Accumulation of cesium-137 Leccinum aurantiacum from podzolic soil

. The accumulation of cesium-137 by the cap and stalk of fruiting bodies of Leccinum aurantiacum (Bull.) Gray from the podzolic soil of an aspen forest 30-35 years old was compared with spruce undergrowth, which resumes after felling and damage to the litter. The average specific activity of caesium-137 for dry caps is 1589±85 Bq/kg and for stems 879±72 Bq/kg. It has been established that the specific activity of cesium-137 does not exceed the allowable values. The coefficients of accumulation of cesium-137 by parts of fruiting bodies from the soil vary in the following range: for caps from 5.4 to 6.8, and for legs from 1.2 to 2.8. The maximum specific activity of caesium-137 was found in the forest litter 461±54 Bq/kg. With increasing depth, it naturally decreases.


Introduction
The source of cesium-137 entering the environment are accidents at nuclear power facilities [1] and nuclear weapons testing.
The migration of cesium-137 and the disinfection of landscapes under the influence of similar environmental factors proceed at different rates. The results of the studies carried out indicate that cesium-137 is retained and, under certain conditions, accumulates in natural landscapes, in contrast to agronomic and urban ones [2]. Currently, ecosystems of natural landscapes are geochemical barriers that retain caesium-137. The most affected by caesium-137 pollution are forest ecosystems.
After the initial fallout on the crowns of trees and flushing into the soil, cesium-137 remains in the forest ecosystem for a long time. The main depot of cesium-137 is the soil. Cesium-137 is located in the upper part of the soil profile, and is practically not carried out with water runoff into the river network [3]. The maximum concentration of cesium-137 is found in the forest litter and the upper organomineral soil layer [4].
This leads to the activity of root systems and fungal mycelium. The specific activity of cesium-137 in the upper part of the soil profile determines its accumulation by the organisms that form the ecosystem. Fungi that form ectomycorrhiza with tree species make a significant contribution to the accumulation and migration of caesium-137 [5]. L aurantiacum is a species that forms ectomycorrhiza with aspen [6]. Cesium-137 absorbed by fungal mycelium is a significant part of soil organic matter [4]. Representatives of the genus Leccinum S.F. Gray are used to monitor caesium-137 in forest communities [7].
Mushrooms, the bottom bodies of which are included in food chains, are a source of further cesium-137 migration. If wild mushrooms are considered edible, then this creates a threat of additional internal exposure of people using them as an additional source of nutrition, which is traditional in some cultures.
The purpose of the study was to assess the accumulation of cesium-137 in parts of the fruiting bodies of mushrooms of the species Leccinum aurantiacum (Bull.) Gray in an aspen forest 30-35 years old with spruce undergrowth recovering after felling and damage to the litter.

Materials and methods
The place of collection of fungal fruiting bodies and soil sampling is located in the vicinity of the point with coordinates (N 59°04.471', E030°26.945', H 50 m) (Russian Federation, Leningrad oblast, Gatchinsky district). The studied ecosystem is located between two arrays of horticultural associations and is a green zone 60 m wide with a drainage ditch 2 m deep on the north side. The mixed aspen-spruce forest in the green zone was completely cut down in 1984-1987. At the same time, the forest floor was completely destroyed.
Preparation of soil and fruiting bodies of fungi for measurements consisted in drying to a constant weight in a stream of warm air at a temperature of 40 °C. The specific activity was measured on a Beta radiometer. The radionuclide composition was determined by gamma spectrometry.
The concentration of cesium-137 in fungal fruiting bodies was assessed using the accumulation coefficient (AC), calculated as the ratio of the specific activity of the cap or stem of the fruiting body to the specific activity of the soil horizon with the maximum number of roots.

