Role of the fungal mycelium in the retention of radiocaesium in forest soils

Evaluation of contribution rate of the infiltrated water collected using zero-tension lysimeter to the downward migration of 137Cs derived from the FDNPP accident in a cedar forest soil

The vertical distribution of ¹³⁷Cs in forest soil is important for predicting air dose rates and future cycling in forest ecosystems. However, there are many unexplained questions about the mechanisms of its downward migration. In this study, the ¹³⁷Cs flux by rainfall infiltration was observed for three years from August 2017 using zero-tension lysimeters in a mature cedar forest where monitoring of the vertical distribution of ¹³⁷Cs has been conducted since 2011. As a result, the ¹³⁷Cs concentration in infiltrated water through the litter layer, 5 cm and 10 cm showed a tendency to be high in summer, but no such seasonal variation was found at 20 cm. Although the ¹³⁷Cs inventory in the litter layer has been exponentially decreasing, the annual ¹³⁷Cs fluxes in infiltrated water through the litter layer were almost the same in three years, and about 0.14-0.17% of the deposition density of ¹³⁷Cs. Comparing these ¹³⁷Cs fluxes with the apparent amounts of downward migration of ¹³⁷Cs estimated from the change in the vertical distribution of ¹³⁷Cs, the contribution rate of the infiltrated water to downward migration of ¹³⁷Cs from litter to soil was calculated to be 8.5-17.7%. Similarly, the contribution rate in mineral soil layers was calculated to be 0.6-0.8% on a measured basis and estimated to be 3.0 ± 0.2% after correcting the amount of collected water, which is a problem with zero-tension lysimeter. It indicates that rainfall infiltration can explain a small part of the downward migration of ¹³⁷Cs, thus further studies are required to clarify the contribution rate of remaining mechanisms such as advection-diffusion, colloidal transport, physical mixing, bioturbation, and growth and death of plant roots.

Effect of drying, blanching, pickling and maceration on the fate of 40K, total K and 137Cs in bolete mushrooms and dietary intake

The effects of blanching, blanching and pickling and maceration on the leaching of ¹³⁷Cs and ⁴⁰K from the flesh of three edible bolete mushroom species-Boletus edulis, Leccinum scabrum and Leccinum versipelle-were investigated. Significant (p < 0.05) decreases in activity were observed but varied depending on the treatment. Relative to fresh mushrooms, blanching decreased the activity concentration of ¹³⁷Cs by 15 ± 13%, and of ⁴⁰K, by 16 ± 7%, but blanching and pickling (vinegar) reduced activity more effectively, by 55 ± 8% and 40 ± 20% respectively. The corresponding losses of ¹³⁷Cs and ⁴⁰K through maceration of dried, powdered mushrooms were 38 ± 11% and 35 ± 14% ww, respectively. These results indicate that traditional domestic processing methods may not be as efficient at excluding ¹³⁷Cs radioactivity as shown in some other studies. The activity concentration of ¹³⁷Cs in a typically sized (100 g) portion of a processed mushroom (sourced from nearshore regions of the southern Baltic Sea coast near Gdańsk in 2015) meal was projected to be low, i.e. in the range of 0.51 to 12 Bq kg^-1 ww. The corresponding effective dose of ¹³⁷Cs from blanched, blanched and pickled and macerated mushrooms per capita was also assessed to be low, from 0.001 to 0.010 μSv. Nutritionally, the median concentration of potassium (330 mg) in 100 g portions of blanched or pickled mushrooms would account for around 7% of the adequate adult daily intake.

Cesium-137 stored on and discharged from banks of an agricultural canal in Iitate, Fukushima

This study aimed to investigate the dynamics of ¹³⁷Cs around banks along an agricultural canal for paddy fields in Iitate, Fukushima, Japan. Five plots (2.4-12.6 m²) on the banks were monitored intermittently during six time periods from May 2018 to November 2019. We directly collected runoff water samples discharged from the banks followed by partitioning it into particulate and dissolved fractions and determining ¹³⁷Cs in them. To investigate the source of ¹³⁷Cs in the runoff water, we sequentially extracted ¹³⁷Cs in various chemical forms from litter samples collected on the banks. The results showed that the discharge rates of the dissolved ¹³⁷Cs per unit area from the plots were lower than those observed at the downstream of the agricultural canal, whereas more than 50% of the ¹³⁷Cs discharged from the plots was in the dissolved fraction. Moreover, the results indicate that ¹³⁷Cs stored in the standing plants and the litter was the primary source of the dissolved ¹³⁷Cs discharged into the agricultural canal. The concentrations of the water-soluble ¹³⁷Cs in the litter per plot area may have been retained by the sufficiently higher concentrations of ¹³⁷Cs in litter in other chemical forms and those in the standing plants, which are the source of the litter.

