Fukushima Forests Research Articles

Changes in air dose rates due to soil moisture content in the Fukushima prefecture forests

Radionuclides released and deposited because of the 2011 Fukushima Dai-ichi Nuclear Power Plant accident caused an increase in air dose rates in Fukushima Prefecture forests. Although an increase in air dose rates during rainfall was previously reported, the air dose rates in the Fukushima forests decreased during rainfall. This study aimed to develop a method to estimate rainfall-related changes in air dose rates, even in the absence of soil moisture data, in Namie-Town and Kawauchi-Village, Futaba-gun, Fukushima Prefecture. Moreover, we examined the relationship between preceding rainfall (Rw) and soil moisture content. The air dose rate was estimated by calculating the Rw in Namie-Town from May to July 2020. We found that the air dose rates decreased with increasing soil moisture content. The soil moisture content was estimated from Rw by combining short-term and long-term effective rainfall using half-live values of 2 h and 7 d and considering the hysteresis of water absorption and drainage processes. Furthermore, the soil moisture content and air dose rate estimations showed a good agreement with coefficient of determination (R2) scores >0.70 and >0.65, respectively. The same method was tested to estimate the air dose rates in Kawauchi-Village from May to July 2019. At the Kawauchi site, variation in estimated value is relatively large due to the presence of water repellency in dry conditions, and the amount of 137Cs inventory was low, so estimating air dose from rainfall remained a challenge. In conclusion, rainfall data were successfully used to estimate soil moisture and air dose rates in areas with high 137Cs inventories. This leads to the possibility of removing the influence of rainfall on measured air dose rate data and could contribute to the improvement of methods currently used to estimate the external air dose rates for humans, animals, and terrestrial forest plants.

Late phase radiocesium dynamics in Fukushima forests post deposition

The long term dynamics of radiocesium in typical forest ecosystems was studied in the radioactive contaminated areas in Fukushima Prefecture. Six observations sites located in Yamakiya Village (Kawamata Town; since 2014), Tsushima Village (Namie Town, since 2015), and Tomioka Town (since 2017) were monitored. The forests consisted of artificial plantations of Japanese cedar (Cryptomeria japonica) at Yamakiya Village, Tsushima Village, and Tomioka Town. Tsushima Village also had a natural mixed forest dominated by Japanese red pine (Pinus densiflora), and Tomioka Town had a young and a mature artificial plantation of Japanese cypress (Chamaecyparis obtuse). Concentrations of 137Cs were monitored in the samples collected from the main aboveground biomass compartments, fresh litterfall, forest litter, and soil. Concentrations of exchangeable forms of 137Cs and stable K were measured in soil samples. During the observation period, the litter radiocesium inventories at all sites decreased significantly to approximately 1% or less of the total ground deposition. Approximately 80% of the total radiocesium inventory is localized in the upper 5-cm layer of soil and there is little downward migration of radiocesium. At the sites with the longest monitoring series (Yamakiya and Tsushima), the radiocesium expectation depths and expectation mass depths were relatively constant at 2–3 cm and 5–6 kg m−2, respectively. Aboveground biomass compartments showed similar decreasing trends in radiocesium aggregated transfer factors, Tag, in the compartments that were exposed to atmospheric fallout in March 2011 (old foliage, small branches, and outer bark). The mean Tag in cedar stand compartments currently are in the range of 10−3-10−2 m2 kg−1 dw. However, the mean Tag and their dynamic trend significantly differed in the wood compartments of the cedar stands, which may indicate root uptake differences of orders of magnitude between observation sites. The difference in radiocesium concentration in wood between the sites becomes less pronounced when normalized by the ratio of exchangeable 137Cs/K in the soils.

Analysis of Indirect Socioeconomic Impacts on East Asia by Recovering Fukushima Forest Watershed Quality

Analysis of Indirect Socioeconomic Impacts on East Asia by Recovering Fukushima Forest Watershed Quality

Intake of Radionuclides in the Trees of Fukushima Forests 5. Earthquake Could Have Caused an Increase in Xyloglucan in Trees

A megathrust earthquake caused the Fukushima–Daiichi nuclear power plant accident, which dispersed abundant radioiodines, causing them to be bound to xyloglucan into forest trees. Nevertheless, targeted xyloglucan was found in increased quantities in the annual rings of forest trees affected by the earthquake. We propose that trees could acclimate rapidly to shaking stress through an increase in xyloglucan deposition as a plant response under natural phenomena.

