Cs and 40 K activity concentrations in edible wild mushrooms from China regions during the 2014–2016 period

: Introduction . Contamination by radiocaesium of edible wild mushrooms after major nuclear accidents is a long-lasting process in some regions of the world. Following greater awareness of radioactive pollution in Asia, particularly after the Fukushima accident, this study investigated the radioactivity of 137 Cs and 40 K contamination in edible wild mushrooms in China. Study objects and methods . The objects of the research were edible wild mushrooms collected during 2014 to 2016, from the Inner Mongolian and Yunnan regions of China. To obtain an insight into any environmental impacts to distant regions of mainland Asia, the mushrooms were analyzed for 137 Cs activity. In parallel, the natural activity of 40 K was also determined and used to estimate the content of total K. The topsoil underneath the mushrooms was also investigated from a few sites in Bayanhushu in Inner Mongolia in 2015. Results and discussion. The results showed that in 4 to 6 mushrooming seasons after the accident, mushrooms from both regions were only slightly contaminated with 137 Cs, which implied negligible consequences. The activity concentrations of 137 Cs in dried caps and whole mushrooms in 63 of 70 lots from 26 locations were well below 20 Bq kg –1 dry weight. Two species ( Lactarius hygrophoroides L. and Lactarius volemus L.), from Jiulongchi in Yuxi prefecture showed higher 137 Cs activities, from 130 ± 5 to 210 ± 13 Bq kg –1 dw in the caps. 40 K activities of mushrooms were around two- to three-fold higher. A composite sample of topsoil (0–10 cm layer) from the Bayanhushu site (altitude 920 m a.s.l.) in Inner Mongolia showed 137 Cs activity concentration at a low level of 6.8 ± 0.7 Bq kg –1 dw, but it was relatively rich in potassium ( 40 K of 595 ± 41 Bq kg –1 and total K of 17000 ± 1000 mg kg –1 dw). Conclusion. Wild mushrooms from the Yunnan and Inner Mongolia lands only slightly affected with radioactivity from artificial 137 Cs. Lack of 134 Cs showed negligible impact from Fukushima fallout. Ionizing radiation dose from 137 Cs in potential meals was a fraction of 40 K radioactivity. The associated dietary exposure to ionizing irradiation from 137 Cs and 40 K contained in mushrooms from the regions studied was considered negligible and low, respectively. Mushroom species examined in this study are a potentially good source of dietary potassium.


INTRODUCTION
Radiocaesium ( 134/137 Cs), if not mention the shortlived radioactive 131 I (t 0.5 = 8.02 days), is the main mass and a long-term source of the toxic radiation, polluting the Earth in the past from the nuclear weapon explosions and nuclear power plant accidents [1,2].
Radiocaesium ( 137 Cs) is an artificial and long-lived (t 0.5 = 30.1 years) nuclide, which appeared in mushrooms after global fallout from nuclear weapons detonations in the atmosphere. High levels of radioactivity reappeared following the collapse of the Chernobyl nuclear power plant in 1986, including massive levels of 134 Cs and 137 Cs emissions [15]. The consequent radioactive fallout caused a long-lasting and substantial contamination with 137 Cs of forest ecosystems including mushrooms in regions surrounding the collapsed plant, especially in the Ukraine, Belarus and Russia, as well as elsewhere in Europe [16][17][18][19][20][21][22][23].
As in Chernobyl, a similar accident occurred in Japan in March 2011, where, following a major earthquake, a 15-meter tsunami disabled the power supply and cooling systems of three Fukushima Daiichi nuclear power plant reactors. All three cores largely melted in the first three days, caused radioactive contamination of the environment on a large scale, including high 137 Cs pollution of fungi growing in the region [24][25][26].