Radiocaesium in Tricholoma spp. from the Northern Hemisphere in 1971-2016

A considerable amount of data has been published on the accumulation of radiocaesium (¹³⁴Cs and particularly, ¹³⁷Cs) in wild fungi since the first anthropogenically influenced releases into the environment due to nuclear weapon testing, usage and subsequently from major accidents at nuclear power plants in Chernobyl (1986) and Fukushima (2011). Wild fungi are particularly susceptible to accumulation of radiocaesium and contamination persists for decades after pollution events. Macromycetes (fruiting bodies, popularly called mushrooms) of the edible fungal species are an important part of the human and forest animal food-webs in many global locations. This review discusses published occurrences of ¹³⁴Cs and ¹³⁷Cs in twenty four species of Tricholoma mushrooms sourced from the Northern Hemisphere over the last five decades, but also includes some recent data from Italy and Poland. Tricholoma are an ectomycorrhizal species and the interval for contamination to permeate to lower soils layers which host their mycelial networks, results in a delayed manifestation of radioactivity. Available data from Poland, over similar periods, may suggest species selective differences in accumulation, with some fruiting bodies, e.g. T. portentosum, showing lower activity levels relative to others, e.g. T. equestre. Species like T. album, T. sulphurescens and T. terreum also show higher accumulation of radiocaesium, but reported observations are few. The uneven spatial distribution of the data combined with a limited number of observations make it difficult to decipher any temporal contamination patterns from the observations in Polish regions. When data from other European sites is included, a similar variability of ¹³⁷Cs activity is apparent but the more recent Ukrainian data appears to show relatively lower activities. ⁴⁰K activity in mushrooms which is associated with essential potassium, remains relatively constant. Further monitoring of ¹³⁷Cs activity in wild mushrooms would help to consolidate these observations.

Radiocesium in Shiitake mushroom: Accumulation in living fruit bodies and leaching from dead fruit bodies

In this paper, cesium (Cs) accumulation by the saprophytic fungus Lentinula edodes (Shiitake) was investigated to contribute to the elucidation of radiocesium-cycling mechanisms in forest environments. Although the ¹³⁷Cs in the mushroom bed before culture was bioavailable, the transfer factor (TF) of Cs (¹³³Cs and ¹³⁷Cs) from the mushroom bed to fruit bodies was low (approximately 1) and the TFs of K (5) and Na (1.5) were higher. Cs and K concentrations in fruit bodies at different maturity stages were almost constant. The concentration ratio of Cs/K is constant in the pileus regardless of the pileus tissues. These results demonstrate that Shiitake non-specifically accumulates Cs while accumulating the essential element K and provide evidence that no selective Cs accumulation (or binding) sites exist within the Shiitake fruit body. Furthermore, the present results show that most accumulated Cs quickly leaches out from the dead fruit body with exposure to water. The leached Cs was largely adsorbable on clay minerals, suggesting that the Shiitake fruit body likely contains Cs in the cation form.

Sorption and desorption experiments using stable cesium: considerations for radiocesium retention by fresh plant residues in Fukushima forest soils

We conducted sorption experiments with stable cesium (133Cs) solution in different organic matter samples, aiming to understand the sorption of radiocesium (134Cs and 137Cs) in the initial throughfall by fresh plant residues (e.g., needles, wood, and bark from Japanese cedar trees) in the Oi horizon in forests in Fukushima. Among the organic matter samples, bark and wattle tannin sorbed relatively large amounts of Cs, whereas wood and cellulose powder sorbed small amounts. In contrast, samples containing clay minerals showed much higher Cs sorption. We also conducted desorption experiments, and suggested that Cs on the organic matter samples were relatively mobile.

