonzom et al, 2016;Hoyos-Hernandez et al, 2019;Ogwu et al, 2019;Ihara et al, 2021).
The results of our research demonstrate the stimulating effect of low RC doses (Range No. 1) on the development and functioning of microorganisms, which are cellulose decomposers (for example, the number of micromycetes increases by 1.6-3.6 times at the Narodychi 3 point compared to the indicators of the Narodychi 1 point).Under high RC doses (Range No. 2), the investigated indices decreased considerably (by an average of 10 times).
Certainly, such comparison of the obtained indices has its degree of conditionality, since the soils of both ranges can be compared only under complete coincidence of all soil and climatic parameters.We believe that though this comparison is conditional, it is still possible since the landscape of both ranges is fl at, the moisture content in the samples was practically on the same level in all the periods of the study, the agrochemical indices were not contrastive.The ranges differ by their history.Though the soil on both ranges is sod-podzolic, yet in the fi rst range it used to be cultivated and served as an agricultural fi eld prior to the Chornobyl catastrophe.The sod podzolic soil in the ChNPP exclusion zone (Range No. 2) is in the forest, and there was a fi re in the area with the highest level of radionuclides contamination in 2020 (see Figure 1, b, sampling point 4).Despite the above-mentioned differences between both experimental Ranges, we consider that one of the important parameters that allows us to distinguish the characteristics of the studied soils is the level of radionuclide contamination since the indicators differ by one or two orders of magnitude.
When the indices of radiation absorbed dose rate are considered, the biological activity of soils depends on the level of radionuclides contamination.It was demonstrated by both methods, used by us (both the TBIindex calculations and the determination of microbial biomass).These indices are in close correlation with the number of microorganisms, which are organic matter decomposers.The specifi cities in the development of microorganisms that are representatives of both saccharolytic and peptolytic modes of plant residuals biodecomposition are similar: their number increased under a relatively low increase in the radiation absorbed dose rate within Range No. 1 (the number of micromycetes increased by 1.6-3.6 times, cellulolytic bacteria by an average of 10 times and ammonifi ers by 2.4-6.3 times) and was low in the soil of Range No. 2 (сellulolytic bacteria were not detected at all during certain periods of research, and the number of ammonifi ers was 17-40 times less than the indicators of Range No 1).The lowest indices of the microorganisms number were remarkable for the pyrogenically transformed area in the ChNPP exclusion zone.Judging by our results, the conclusion may be made that relatively low doses of ionizing radiation do not inhibit the development of microbiota in soil.It is confi rmed to some degree by the publication of Bonzom et al (2016) who studied the intensity of forest fl oor decomposition in the ChNPP exclusion zone.According to their results, the mass of the forest fl oor was mainly lost with the increase in the intensity of the ambient dose rate of γ-radiation from 0.22 to 29 μGy/h.The authors came to the conclusion that the radioactive contamination of forest ecosystems for more than two decades did not necessarily have a negative effect on organic matter decomposition.Our results also indicate increasing of percentage of rooibos tea reduced weight with increasing radionuclide contamination for both Ranges (except point ChEZ-4, but this territory was pirogenically transformed).Also we can compere ranges in radiation levels in our study and that of Bonzom et al. (2016).The sites selected by Bonzom et al. (2016) differed between 0.22 to 29 μGy/h, compared to our study sites ranging from 0.127 to 34.8 μSv/h (if we consider only γ-radiation dose rate we can compere Gy and Sv).Chapon et al. (2012), using both molecular-and culture-based approaches, demonstrated that in the radionuclides contaminated soil with 137 Cs activity concentration within the range from 61 to 750 Bq/g did not lead to signifi cant changes of bacterial diversity which was similar to that observed in control points of soil with signifi cantly lower 137 Cs activity concentration from 0.35 to 1.5 Bq/g, 25 years since the accident which may show the restoration of bacterial community.
However, studies by Mousseau et al (2014) demonstrated a decrease in the rate of forest fl oor decomposition by microorganisms in response to the increase in the radioactivity level, which resulted in enlarging the thickness of forest fl oor layer with an increase in the radiation dose rate for two sites among the most contaminated.This indicates a decrease in the activity of soil microbiota.It was also confi rmed by the conclusions of Theodorakopoulos et al (2017), who conducted the investigation of the development of prokaryotes under different radioactivity levels and demonstrated that the total absorbed dose rate by the cell and decrease the organic matter content of the soil were the main impact factors which had the effect of reducing the diversity of prokaryotic communities.
At fi rst glance, obtaining somewhat contradictory results to those of the above-mentioned authors can perhaps be explained both by the received radiation dose and by the different composition of radionuclides IMPACT OF RADIOACTIVE CONTAMINATION OF SOILS ON THE DIVERSITY OF MICROPOPULATION in the soils.In addition, the different composition of the dead mass entering the soils may be important (due to the different structure and species composition of plant communities in the places of analysis), since the quality of plant remains is a powerful factor infl uencing the formation of microbial communities (Almagro et al, 2021).
In our study, noteworthy were extremely low numbers of cellulose-decomposing bacteria, especially compared to the number of micromycetes.Taking this fact into consideration, the conclusion can be made about the dominant role of fungi in the processes of cellulose biodecomposition in the soil contaminated with radionuclides.A similar effect was observed by Ogwu et al. (2019) while conducting the studies under targeted γ-radiation of a sandy loam soil.The authors assumed that the more active development of fungi and algae on the background of a decrease in the intensity of bacterial reproduction could be explained by the removal of competition from the ecologic niche.