The nuclear accidents caused long-term psychosocial consequences on exposed individuals. One of the consequences was that big game and domesticated ruminants that eat contaminated mushrooms could be also heavily loaded with 137 Cs [27][28][29]. In humans, mushrooms can be also the most important exposure route to 137 Cs when there is elevated consumption of wild species [30]. As mentioned, contamination by 137 Cs after the Chernobyl accident as well as atomic weapon testing is a long-lasting process in some mushroom species even collected relatively away from this source [12][13][14].
The contribution of the 137 Cs fallout from the Chernobyl accident to ecosystems in distant places like the Japanese islands was considered small compared to the previous global fallout [31]. The Chernobyl fallout had also some impacts on continental Asia. In China, soils (layer 0-10 cm) sampled from 56 sites in the Inner Mongolia province in 1982-1987 showed 137 Cs mean activity concentration of 13.6 ± 6.6 Bq kg -1 dry weight (dw) (from 5.8 ± 4.4 to 23.4 ± 13.4 Bq kg -1 dw) [32]. Soil from Yunnan province was also contaminated, showing activity of 6.2 ± 5.4 Bq kg -1 dw (from 1.9 ± 0.3 to 31.6 ± 0.8 Bq kg -1 dw) in 1982-1987 [33].
The accident in the Fukushima nuclear power plant caused a high alert on a direct and indirect radioactive pollution consequences regarding to exposed staff and local residents. It affected public health and foods safety in Japan, as well as continental Asia from serious accidental discharge and included studies on the consequence to various types of environmental media including soils, vegetation and wild growing mushrooms [25,[34][35][36][37][38][39][40][41][42][43][44][45][46].
Edible mushrooms collected from the wild are common foodstuffs in Yunnan, a land diverse in climate, soil, forest types and landscape topography and with a high biodiversity of mushroom species [47,48]. Certain species are conditionally edible or medicinal mushrooms, e.g. Caloboletus calopus (Pers.) Vizzini or Tricholoma sejunctum (Fr. ex Sow.) Quél. Inner Mongolia has an area of 1 183 000 km 2 (457 000 sq mi) with a landscape made up largely of meadows with an abundance of saprobic mushrooms. This region is poor in ectomycorrhizal mushrooms, a result of the limited wooded areas, apart from the thickets along the Huang He River [49].
To get greater awareness of radioactive pollution in Asia, particularly after the Fukushima accident, this study investigated the radioactivity contamination with 137 Cs and 40 K of edible wild mushrooms from the Inner Mongolian and Yunnan provinces of China. The activity concentrations of 137 Cs and 40 K were studied for the first time in wild mushrooms (five species) from Inner Mongolia and also in more than 26 species, including taxa without previous data on 137 Cs Soil samples were collected in parallel as two pooled samples of topsoil (0-10 cm layer) beneath the fruiting bodies of A. arvensis from grassy stands in the Bayanhushu site in Inner Mongolia. Details of the geographical locations of the sampling sites from which mushrooms and topsoil were collected are given in Fig. 1 and Table 1.
Preparation of materials. To examine the distribution of 137 Cs and 40 K and total K between the morphological parts, individual fruiting bodies were rinsed and separated into caps (with skin) and stipes, but some were examined as whole (Table 1). Before drying, the fungal materials were sliced into pieces using a ceramic knife and pooled to create composite samples representing each species, sampling location and time of collection. Mushroom parts were dried at 65°C to constant mass (Ultra FD1000 dehydrator, Ezidri, Australia), finely powdered in a porcelain mortar, passed through an 80-mesh sieve, and stored in screw sealed plastic (low density polyethylene) bags under dry conditions.
Two pooled samples of topsoil (0-10 cm layer; 150 g whole weight each) were cleaned from any visible pebbles, leaves and twigs, soil samples, air dried under clean condition, ground (porcelain mortar), sieved (2 mm mesh plastic sieve), and stored in sealed polyethylene bags.
Directly before analysis, the mushroom and soil materials were prophylactically deep frozen and lyophilized (Labconco Freeze Dry System, Kansas City, MO, USA) for three days to ensure full dehydration.