Results and Discussion
Aspen and spruce, unlike birch and pine, prefer richer soils. The initial soil conditions determine the emerging type of forest community. At the same time, tree species have a significant impact on the soil. It has been established that the reserves of soluble organic carbon in the soils of aspen forests are higher than in the soils of coniferous forests [8]. This is a very important parameter that affects the fruiting of mushrooms, the mycelium of which absorbs organic matter throughout the surface.
At the initial stage of restoration of the forest community after felling or fire, aspen dominates. The faster growth of aspen is based on the fact that its renewal occurs by shoots from roots preserved in the ground, while spruce is restored due to seed reproduction. Then comes the gradual displacement of aspen by spruce, which leads to a complex change in a number of key parameters, including illumination, the composition of nutrients and soil acidity.
On the basis of the experimental material, the authors of [9] analyzed the dynamic interactions between aspen and spruce stands as a result of the succession of the forest community. The change in the composition of tree species in the plant community is influenced by the species composition of fungi that form ectomycorrhizal endings with aspen.
The choice of L. aurantiacum for the study is due to the fact that it is included in the state standards for the harvesting of wild-growing edible mushrooms and is recommended for sale through retail chains. In addition, representatives of the genus Leccinum are promising biomonitors of caesium-137 in forest and marsh ecosystems [7].
The choice of the forest community is associated with the mass formation of fruiting bodies of Leccinum aurantiacum. In addition, one of the objectives of the study was to study the influence of the process of forest litter disturbance on the distribution of caesium-137 in the soil profile. It is known that the maximum concentration of caesium-137 is confined to the forest litter. For the study, a stand area was chosen that spontaneously recovers over 30-35 years after cutting and damage to the litter.
The results of specific activity measurements in the upper part of the podzolic soil profile are given in Table 1 and graphically presented in figure 1.
The maximum specific activity of caesium-137 was registered for a thin bedding of 461±54 Bq/kg. In the upper part of the podzolic horizon, at a depth of 3-5 cm, the main mass of aspen roots is concentrated. In the soil profile, the value of specific activity decreases with increasing depth (Figure 1).   The felling of the forest stand in 1984-1987 and the creation of an array of horticultural associations led to the fact that the formation of litter in the green zone began anew. This is evidenced by its insignificant thickness of 0 -1 cm. The thickness of the forest litter in undisturbed aspen-spruce forests, where succession processes proceed naturally, reaches 6 cm. Thus, in the aspen-spruce forest located between the Kremenka River and the Oredezh River (N 59°04.673', E030°28.099', H 58 m), the litter is a peat mat densely permeated with aspen roots.
The upper mineral soil horizons are represented by boulder loam. If we evaluate the projection of boulders in three-dimensional space, then they are superimposed on each other in such a way that they form a kind of solid paved pavement, which has a significant impact on the placement of aspen roots in the soil. At the same time, only a small layer of loam in the upper part of the soil profile is available for the development of aspen root systems.
The bulk of aspen roots, with which L. aurantiacum forms ectomycorrhizal endings, is located in the podzolic horizon at a depth of 3.0 to 5.0 cm. Therefore, to calculate the accumulation coefficient, we used the average value of the specific activity of cesium-137 in the podzolic horizon at a 5 cm (Table 1, Figure 1), amounting to 267±16 Bq/kg. The data obtained from the results of measuring the specific activity of cesium-137 in the cap and stalk of the fruiting bodies of mushrooms of the species L. aurantiacum are presented in table 2.
The coefficient of variation of specific activity for dry caps of fruiting bodies (table 2) is 6.8%, which makes it possible to characterize its variability as insignificant. The variation coefficient of specific activity for dry stems of fruiting bodies (Table 2) is 32%, which makes it possible to characterize its variability as significant. The coefficients of accumulation by caps of fruiting bodies in relation to the soil horizon with the maximum content of roots vary from 5.4 to 6.3 (Table 2), which is higher than the values for the legs of fruiting bodies of fungi from 2.0 to 5.3 ( Table 2). 2±0.2 Notes: * -the calculation of the confidence interval of the average activity was carried out at a significance level of p<0.05; ** -coefficient of accumulation of cesium-137 by the cap and stem of the fruiting body from the rootinhabited soil layer; *** is the ratio of specific activities in the cap and stem of the fruiting body of the fungus.
The ratio of specific activities in the cap and stem of L. aurantiacum fruit bodies ( Table  2) shows that the cap accumulates 1.2-2.8 times more cesium-137 than the stem.
According to the sanitary rules of the Russian Federation, the permissible activity of caesium-137 for dry mushrooms is 2500 Bq/kg [10]. For mushrooms of the species L. aurantiacum, the average specific activity of caesium-137 in the studied fruiting bodies for caps is 1589±85 Bq/kg, and for legs 879±72 Bq/kg.
The specific activity of cesium-137 in fungi is an integral value that not only shows the accumulation of the radionuclide by fruiting bodies, but also characterizes its distribution in the soil profile. It is necessary to conduct long-term observations of the dynamics of changes in the specific activity in the fruiting bodies of fungi, since this allows you to control the processes occurring in the ecosystem associated with the migration of cesium-137 in the soil profile and ecosystem components.
Researchers analyzing the consequences of the accident at the Nuclear Power Plant Fukushima Daiichi in forest ecosystems come to similar conclusions about the need to conduct long-term observations of the dynamics of the specific activity of caesium-137 in the fruiting bodies of mushrooms. If we consider economically valuable species of fungi, then monitoring the dynamics of the specific activity of cesium-137 in fruiting bodies helps to reduce the risks from internal human exposure [11].

Conclusion
For the forest ecosystem of the aspen forest with spruce undergrowth recovering after felling, accompanied by a significant disturbance of the litter, it was found that the maximum specific activity of caesium-137, which is 461±54 Bq/kg, is associated with a thin litter. The maximum number of aspen roots is confined to the podzolic horizon at a depth of 3-5 cm. This distribution of roots is affected by a large number of boulders and the absence of peat litter, which is typical for undisturbed aspen-spruce forests. The specific activity of cesium-137 in the soil profile naturally decreases with increasing depth. As a result of the analysis of the specific activity of cesium-137 in the caps and stalks of the fruiting bodies of fungi of the species Leccinum aurantiacum (Bull.) Gray, it was found that for dry caps it is 1589 ± 85 Bq / kg, and for stalks 879 ± 72 Bq / kg, which does not exceed the established allowable values. For the caps and pedicels of the fruiting bodies of fungi, the accumulation coefficients (AC) were determined in relation to the soil horizon containing the maximum amount of aspen roots, the specific activity of caesium-137 in which is 267 ± 16 Bq/kg. For the caps of the fruiting bodies of mushrooms, the accumulation coefficients vary in the range from 5.4 to 6.8, and for the legs from 1.2 to 2.8.