The determination of the number of ammonifi ersthe agents of the peptolytic mode of organic matter biodecomposition in soil -mainly demonstrated the course of development, similar to that of representatives of the saccharolytic mode, depending on the RC level.For instance, in the soil of Range No. 1, the number of these bacteria is rather high and increases with a higher dose of ionizing radiation.In the soil of Range No. 2, the number of ammonifi ers is one-two orders lower than the indices of Range No. 1 which may demonstrate a negative effect of high RC levels on their development.
A destructive effect of high RC levels on the development of bacteria was noted in the studies, conducted soon after the accident.According to the data of Romanovska et al. (1998), the diversity of the cultivated bacteria, isolated in the 10-km zone around the ChNPP was two orders lower than that from the control noncontaminated areas.Similarly, the diversity of soilborne bacterial communities was poorer in the samples from the areas of Fukushima with the highest RC levels (Ihara et al, 2021).Theodorakopoulos et al (2017) predicted that chronic exposure with low doses of ionizing radiation may selectively affect the development of bacteria resistant to ionizing radiation in soils.As over thirty years have passed since the accident on the ChNPP, it would be quite expectable not to fi nd any difference in the number of bacteria depending on the RC gradient in the soil of Range No. 1.The results obtained confi rm these assumptions and even demonstrate the increase in the total microbial biomass from 1575,9 ± 232,1 to 3806,2 ± 214,6 mg/kg of soil (Table 7).Another situation was observed for microbial development under high exposure doses.In the soil of Range No. 2, the number of representatives of the investigated groups of bacteria was one-two orders lower than the corresponding indices of Range No. 1.Therefore, our research shows that a high level of soil RC affects the development of the studied groups of microorganisms not only in the fi rst years after accidental exposure, but also over a rather long period of time.During certain sampling periods, we did not manage to detect the development of cellulose-destroying bacteria in the soil of Range No. 2 at all.The number of ammonifi ers was also one or two orders of magnitude lower compared to the parameters of the Range No. 1 soil.
Compared to the cellulose-decomposing bacteria, the fungi are not so sensitive to the negative effect of the soil RC.The intense development of micromycetes in the radionuclides contaminated soils was already noted by Zhdanova et al (1991), andTugay et al (2005).Malo and Dadachova (2019) reported that some strains of micromycetes (genera Cladosporium and Penicillium) were capable of surviving under a high level (up to 100-50 Gy/sample) of ionizing radiation due to the accident in the ChNPP, using melanin synthesis as a radioprotector.But there are also dissenting statements.For instance, Panahova (2009) demonstrated on the sample of gray-brown soil of Absheron (Azerbaijan) that microscopic fungi were the most γ-radiation sensitive representatives of soil microbiota.However, it may be incorrect to compare the results of studies of the abovementioned authors since some of them were obtained in the ChNPP zone with constant selective stress of ionizing radiation, while others (Panahova, 2009) -under targeted single-time radiation.After all, the total doses absorbed by groups of microorganisms are probably more signifi cant compared to the effect of instantaneous acute γ-radiation.
The results of our study confi rmed a relative resistance of fungi to the ionizing radiation, but their development also depended on the gradient of soil radionuclides contamination.
Summing up, ionizing radiation is potentially lethal for microorganisms since the exposure energy may be suffi cient to induce breaks in DNA strands (Ravanat, Douki, 2016;Song, Kuai, 2017).However, most microorganisms code normal enzymatic mechanisms of DNA restoration, due to which a large part of the damage is reparable (Jung et al, 2017).There were also re-ports about some potential mechanisms of microbial resistance to radiation which turn on the detoxication systems of reactive oxygen intermediaries and enzymatic antioxidant processes (Pavlopoulou et al, 2016;Jung et al, 2017).The dose, under which it occurs for each specifi c species, is very variable (Ghosal et al, 2005), so theoretically, the survival of microorganisms and the formation of specifi c communities in soil, for instance, may depend on the gradient of RC.This is practically confi rmed by the results of our study.On the other hand, in addition to the impact of radiation on the biological properties of soil, there is a known effect of this factor on the changes in physical and chemical characteristics of soil which may affect the development of microbiota.This point was substantiated in the publication of McNamara et al (2003): the radiation may affect both cellular physiology and bioavailability of growth substrates, i.e. the donors of electrons, acceptors, and possibly nutrients.It was also confi rmed by other investigations.For instance, it was demonstrated that the ionizing radiation destroyed natural organic substances in soils which resulted in the increase in the content of soluble organic carbon therein (Bank et al, 2008;Schaller et al, 2011).This radiolytic degradation of organic matter may enhance the bioavailability of organic carbon for microbial metabolism and thus impact both the development of microbiota and its functional activity.Theodorakopoulos et al (2017) determined that, fi rst and foremost, the development of microorganisms is affected by the total intensity of the exposure dose, absorbed by the cell, compared to the effect on organic matter.
The general conclusion can be made as follows: radioactive contamination affects the development of separate ecological and trophic groups of microorganisms in soil and depends on its gradient.Moreover, it is not known whether radiation pollution selectively affects the development of microorganisms at the level of individual taxa.This requires additional research using molecular biological methods.These studies are planned to be conducted as soon as there is an opportunity to take soil samples in the ChNPP exclusion zone.