Instrumental analysis. The analytical methodology applied has been presented in detail before [43,67,68] but a summarized description is given below. In brief, activity concentrations of 137 Cs, 134 Cs and 40 K were measured using a γ-spectrometer with a coaxial HPGe detector with a relative efficiency of 18% and a resolution of 1.9 keV at 1.332 MeV of 60 Co (with associated electronics) (Detector GC 1819 7500 SL, Canberra Packard, Poland, Warsaw). The measurements of the fungal materials in this study were preceded by   137 Cs (1.5%), 54 Mn (1.55%), 65 Zn (1.2%) and 60 Co (0.8%). The same geometry of cylindrical dishes with a 40-mm diameter was used for the analysis of the fungal material extracts as well as for the reference samples during equipment calibration organized by IAEA-RML-2018-01. Detailed results of the intercalibration are available in the publication [50].
Minimum detectable activity was determined by the Currie method. This method is based on two basic parameters: (a) critical level, which is defined as a level below which the detection signal cannot be reliably recognized and (b) detection limit specifying the smallest signal that can be quantitatively reliable. The measurement results obtained were recalculated for dehydrated materials and decay corrected back to the time of collection. Total potassium content was calculated from the original 40 K activity concentration data (using mean value of 29.32 Bq g -1 ) in natural K, which is in the range from 27.33 to 31.31 Bq g -1 of K (percentage abundance of 40 K atoms in natural K is 0.0117%) [51]. 137 Cs and 134 Cs in mushrooms and soil. All species collected from Inner Mongolia in this study were saprobic. 134 Cs activity was not detected in any of the study samples. It was possibly due to the negligible impact from the Fukushima's fallout in 2011 as wells as a relatively short half-life of this isotope (t 0.5 = 2.1 years) and small impacts from the Chernobyl's fallout in 1986 and preceding, the nuclear weapons detonations in the atmosphere.

RESULTS AND DISCUSSION
The values of the activity concentration of 137 Cs in caps and stipes of the fruiting bodies of Agaricus arvensis, Calocybe gambosa, Lepista personata and Macrolepiota excoriata and in the whole fruiting bodies of C. gigantea were in the range from < 4.1 to 19 ± 4 Bq kg -1 dw (Table 1). There is no prior data for these species from regions of Asia other than Inner Mongolia [44,53,54]. The low levels of 137 Cs contamination in the studied mushrooms from the Inner Mongolian region reflects low activities of this nuclide in local soils as well as a lower potential of these species to bio-accumulate this nuclide.
In this study, a composite sample of the upper (0-10 cm) layer of soil collected in parallel with A. arvensis from the Bayanhushu site (altitude 920 m a.s.l.) showed 137 Cs activity concentration of 6.8 ± 0.7 Bq kg -1 dw. This result obtained for the sample from 2015 is around 2 to 4-fold lower than earlier results cited for topsoils collected in Inner Mongolia in 1982-1987, and is close to the activity values reported in 1-5 cm layer of forest topsoils sampled from the Changning and Mengman sites in Yunnan in 2016 (4.9 ± 0.6 and 7.5 ± 0.7 Bq kg -1 dw) [53].
Because of colder weather in the mountains, soil and the mushrooms can be specifically affected with radiocaesium, which is scavenged from the contaminated plumes by wet precipitation [53][54][55]. Forest topsoil collected at 3000 m above sea level from the Minya Konka (Gongga Shan) mountain in Sichuan province of China in 2012 showed 137 Cs at level from 41 ± 1 to 79 ± 2 Bq kg -1 dw. This result is well in excess 137 Cs, Bq kg -1 dw 40  Continuation of Table 1 of what has been noted in topsoil from Inner Mongolia in this study or other studies of soils from China [32,33,53]. As given in Table 1, the determined activity concentrations of 137 Cs in fruiting bodies of the saprobic and perhaps a little parasitic species of Auricularia delicate, the caps and stipes of fruiting bodies of the saprobic decomposer Lentinula edodes, the saprobic Morchella esculenta as well as over 20 species of mycorrhizal mushrooms collected in Yunnan were low and roughly in the range of values noted in mushrooms from Inner Mongolia.