Chemical sequential extraction of O horizon samples from Fukushima forests: Assessment for degradability and radiocesium retention capacity of organic matters

To investigate how radiocesium (¹³⁷Cs) is retained in the O horizon via interactions with organic matter, we collected O horizon samples in Japanese cedar (Cryptomeria japonica) and konara oak (Quercus serrata) forest sites in Fukushima during the 8 years following the Fukushima Dai-ichi Nuclear Power Plant accident. To assess degradability and ¹³⁷Cs retention capacity of organic matter, we conducted chemical sequential extraction with organic solvent and sulfuric acid, collecting the following fractions: organic solvent extractives (Fraction 1), acid-soluble carbohydrates (Fraction 3), and acid-insoluble residue (Fraction 4). In all samples, across sampling years and sites, ¹³⁷Cs content in Fractions 1, 3, and 4, as a proportion of the total ¹³⁷Cs content, was 0.0-23.6%, 18.4-42.9%, and 44.8-76.0%, respectively. Generally, ¹³⁷Cs is considered to be electrostatically bound to organic matter and relatively mobile, making it easily extractable by sulfuric acid treatment. However, we observed a relatively high proportion of ¹³⁷Cs in Fraction 4, suggesting strong retention of ¹³⁷Cs and their immobility in the O horizon. Complex organic matter such as lignin or tannin may contribute this retention. We also noted that some part of ¹³⁷Cs may be also retained by clay minerals in the O horizon. Although organic matter in Fractions 1 and 3 is considered to decompose faster than that in Fraction 4, over the observation period the ¹³⁷Cs proportion and net rate of decrease in ¹³⁷Cs content (in total and in each fraction) remained nearly constant. This result implies that decomposition of organic matter and the consequent release of bound ¹³⁷Cs may be partly compensated by additional input of ¹³⁷Cs from the canopy and ¹³⁷Cs recycling by soil microorganisms. Our study highlights the potential role of organic matter in the O horizon as a temporary reservoir of ¹³⁷Cs and a driver of the ¹³⁷Cs cycle in forest ecosystems.

In vivo removal of radioactive cesium compound using Prussian blue-deposited iron oxide nanoparticles

Aim: To mitigate the side effects of medical treatment by Prussian blue (PB), a well-known adsorbent for radioactive cesium (Cs), PB-deposited magnetic nanoparticles (MNPs), were prepared and analyzed on the adsorbent capacity for Cs removal. Materials & methods: The PB-deposited MNPs were prepared by photo-deposition method and investigated for their Cs adsorption properties in vitro and in vivo. The distribution of the adsorbents was also evaluated in C57BL/6 mice. Results: PB-deposited MNPs provided an improved adsorbent capacity for Cs removal and reduced toxicity to blood cells compared with those of bulk PB. Conclusion: PB-deposited MNPs could be considered as an alternative of PB-based medicine to reduce the possible hazards caused by high dose of PB intake.

Fit-for-purpose modelling of radiocaesium soil-to-plant transfer for nuclear emergencies: a review

Numerous radioecological models have been developed to predict radionuclides transfer from contaminated soils to the food chain, which is an essential step in preparing and responding to nuclear emergencies. However, the lessons learned from applying these models to predict radiocaesium (RCs) soil-to-plant transfer following the Fukushima accident in 2011 renewed interest in RCs transfer modelling. To help guide and prioritise further research in relation to modelling RCs transfer in terrestrial environments, we reviewed existing models focussing on transfer to food crops and animal fodders. To facilitate the review process, we categorised existing RCs soil-to-plant transfer models into empirical, semi-mechanistic and mechanistic, though several models cross the boundaries between these categories. The empirical approach predicts RCs transfer to plants based on total RCs concentration in soil and an empirical transfer factor. The semi-mechanistic approach takes into account the influence of soil characteristics such as clay and exchangeable potassium content on RCs transfer. It also uses 'bioavailable' rather than total RCs in soil. The mechanistic approach considers the physical and chemical processes that control RCs distribution and uptake in soil-plant systems including transport in the root zone and root absorption kinetics. Each of these modelling approaches has its advantages and disadvantages. The empirical approach is simple and requires two inputs, but it is often associated with considerably uncertainty due to the large variability in the transfer factor. The semi-mechanistic approach factorises more soil and plant parameters than the empirical approach; therefore, it is applicable to a wider range of environmental conditions. The mechanistic approach is instrumental in understanding RCs mobility and transfer in soil-plant systems; it also helps to identify influential soil and plant parameters. However, the comlexity and the large amount of specific parameters make this approach impractical for nuclear emergency preparedness and response purposes. We propose that the semi-mechanistic approach is sufficiently robust and practical, hence more fit for the purpose of planning and responding to nuclear emergencies compared with the empirical and mechanistic approaches. We recommend further work to extend the applicability of the semi-mechanistic approach to a wide range of plants and soils.