Intake of Radionuclides in the Trees of Fukushima Forests 4. Binding of Radioiodine to Xyloglucan

The 1,4-linked glucans such as xyloglucan and amylose are known to form a complex with iodine/iodide ions and to also be precipitated with CaCl2 in the presence of iodine. Here, we show that iodine gas could be specifically incorporated into xyloglucan. Furthermore, we show that [125I]I2 gas is, over time, incorporated at high levels into the entire outer surface of poplar seedlings but that spraying seedlings with abscisic acid to close stomata decreases the incorporation of the gas. There was less incorporation of the gas in a transgenic poplar overexpressing xyloglucanase at the early stages when compared with a wild type. This shows that xyloglucan serves as a key absorber of iodine gas into a plant body. After individual leaves of cultured seedlings were exposed to the gas for 30 min, no radioiodine was emitted from those leaves over the following two weeks, indicating that no turnover occurs in radioiodine once it is bound to the polysaccharides in plant tissues. We conclude that forest trees could serve as one of the largest enormous capture systems for the radioiodine fallout following the nuclear power plant accident in Fukushima.

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.

Intake of Radionuclides in the Trees of Fukushima Forests 3. Removal of Radiocesium from Stem Wood, Cryptomeria Japonica (L.f.) D. Don.

Nuclear power plant accidents have dispersed radiocesium into the atmosphere to contaminate trees with no turnover in heartwood, as occurred in Fukushima, and as has persisted for over 30 years around Chernobyl. Here we employ the ponding method, in which radiocesium can be flushed out from the cross-cut edges of Japanese cedar, Cryptomeria japonica (L.f.) D. Don., stem with water due to xyloglucan degradation in tracheids. Furthermore, lab-scale ponding experiments have shown that a non-detectable level of radiocesium has been observed not only in the pool water used for 575 days but also in the water containing recombinant xyloglucanase. This traditional technology is now a new biotechnology.

Radiation Dose Assessment Model for Terrestrial Flora and Fauna and Its Application to the Environment near Fukushima Accident

Background: To investigate radiological effects on biota, it is necessary to assess radiation dose for flora and fauna living in a terrestrial ecosystem. This paper presents a dynamic model to assess radioactivity concentration and radiation dose of terrestrial flora and fauna after a nuclear accident. Materials and Methods: Litter, organic soil, mineral soil, trees, wild crops, herbivores, omnivores, and carnivores are considered the major components of a terrestrial ecosystem. The model considers the physicochemical and biological processes of interception, weathering, decomposition of litter, percolation, root uptake, leaching, radioactive decay, and biological loss of animals. The predictive capability of the model was investigated by comparison of its predictions with field data for biota measured in the Fukushima forest area after the Fukushima nuclear accident. Results and Discussion: The predicted radioactive cesium inventories for trees agreed well with those for evergreens and deciduous trees sampled in the Fukushima area. The predicted temporal radioactivity concentrations for animals were within the range of the measured radioactivity concentrations of deer, wild boars, and black bears. The radiation dose for the animals were, for the whole simulation time, estimated to be much smaller than the lower limit (0.1 mGy∙ d-1) of the derived consideration reference level given by the International Commission on Radiological Protection for terrestrial flora and fauna. This suggested that the radiation effect of the accident on the biota in the Fukushima forest would be insignificant. Conclusion: The present dynamic model can be used effectively to investigate the radiological risk to terrestrial ecosystems following a nuclear accident.

Intake of Radionuclides in the Trees of Fukushima Forests 2. Study of Radiocesium Flow to Poplar Seedlings as a Model Tree

After the nuclear power plant accident in Fukushima, radionuclides were deposited over a large area of local forest. However, almost nothing is known about radionuclide infiltration into trees. Here, we used poplar seedlings as a model to show that radiocesium can enter directly into leaves and bark, moving via ray cells through the symplastic pathways to the xylem and concentrating around the meristems, cork, and vascular cambium. During induced potassium incorporation and reduced seasonal growth, the radiocesium in the meristems of stems mainly passes into abscission tissues such as leaves and heartwood. There is no turnover of radiocesium after it enters the heartwood.