The only exception was individuals of Lactarius hygrophoroides collected from the region of Jiulongchi in Yuxi prefecture in central Yunnan in the summer of 2016. They showed activity concentrations of 137 Cs from 130 ± 5 to 210 ± 13 Bq kg dw -1 in caps and from 60 ± 5 to 67 ± 7 Bq kg dw -1 in stipes (Table 1). These relatively high levels of 137 Cs activity in L. hygrophoroides from the Jiulongchi site were in the range of activities determined previously in several species of ectomycorrhizal mushrooms collected at 2900-3600 m above sea level from the Minya Konka summit in 2012 [53].
Many other species of mushrooms collected from the prefecture of Yuxi and across other regions from Yunnan and elsewhere in China (Zhangzhou in the Fujian province) in 2010-2018 were substantially less contaminated than L. hygrophoroides from the Jiulongchi site or even mushrooms from the subalpine regions on the eastern slope of the Minya Konka summit [12,16,42,44,47,52,53,56]. The exception was  [44,57].
Elevated activity concentrations of 137 Cs in L. hygrophoroides from the Jiulongchi site in this study can possibly be explained by weather conditions (episodic rain) scavenging nuclides from the radioactive plume after the Fukushima (Japan) nuclear power plant accident in early 2011.
The radioactive incident took place in Tongchuan, Shaanxi Province, south of the central region of Inner Mongolia (approximate distance from the sampling sites mushrooms there is 1200 km). Some 137 Cs from a measuring instrument (lead ball -a major component of a nuclear scale) when dismantling a cement factory has gone missing. In a later investigation, radioactivity from 137 Cs was found at a steel refinery in Shaanxi's Fuping county. Possibly, a lead ball with scrap metal was melted down into the steel [58]. Information on possible, if any, ground pollution in the region from this accident is not available.
A recent (2021) study showed that the activity concentration of 137 Cs in 66 out of 68 of wild mushrooms (17 species) collected from the northeast regions of China in 2017-2020 ranged from < 0.6 to 26 Bq kg -1 dw (data rounded), and only in single Lactarius deliciosus and Lepista nuda (Bull.) Cooke specimens collected in 2020, was 46 ± 3 Bq kg -1 dw and 130 ± 9 Bq kg -1 dw, respectively [59].
The maximum activity concentration of 137 Cs noted in L. nuda in the above mentioned study was close to values determined in Lactarius hygrophroides and Lactarius volemus from Jiulongchi, Yuxi (Yunnan) ( Table 1), while the results are not very comparable due to only two single specimens examined by Wang et al. [59].
The radiocaesium contamination of land, the oceans and biota, including edible wild growing mushrooms has thus far, occurred in three main waves. The first one arose from the nuclear weapons detonations in the atmosphere in the period from 1945 to 1980 and resulted in wide-spread aerial diffusion of radiocaesium and other nuclides including 14 C, 137 Cs, 90 Sr, 239-240 Pu, 241 Am and 3 H [60]. With time, the resulting depositions of longer lasting 137 Cs affected every region of the world [1,60].
Data on radiocaesium in mushrooms for the period before 1986 is scarce [10-13, 42, 61]. Fifteen years before the Chernobyl accident, a solely fruiting body of Tricholoma terreum collected from the Czech Republic in 1971 showed 137 Cs at a level of 40 Bq kg -1 dw [61]. Additional historical data on 137 Cs in mushrooms was recorded in 1984, in Poland for the Poison Pax (Paxillus involutus), which showed 137 Cs at a level of 2700 Bq kg -1 dw, with lower levels noted for the King Bolete (Boletus edulis) (95 and 104 Bq kg -1 dw) and Slippery Jack, Suillus luteus (125 and 150 Bq kg -1 dw) collected in 1984 and 1985, respectively [10].