Effect of Pseudomonas fluorescens and pyoverdine on the phytoextraction of cesium by red clover in soil pots and hydroponics

With the aim of improving the phytoextraction rate of cesium (Cs), the effect of Pseudomonas fluorescens ATCC 17400 and its siderophore pyoverdine (PVD) on the uptake of Cs by red clover was studied in soil pots. This work also provides a mechanistic understanding of the Cs-bacteria (or PVD)-illite-plant interactions by using a simplified experimental design, i.e., hydroponics with either Cs in solution or Cs-spiked illite in suspension. For soil spiked with 11.2 mmol kg^-1 (1480 mg kg^-1) of Cs, 0.43% of total Cs was taken up by red clover in 12 days (119 μmol g^-1 (16 mg g^-1) of Cs dry matter in roots and 40 μmol g^-1 (5 mg g^-1) in shoots). In hydroponics with Cs in solution (0.1 mmol L^-1 or 13 mg L^-1), 75% of Cs was taken up vs. only 0.86% with Cs-spiked illite suspension. P. fluorescens and PVD did not increase Cs concentrations in aboveground parts and roots of red clover and even decreased them. The damaging effect of PVD on red clover growth was demonstrated with the biomass yielding 66% of the control in soil pots (and 100% mortality after 12 days of exposition) and only 56% in hydroponics (78% with illite in suspension). Nonetheless, PVD and, to a lesser extent, P. fluorescens increased the translocation factor up to a factor of 2.8. This study clearly showed a direct damaging effect of PVD and to a lower extent the retention of Cs by biofilm covering both the roots and illite, both resulting in the lower phytoextraction efficiency.

Behaviour of 137Cs in forest humus detected across the territory of the Czech Republic

The activity concentration of ¹³⁷Cs in samples of coniferous forest humus collected across the territory of the Czech Republic in 1995 and 2005 was analysed, and it was found significantly correlated with the surface deposition caused by the Chernobyl accident. The effective (12.8 y) and environmental (22.3 y) half-lives of radiocaesium in humus were calculated and compared with those in spruce bark. The impact of important forest stand factors, that is, precipitation, content of organic matter, age of trees and pH, on the behaviour of ¹³⁷Cs in humus was studied. It was observed that humus samples with a higher proportion of organic matter, higher pH(H2O) and pH(CaCl2) contained higher amounts of ¹³⁷Cs. Conversely, with the age of trees, the activity concentration of ¹³⁷Cs in humus is decreasing. Higher precipitation and humus acidity decrease the reduction rate of the ¹³⁷Cs in humus. These stand factors increase bioavailability of ¹³⁷Cs in humus. The transfer and retention of available ¹³⁷Cs in biomass of organisms living in humus for a long time can satisfactorily explain the longer residence time of ¹³⁷Cs in humus affected by the studied factors.

Direct accumulation pathway of radioactive cesium to fruit-bodies of edible mushroom from contaminated wood logs

This paper presents the accumulation process of radioactive Cs in edible mushrooms. We here first report the direct accumulation pathway of radioactive Cs from contaminated wood logs to the fruit-bodies of shiitake mushrooms through the basal portion of the stipe. In this pathway, radioactive Cs is not transported through the hyphae. This pathway results in a high accumulation of radioactive Cs in the fruit-body, more by the excess accumulation of radioactive Cs from the wood logs than that through the hyphae. We grew the fruit-bodies of Shiitake mushroom from radioactive-Cs-contaminated wood logs. The spatial distributions of radioactive Cs and Prussian blue as a tracer of interstitial water in the cross section of the wood log measured after the harvest of the fruit-body from the inoculated sawdust spawn area indicated that some fraction of the radioactive Cs and Prussian blue were transported directly to the basal portion of the stipe during the growth of the fruit-bodies.