Intake of Radionuclides in the Trees of Fukushima Forests 1. Field Study

The earthquake and tsunami on 11 March 2011 led to a meltdown followed by a hydrogen explosion at the Fukushima–Daiichi nuclear power plant in Japan, causing the dispersal of abundant radionuclides into the atmosphere and ocean. The radionuclides were deposited onto trees and local residences in aerosol or gaseous forms that were partly absorbed by rain or melting snow. Here, we show that the radionuclides attached to the surfaces of trees, in which some radiocesium was incorporated into the xylem through ray cells and through symplastic pathways. The level of incorporated radiocesium varied based on tree species and age because of the ability of radiocesium to attach to the surface of the outer bark. After four years, the radiocesium level in the forest has been decreasing as it is washed out with rainwater into the sea and as it decays over time due to its half-life, but it can also be continuously recycled through leaf tissue, litter, mulch, and soil. As a result, the level of radiocesium was relatively increased in the heartwood and roots of trees at four years after the event. In private forest fields, most trees were left as afforested trees without being used for timber, although some trees were cut down. We discuss an interdisciplinary field study on the immediate effects of high radiation levels upon afforested trees in private forest fields.

Rice Bran Addition to Leaf Compost Can Reduce Radiocesium Concentration and Its Uptake by Crops After the Fukushima Daiichi Nuclear Power Plant Accident

Leaf composting is an essential technique in organic farming; it improves the physicochemical properties of soil such as texture, structure, water-holding capacity, and nutrient content. However, the use of leaf compost is prohibited in the Fukushima and Ibaraki prefectures because large areas of the Fukushima and Ibaraki forests were contaminated by radiocesium (134Cs and 137Cs) after the Fukushima Daiichi nuclear power plant (FDNPP) accident. We examined the changes in radio Cs concentration and other physicochemical properties in leaf compost made from Ibaraki and Fukushima forest leaves. At the beginning of the composting process, rice bran-treated compost showed 25%–32% lower radio Cs concentration than the leaf-only compost; however, 2 years after composting, the difference in concentration between these treatments had increased to 35%–63%. Moreover, the incorporation of rice bran significantly increased the compost temperature, moisture, electrical conductivity, bulk density, and total nitrogen during the composting process. Plant uptake of radio Cs was significantly lower in rice bran-treated compost than the leaf-only compost at each level of application; furthermore, the levels of soil radio Cs showed a similar trend. Potassium application combined with leaf compost resulted in a significant reduction of radio Cs plant uptake. Our data revealed that adding rice bran to leaves positively affects radio Cs reduction in leaf compost and also reduces its uptake by plants. Our findings may improve the management of leaf composting after the FDNPP accident.

Web-building spider Nephila clavata (Nephilidae: Arachnida) can represent 137Cs contamination of arthropod communities and bioavailable 137Cs in forest soils at Fukushima, Japan

Large areas of Fukushima's forests were contaminated with radiocesium (137Cs) after the Fukushima Dai-ichi Nuclear Power Plant accident. Most of the contaminated forests have not been decontaminated, and bioavailable 137Cs is likely to circulate within the forest environment's food web. Nephila clavata (Nephilidae: Arachnida) is a top predator in the forest arthropod community, and this web-building spider potentially consumes many arthropod species presented in the grazing and detrital food chains. We tested whether 137Cs in the spider could serve as a proxy for 137Cs contamination of these arthropod communities. We also examined whether N. clavata could serve as a proxy for soil bioavailable 137Cs. Nephila clavata was similarly or more contaminated with 137Cs compared with lower-trophic-level arthropods such as herbivores and other predators at the same trophic level. Thus, the 137Cs activity of N. clavata could represent the extent to which the arthropod community was contaminated with 137Cs. Data from nine 137Cs-contaminated sites in Fukushima showed a significant positive correlation between soil bioavailable 137Cs and N. clavata's 137Cs activity05 but the coefficient of determination was only moderate (R2 = 0.43), suggesting that N. clavata is only a weak proxy of soil bioavailable 137Cs. Our results also showed that the bioavailable fraction of 137Cs in Fukushima was strongly correlated with the total inventory and that the K and Na contents of the soil determined the soil-to-spider transfer factor for 137Cs and the 137Cs activity in N. clavata, respectively. These results improve our understanding of 137Cs transfer from the soil to arthropod species.

APPLICATION OF THE FOREST SHIELDING FACTOR TO THE MAXIMUM-LIKELIHOOD EXPECTATION MAXIMIZATION METHOD FOR AIRBORNE RADIATION MONITORING.