Data on the radiocaesium concentration activities accumulated in wild mushrooms growing in Asia from the period before the Chernobyl accident are absent in the available literature. Effectively, there is also nothing published on radiocaesium in wild mushrooms from mainland Asia in the period between the Chernobyl and Fukushima incidents.
The Chernobyl emission of radioactivity caused an extreme and long-lasting radiocaesium pollution of wild growing mushrooms in the regions of Europe, and particularly in the neighbor areas collapsed nuclear power plant [12,16,17,[62][63][64][65]. Japanese researchers have published a large volume of data on artificial radioactivity accumulated in wild mushrooms growing in the country, both from the post-Chernobyl and post-Fukushima emissions, which have recently been evaluated by Komatsu et al. and Prand-Stritzko and Steinhauser [25,66]. The activity in these wild mushrooms collected in the period up to March 2011 was largely from accumulated radiocaesium ( 137 Cs) due to the global fallout from nuclear weapons detonations, with a small proportion being attributed to the Chernobyl emissions [54]. The more recent emissions from the Fukushima incident changed the pattern of radionuclide contamination of wild mushrooms in Japan. However, as shown in this study (Table 1) and in a few other reports, the emissions could have only a small impact on mainland Asia or elsewhere [44,53,[68][69]. 40 K and K in mushrooms and soil. The topsoil from the Bayanhushu site showed 40 K activity concentration of 595 ± 41 Bq kg -1 dw and total K content of 17 000 ± 1000 mg kg -1 dw, which were higher than previously determined in topsoils sampled from several forested areas in Yunnan (150 ± 14 to 340 ± 19 Bq kg -1 dw) [53].
In the study by Zhang et al., the means of 40 K activity concentrations in topsoils (0-10 cm) in Inner Mongolia and Yunnan in 1982-1987 were 755 (866-1066 Bq kg -1 dw) and 487 Bq kg -1 dw (149-1010 Bq kg -1 dw), respectively [70]. In another national survey performed during 1983-1990, the area-weighted mean and the point-weighted mean of 40 K were 655.6 and 624.6 Bq kg -1 dw, respectively, for soils in Inner Mongolia, while the two values for soils from Yunnan were 532.0 and 518.6 Bq kg -1 dw, respectively [71].
The activity concentrations of 40 K in mushrooms from Inner Mongolia were in the range of 875 ± 140 to 1600 ± 320 Bq kg -1 dw in caps and from 1100 ± 180 to 1400 ± 620 Bq kg -1 dw in stipes (Table 1). In the case of mushrooms from Yunnan, A. delicate (ear-like jelly fungus), which grows on wood, they had a lower activity concentration of 40 K (540 ± 61 Bq kg -1 dw) than L. edodes (Table 1), which also grows on wood. The L. edodes showed activities in the range of 790 ± 110 to 1200 ± 140 Bq kg -1 dw in the caps, which are culinary valued, and from 640 ± 88 to 1100 ± 240 Bq kg -1 dw in the stipes, which are largely discarded. This species collected from Yunnan and examined by other authors, demonstrated the mean value of 40 K activity concentration to be 629 Bq kg -1 dw (from 396 to 1010 Bq kg -1 dw; n = 11) [44]. 40 K values in the caps of terrestrial mushrooms from Yunnan were from 580 ± 110 Bq kg -1 dw in L. deliciosus to 4000 ± 680 Bq kg -1 dw in Boletus tomentipes, while stipes showed activities from 380 ± 78 Bq kg -1 dw in L. deliciosus to 1900 ± 340 Bq kg -1 dw in Tricholoma sejunctum.
Potassium (total K) is the major metallic element in mushrooms and occurs in dried fungal materials in quantities of up to several percent, while the natural nuclide 40 K forms only a small proportion (makes up 0.012%) of the total. Hence, mushrooms collected from areas that are only mildly affected by 137 Cs depositions or mushrooms without a high species-specific ability to bioconcentrate this nuclide, e.g. like some species from the genus Cortinarius, contained natural 40 K in high excess relative to 137 Cs (Table 1) [12].