Long-term retention of (137)Cs in three forest soil types with different soil properties

Current (137)Cs activity concentrations were studied at three localities in individual soil horizons of Stagnosol, Arenic Podzol and Haplic Cambisol soil units in soil blocks with dimensions of 20 × 20 × 40 cm situated below pine canopies (n = 3) and spruce canopies (n = 3), and below small canopy gaps, at least 15 × 15 m in area (n = 3 + 3), which have probably endured since 1986. The main zone of (137)Cs accumulation in all the localities was found to be in the organic horizons (H and F). No significant transport and accumulation of (137)Cs into illuvial soil horizons (Bm, Bs or Bhs, Bv and Bv/IIC) was found. The estimated current total (137)Cs activity concentrations in the soil blocks 40 cm in depth were only slightly higher below the coniferous canopy than they were below nearby canopy gaps. The inventory of (137)Cs in the soils was found to be in accordance with the estimated (137)Cs inputs from the Chernobyl fallout and from global fallout. The low amounts of (137)Cs found accumulated in the aboveground biomass (mosses, grasses, needles) did not substantially bias the studied radiocaesium balance in the soils. The vertical migration rate of (137)Cs in soils (cm/year) had a tendency to be higher below canopies than below canopy gaps and below pine canopies than below spruce canopies. We expected the current (137)Cs activity concentrations in the individual soil horizons to be related to the studied soil parameters: pH (H2O), pH (CaCl2), content of organic matter and mineral portion and portion of humic and fulvic acid contents (Q4/6). However, this was not confirmed. Similarly, we observed a weak tendency toward higher (137)Cs activity in soils below the canopy than in soils below canopy gaps. The available gaps used in our study may have been too small, and they may have been affected by an accumulation of litter and humus containing (137)Cs from the surrounding plots situated below neighbouring canopies.

Absorption of radionuclides from the Fukushima nuclear accident by a novel algal strain

Large quantities of radionuclides have leaked from the Fukushima Daiichi Nuclear Power Plant into the surrounding environment. Effective prevention of health hazards resulting from radiation exposure will require the development of efficient and economical methods for decontaminating radioactive wastewater and aquatic ecosystems. Here we describe the accumulation of water-soluble radionuclides released by nuclear reactors by a novel strain of alga. The newly discovered green microalgae, Parachlorella sp. binos (Binos) has a thick alginate-containing extracellular matrix and abundant chloroplasts. When this strain was cultured with radioiodine, a light-dependent uptake of radioiodine was observed. In dark conditions, radioiodine uptake was induced by addition of hydrogen superoxide. High-resolution secondary ion mass spectrometry (SIMS) showed a localization of accumulated iodine in the cytosol. This alga also exhibited highly efficient incorporation of the radioactive isotopes strontium and cesium in a light-independent manner. SIMS analysis showed that strontium was distributed in the extracellular matrix of Binos. Finally we also showed the ability of this strain to accumulate radioactive nuclides from water and soil samples collected from a heavily contaminated area in Fukushima. Our results demonstrate that Binos could be applied to the decontamination of iodine, strontium and cesium radioisotopes, which are most commonly encountered after nuclear reactor accidents.

Accumulation of potassium, rubidium and caesium (133Cs and 137Cs in various fractions of soil and fungi in a Swedish forest

Radiocaesium ((137)Cs) was widely deposited over large areas of forest in Sweden as a result of the Chernobyl accident in 1986 and many people in Sweden eat wild fungi and game obtained from these contaminated forests. In terms of radioisotope accumulation in the food chain, it is well known that fungal sporocarps efficiently accumulate radiocaesium ((137)Cs), as well as the alkali metals potassium (K), rubidium (Rb) and caesium (Cs). The fungi then enhance uptake of these elements into host plants. This study compared the accumulation of these three alkali metals in bulk soil, rhizosphere, soil-root interface, fungal mycelium and sporocarps of mycorrhizal fungi in a Swedish forest. The soil-root interface was found to be distinctly enriched in K and Rb compared with the bulk soil. Potassium concentrations increased in the order: bulk soil<rhizosphere<fungal mycelium<soil-root interface<fungal sporocarps; and Rb concentration in the order: bulk soil<rhizosphere<soil-root interface<fungal mycelium<fungal sporocarps. Caesium was more or less evenly distributed within the bulk soil, rhizosphere and soil-root interface fractions, but was actively accumulated by fungi. Fungi showed a greater preference for Rb and K than Cs, so the uptake of (137)Cs could be prevented by providing additional Rb or K at contaminated sites. The levels of K, Rb, and Cs found in sporocarps were at least one order of magnitude higher than those in fungal mycelium. These results provide new insights into the use of transfer factors or concentration ratios. The final step, the transfer of alkali metals from fungal mycelium to sporocarps, raised some specific questions about possible mechanisms.