The maximum-likelihood expectation maximization (ML-EM) method is expected to improve the accuracy of airborne radiation monitoring using an unmanned aerial vehicle. The accuracy of the ML-EM method depends on various parameters, including detector efficiency, attenuation factor, and shielding factor. In this study, we evaluate the shielding factor of trees based on several field radiation measurements. From the actual measurement, the shielding factors were well correlated with the heights of the trees. The evaluated shielding factors were applied to the ML-EM method in conjunction with the measured data obtained from above the Fukushima forest. Compared with the conventional methods used for calculating the dose rate, the proposed method is found to be more reliable.

Six-year trends in exchangeable radiocesium in Fukushima forest soils

We investigated the changes in 137Cs exchangeable fraction in the soils of contaminated forest ecosystems until 6 years after the Fukushima radioactive atmospheric deposits. For this investigation, we performed chemical extractions of 137Cs from both organic and mineral soil layer sampled from two forest stands, Japanese cedar (Cryptomeria japonica) and broadleaf deciduous, mainly konara oak (Quercus serrata), which are located 26–27 km inland from the Fukushima Dai-ichi Nuclear Power Plant. The exchangeable 137Cs as a proportion of the total 137Cs inventory was only 10% in the organic layer and 6.1% in the mineral soil layer about 5 months after the accident at the Japanese cedar site. We observed an exponential decrease in the proportion of exchangeable 137Cs in both organic and mineral soil layer samples over the 6-years observation period at both sites. The proportion significantly decreased within 2–4 years after the accident, becoming almost constant (2–4%). These results support the interpretation that contaminated forests have entered a steady-state phase of 137Cs cycling, although we need more integration for an improved vision of the future of Fukushima forests.

Effects of radiocesium fixation potentials on 137Cs retention in volcanic soil profiles of Fukushima forests

Radiocesium is well-known to be stabilized by clay minerals in soils, while volcanic soils could typically be poor in micaceous clays that fix 137Cs effectively. We investigated 137Cs fixation potentials [radiocesium interception potential (RIP)] and depth distribution of 137Cs stocks in volcanic soils to analyze effects of clay content and mineralogy on soil retention and migration of 137Cs after the Fukushima nuclear accident. Clay minerals of the volcanic soils were dominated by hydroxy-interlayered vermiculite (HIV) and short-range-order minerals, irrespective of bedrocks. The soil RIPs were positively correlated with mass of clay fraction among the HIV-dominated volcanic soils, but RIP per clay content in the HIV-dominated volcanic soils were lower than in the soils rich in illite or vermiculitic clays. The small RIPs in the organic horizons resulted in rapid 137Cs migration from the organic horizons and their accumulation in the upper 0–5 cm mineral soil. Although RIPs of the studied volcanic soils fall within the lower class among major soil types, vertical 137Cs migration in the mineral soil is small unless RIP is less than 300–400 mmol kg−1 due to the low clay contents and shortage of illite or vermiculite.

Spatial and temporal variation in vertical migration of dissolved 137Cs passed through the litter layer in Fukushima forests

We examined spatial variation in vertical 137Cs flux from the litter layer using lysimeters combined with copper-substituted Prussian blue in two forests (deciduous broad-leaved and Japanese cedar (Cryptomeria japonica)), approximately 40 km northwest of the Fukushima Daiichi Nuclear power plant. The study ran from August 2016 to February 2017 in three periods; summer (10 Aug–4 Oct), autumn (5 Oct–30 Nov) and winter (1 Dec–27 Feb). Twenty-five and 15 lysimeters were installed in the deciduous broad-leaved and the Japanese cedar sites within 400 and 300 m2 areas with 3–5 m intervals, respectively. The geometric means of the flux in the deciduous broad-leaved site were 0.51, 0.085 and 0.060 kBq/m2/month in summer, autumn and winter periods, respectively. In the Japanese cedar site, the mean fluxes were 0.45, 0.036 and 0.023 kBq/m2/month. The ratio of 137Cs flux during the survey period to litter 137Cs inventory was 6% and 1% on average in the deciduous broad-leaved and Japanese cedar sites, respectively. The 137Cs flux in the summer period was much larger than those in other periods, resulting from higher precipitation in the summer. Our fine scale observation with 5 m interval showed very large spatial variation in the 137Cs flux and the differences between maximum and minimum range from 8 to 104 times, but were mostly 20–25 times. The spatial variations in the 137Cs flux were affected positively by those in the litter 137Cs inventory and negatively by canopy openness.