The amounts of K in the caps, stipes, or whole fruiting bodies of the species in this study were in the range 16 000 to 120 000 mg kg -1 dw (1.6 to 12 g kg -1 dw). Potassium is indispensable for mushrooms, for the uptake and osmotic regulation of water in the cytoplasm of cells and is a co-factor in certain enzymes [72]. However, the same species, i.e. A. arvensis, Boletus bainiugan, Retiboletus griseus, Rubroboletus sinicus, Caloboletus calopus, L. hygrophoroides, L. edodes and T. sejunctum collected from different sites could differ around twofold in the content of K (Table 1).
The daily adequate intake of K for adults is 2300 mg for females and 3400 mg for males [73]. Thus, the mushroom species examined in this study and assuming absorption rate at around 90% could be considered as potentially good sources of dietary potassium, especially when stir-fried with oil, which is a common culinary technique in SW China [67].
Potential risk from ionizing radiation doses. In this study, a total of 70 lots of several species of edible mushrooms collected from 26 locations in Yunnan were examined and in 63 lots, the contamination with 137 Cs of the caps or the whole mushrooms was well below 20 Bq kg -1 dw (Table 1). There were three of 70 lots that were more contaminated with 137 Cs than the others. Those lots were the gilled mushroom B. tomentipes (of 69 ± 4 Bq kg -1 dw), caps of the lamellar mushroom L. hygrophoroides (130 ± 5 Bq kg -1 dw), and caps of lamellar L. volemus (210 ± 13 Bq kg -1 dw) ( Table 1). Assuming that the moisture content in fruiting bodies is 90%, the estimated 137 Cs activities in these three species were 6.9, 13, and 21 Bq kg -1 on a wet weight basis. Therefore, these amounts were much lower than the maximum permitted levels for import of mushrooms from third countries [specific 13 countries affected by the Chernobyl's radioactive fallout for which the regulation applies] to the European Union (600 Bq kg -1 ) [74].
In Yunnan, the main way to cook mushrooms is stirfrying in vegetable oil in a wok pan [75]. It is interesting that stir-fried mushroom meals showed about 2 to 5-fold higher activity concentrations of 137 Cs than the raw mushrooms on a whole weight (wet) basis [67,68]. Therefore, a 100-g portion of stir-fried L. volemus caps from the most contaminated lot in this study could include from 4.2 to 10.5 Bq of 137 Cs (equivalent to ionizing radiation dose from 56×10 -3 to 140×10 -3 µSv per capita or 0.49×10 -3 to 2.35×10 -3 µSv per kg body mass; 60 kg body mass). These estimates are low, taking into account the risk associated with the doses of ionizing radiation received by consumers in Yunnan, even if stirfried mushrooms are consumed daily for longer periods during the mushrooming season.
In comparison, the natural 40 K nuclide contained in mushrooms (Table 1) introduces much higher doses of ionizing radiation than 137 Cs for locals in Inner Mongolia and Yunnan provinces but is not considered as a hazardous nuclide for consumers due to homeostasis of K in human body.

CONCLUSION
The activity concentrations of 137 Cs in lamellar mushrooms from the Inner Mongolia province of China and the local soil were low. 137 Cs contamination of the lamellar and gilled mushrooms from Yunnan province in China was also low, i.e. well below one tenth of statutory limits, and mushroom meals there can be considered as a negligible source of 137 Cs for their consumers.
In view of the results from this study, the accident in the Fukushima nuclear power plant had little or negligible effect on radioactive contamination of edible and medicinal fungi in the regions of China. Natural nuclide 40 K contained in mushrooms is not considered as hazardous for mushroom meal consumers. Wild mushrooms can be considered as a good source of dietary potassium for consumers.

CONFLICT OF INTEREST
The authors declare no conflict of interests regarding the publication of this article.