CONCLUSIONS
Our study demonstrated the dependence of the development and functioning of microorganisms -the representatives of saccharolytic and peptolytic modes of plant residues biodecomposition on the level of ionizing radiation.The relatively low intensity of the absorbed dose in the soil of Range No. 1 (up to 1.6 μGy/h) stimulated the development of micromyce-tes (by 1.2-2.3times), cellulose-decomposing bacteria (by 7.1-7.9times), and ammonifying bacteria (by 1.8-6.3times) and promoted the increase more than twofold their biomass accumulation.High intensity of the absorbed doses rate in the sod podzolic soil of Range No. 2 (from 3.7 to 61.6 and especially 84.0 μGy/h) had a negative effect on the indices under investigation: the biomass of microorganisms decreased by 5.4-10.1 times, the number of micromycetes -by 2.1-3.3 times, ammonifi ers -by 10-250 times compared to the indicators of Range No 1.
Under all the investigated levels of radionuclide contamination of soil, microscopic fungi dominated in the community of cellulose-decomposing microorganisms, which may demonstrate the main role of eukaryotic hyphal microorganisms in the decomposition of vegetative material under these conditions.
Confl ict of interests.The authors declare the absence of any confl icts of interests.

Table 1 .
The coordinates and the values of radiologic indices in the layouts of vegetative substrate and soil sampling

Table 2 .
The сharacteristics of the main agrochemical indicators of soils in the layouts of the vegetative substrate and soil sampling.(given in the "Methods" section)

Table 4 .
The change in the TBI index depending on the level of contamination with radionuclides on Range No. 2 (exposure in April-June 2021) a Note.Letters a, b, c, and d indicate statistically signifi cance at p ≤ 0.05.Each coeffi cient should be compared separately.

Table 3 .
The change in the Tea Bag Index (TBI index) as indicator of decomposition of organic matter, depending on the level of contamination with radionuclides in soil plots of Range No. 1 (exposure in April-June 2021) While analyzing the obtained results for the rate of tea bag decomposition on Range No. 1, we did not fi nd any signifi cant difference in the variants, but the lowest index was noted for the lowest dose of RC.

Table 5 .
The change in the TBI indices depending on the level of contamination with radionuclides on Range No. 1 (exposure in July-September 2021) Note.Letters a, b, c, and d indicate statistically signifi cance at p ≤ 0.05.Each coeffi cient should be compared separately.

Table 6 .
The change in the TBI indices depending on the level of contamination with radionuclides on Range No. 2 (exposure in July-September 2021)

Table 7 .
The total microbial biomass of the soil depending on the level of contamination with radionuclides, mg/kg of soil Note: Hereinafter: І -April 2021, ІІ -July 2021, ІІІ -September 2021.

Table 8 .
The dynamics of the development of micromycetes depending on the level of contamination with radionuclides, thousand CFU/g of dry soil IMPACT OF RADIOACTIVE CONTAMINATION OF SOILS ON THE DIVERSITY OF MICROPOPULATION

Table 9 .
The number of cellulose-decomposing bacteria depending on the level of contamination with radionuclides, thousand/g of dry soil

Table 10 .
The dynamics of the number of ammonifi ers depending on the level of contamination with radionuclides, million CFU/g of dry soil