Development and assessment of a simple ecological model (TRIPS) for forests contaminated by radiocesium fallout

The management of vast forested zones contaminated by radiocesium (rCs) following the Chernobyl and Fukushima fallout is of great social and economic concern in affected areas and requires appropriate dynamic models as predictive or questioning tools. Generally, the existing radio-ecological models need less fragmented data and more ecological realism in their quantitative description of the rCs cycling processes. The model TRIPS (“Transfer of Radionuclide In Perennial vegetation Systems”) developed in this study privileged an integrated approach which makes the best use of mass balance studies and available explicit experimental data for Scots pine stands. A main challenge was the differentiation and calibration of foliar absorption as well as root uptake in order to well represent the rCs biocycling. The general dynamics of rCs partitioning was simulated with a relatively good precision against an independent series of observed values. In our scenario the rCs biological cycling enters a steady-state about 15 years after the atmospheric deposits. At that time, the simulations showed an equivalent contribution of foliage and root uptake to the tree contamination. But the root uptake seems not sufficient to compensate the activity decline in the tree. The initial foliar uptake and subsequent internal transfers were confirmed to have a great possible impact on the phasing of tree contamination. An extra finding concerns the roots system acting as a buffer in the early period. The TRIPS model is particularly useful in cases where site-specific integrated datasets are available, but it could also be used with adequate caution to generic sites. This development paves the way for simplification or integration of new modules, as well as for a larger number of other applications for the Chernobyl or Fukushima forests once the appropriate data become available. According to the sensitivity analysis that involves in particular reliable estimates of net foliar uptake as well as root uptake not disconnected from rCs exchange reactions in soil.

Radiocesium concentration in seeds of Japanese fir (Abies firma Sieb. et Zucc.) growing in Fukushima forests 4.5 years after the Fukushima Daiichi Nuclear Power Plant accident

ABSTRACTSeed soundness is of paramount importance for all plants. Seed soundness of forest trees might have been negatively affected after the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident in March 2011, when a massive amount of artificial radionuclides deposited on forest areas. However, information on seeds of forest tree species contaminated with radionuclides is limited because they are rarely encountered in forest ecosystems. Japanese fir (Abies firma Sieb. et Zucc.) is a predominant tree in natural secondary forests in the Abukuma Mountains in Fukushima, and this species shows masting; cone production occurs every 2 or 3 years, meaning that the contamination levels of A. firma seeds remain unknown. We investigated the contamination levels of 137Cs in cone components (seeds, wings and cone scales) of A. firma approximately 4.5 years after the FDNPP accident. The 137Cs concentration in seeds showed the lowest contamination level of the three cone components. In addition, there was a difference in 137Cs concentration between seeds and cone scales, suggesting a difference in 137Cs contamination levels between strict seed feeders and seed-cone feeders. Our results indicate that the combination of observed low contamination levels of seeds of A. firma and expected small root uptake of 137Cs from soil might lead to low contamination levels of 137Cs in recruits of this species.

The migration of 137Cs from different contaminated Fukushima forest litters in two soil conditions

The migration of 137Cs from different contaminated Fukushima forest litters in two soil conditions

Radiocesium distribution and fluxes in the typical Cryptomeria japonica forest at the late stage after the accident at Fukushima Dai-Ichi Nuclear Power Plant

The Fukushima-derived radiocesium distribution in the typical Japanese cedar (Cryptomeria japonica D. Don) forest ecosystem was determined. In four years after the Fukushima accident, about 74% of the total radiocesium inventory was localized in soil, 20% was in the litter, and only 6% was associated with the aboveground biomass. Most of the radiocesium that was initially intercepted by the tree canopies has been already transported to the ground surface. The importance of the processes for removal of radiocesium from the tree canopies decreased in the order litterfall > throughfall >> stemflow. Within the tree compartments, the largest radiocesium activity fraction, about 46%, was observed in old foliage, which indicates that the process of removal of the initial deposit from the tree crowns has not yet completed. The aggregate soil-to-wood transfer factor was 1.1⋅10−3 m2 kg−1 d.w., which is close to the geometric means of transfer factors recommended by IAEA for other coniferous tree species. Further studies in Fukushima forest are necessary to assess the variation of this parameter under various soil-landscape conditions. Presence of the residues of the initial deposits does not allow to obtain the accurate values of the annual radiocesium fluxes in the ecosystem. Based on the conservative assumptions, the ranges of the fluxes were estimated. Analysis of the flux structures shows that up to percents of the total radiocesium activity in the ecosystem may be involved into biogenic cycling.