Availability and immobilization of 137Cs in subtropical high mountain forest and grassland soils

To understand the behavior of (137)Cs in undisturbed soils after nuclear fallout deposition between the 1940s and 1980s, we investigated the speciation of (137)Cs in soils in forest and its adjacent grassland from a volcano and subalpine area in Taiwan. We performed sequential extraction of (137)Cs (i.e., fractions readily exchangeable, bound to microbial biomass, bound to Fe-Mn oxides, bound to organic matter, persistently bound and residual). For both the forest and grassland soils, (137)Cs was mainly present in the persistently bound (31-41%) and residual (22-62%) fractions. The proportions of (137)Cs labile fractions--bound to exchangeable sites, microbial biomass, Mn-Fe oxides, and organic matter--were lower than those of the recalcitrant fractions. The labile fractions in the forest soils were also higher than those in the grassland soils, especially in the volcanic soil. The results suggest that the labile form of (137)Cs was mostly transferred to the persistently bound and resistant fractions after long-term deposition of fallout. The readily exchangeable (137)Cs fraction was higher in soils with higher organic matter content or minor amounts of 2:1 silicate clay minerals.

Soil microorganisms determine the sorption of radionuclides within organic soil systems

The potential of soil microorganisms to enhance the retention of (137)Cs and (85)Sr in organic systems was assessed in a series of experiments. A biologically active, 'mineral-free', organic material, produced under laboratory conditions from leaves, was used as the uptake matrix in all experiments to minimise potential interference from competing clay minerals. Biological uptake and release were differentiated from abiotic processes by comparing the sorption of radionuclides in sterilised organic material with sterile material inoculated with soil extracts or single fungal strains. Our results show conclusively that living components of soil systems are of primary importance in the uptake of radionuclides in organic material. The presence of soil microorganisms significantly enhanced the retention of Cs in organic systems and approximately 70% of the Cs spike was strongly (irreversibly) bound (remained non-extractable) in the presence of microorganisms compared to only approximately 10% in abiotic systems. Sorption of (85)Sr was not significantly influenced by the presence of soil microorganisms. A non-linear temperature response was observed for the retention in biotic systems with increased uptake at between 10 and 30 degrees C and lower retention at temperatures above or below the optimum range. The optimum temperatures for biological uptake were between 15 and 20 degrees C for Cs, and 25 and 30 degrees C for Sr. Our results indicate that single strains of soil and saprotrophic fungi make an important contribution to the sorption of Cs and Sr in organic systems, but can only account for part of the strong, irreversible binding observed in biotic systems. Single strains of soil fungi increased the amount of non-extractable (137)Cs (by approximately 30%) and (85)Sr (by approximately 20%) in the organic systems as compared to abiotic systems, but the major fraction of (137)Cs and (85)Sr sorbed in systems inoculated with saprotrophic fungi remained extractable.

Is there a future for sequential chemical extraction?

Since their introduction in the late 1970s, sequential extraction procedures have experienced a rapid increase in use. They are now applied for a large number of potentially toxic elements in a wide range of sample types. This review uses evidence from the literature to consider the usefulness and limitations of sequential extraction and thereby to assess its future role in environmental chemical analysis. It is not the intention to provide a comprehensive survey of all applications of sequential extractions or to consider the merits and disadvantages of individual schemes. These aspects have been covered adequately in other, recent reviews. This review focuses in particular on various key issues surrounding sequential extractions such as nomenclature, methodologies, presentation of data and interpretation of data, and discusses typical applications from the recent literature for which sequential extraction can provide useful and meaningful information. Also covered are emerging developments such as accelerated procedures using ultrasound- or microwave energy-assisted extractions, dynamic extractions, the use of chemometrics, the combination of sequential extraction with isotope analysis, and the extension of the approach to non-traditional analytes such as arsenic, mercury, selenium and radionuclides.