Fig. 8.8 Arrhenius plot of the logarithm of k (rate of decrease in the CsMP radii) as a function of the reciprocal temperature 1/T for pure water and seawater. (Reprinted from Okumura et al. 2019b)

solution. Therefore, CsMP solubility should be further investigated to predict the environmental fates of these particles.

Dissolution experiments were performed using CsMPs in pure water and artificial seawater at several temperatures and the dissolution progress was monitored by the decrease in the 137 Cs radioactivity originating from the CsMPs, allowing the dissolution rate and activation energy to be estimated (Okumura et al. 2019b). The dissolution rate of the CsMPs in seawater was approximately one order of magnitude higher than that in pure water (Fig. 8.8). Assuming that a 1 μm radius CsMP is immersed in seawater at 13 °C (the approximate annual mean temperature in Fukushima City), it will completely dissolve within 10 years. Furthermore, the CsMPs were observed using SEM, TEM, and STEM after the dissolution experiments. The shapes of the CsMPs dissolved in pure water were considerably altered, suggesting that the decreased radioactivity was caused not by elution of 137 Cs from the silicate glass through ion-exchange, but by dissolution of the glass itself. Tin and iron originally in the glass matrix formed oxide nanoparticles on the CsMP surfaces owing to their low solubility (Fig. 8.9). This is similar to observations of CsMPs collected in Fukushima Prefecture (Yamaguchi et al. 2017), indicating that CsMP dissolution actually occurs in the environment. For CsMPs dissolved in seawater, a crust composed of secondary Mg- and Fe-rich minerals was formed on the surface and glass matrix dissolution proceeded inside the crust (Fig. 8.9).

Subsequently, further dissolution experiments in solutions of various compositions and pH values were conducted at 60 °C. The dissolution behavior of CsMPs was largely comparable to that of silica-rich glass under these variable conditions (Fig. 8.10). In neutral and basic solutions, dissolution was accelerated by alkali ions such as Na+ , which played a catalytic role. In contrast, dissolution in acid was slow,

Fig. 8.9 STEM images and corresponding element maps of CsMPs after the dissolution experiments in pure water (a) and artificial seawater (b). (Adapted from Okumura et al. 2019b)

Fig. 8.10 Dependence of dissolution rates (k) on pH at 60 °C, where k (m/s) is the decrease rate of the CsMP radii in the presence (open symbols) and absence (closed symbols) of Na ions

even in the presence of alkali ions. In addition, the dissolution rate in Ringer's solution at 37 °C was determined to be 1.00 ± 0.37 μm/year, implying that CsMPs with radii of approximately 1 μm will dissolve completely within a few years at most if they are inhaled.

#### 8.4 Discrimination of CsMPs in Contaminated Samples

CsMPs have spread widely in eastern Japan, with several atmospheric plumes emitted from damaged reactors on March 14–15, 2011 (Tsuruta et al. 2015; Nakajima et al. 2017). Hence, their deposition areas and abundance were controlled by environmental factors such as the wind transporting the plumes, rainfall, and landscapes. Furthermore, CsMPs deposited in fields, forests, and cities have been exposed to environmental factors that may have caused movement during approximately a decade after the accident. However, abundance and distribution of CsMPs in the environment remains largely unknown. Autoradiography using an imaging plate (IP) is a common method to identify CsMPs as "radioactive particles," but a large number of CsSMs are also present in environmental samples. Recently, a method to distinguish CsMPs from CsSMs using autoradiography was developed based on the assumption that radioactivity per individual particle is higher in CsMPs than in CsSMs (Ikehara et al. 2018). However, CsSMs with higher radioactivity than that of CsMPs have been reported (Okumura et al. 2019c), indicating that this distinction method could be inaccurate because of the overlapping radioactivity distributions between CsMPs and CsSMs.

As an alternative approach, a discrimination method based on the difference in response to acidic solutions between CsMPs and CsSMs was developed (Okumura et al. 2020a). The dissolution experiments of the CsMPs mentioned above indicated that the dissolution rate of CsMPs in acidic solutions is very slow, with only mild reductions of radioactivity. In contrast, CsSMs treated with acidic solutions under the same conditions completely lost their radioactivity (Okumura et al. 2020a). Radiocesium sorbed or fixed on the surface of the CsSMs was almost completely released, because acidic solutions dissolve the surface atomic layers. Furthermore, contaminated samples (nonwoven fabric cloth and wheat leaves) were immersed in 1 mM HCl at 90 °C for 24 h, and then in 100 mM HCl at 90 °C for 24 h. The samples were exposed to IP after each treatment (Fig. 8.11) and the IP read-out image before treatment showed several hot spots and uniform luminescence intensity throughout the cloth. However, uniform luminescence was almost completely extinguished by immersion in 1 mM HCl, likely due to the release of radiocesium from the CsSMs. Meanwhile, almost all hot spots remained even after immersion in 100 mM HCl, which could be attributed to CsMPs, because these hot spots would have been extinguished if they originated from CsSMs. In addition to the discrimination of CsMPs using IP autoradiography, their contribution to the total radioactivity of contaminated materials can be estimated quantitatively using γ-ray measurements. When the contaminated samples were immersed in acidic solutions and radioactivity periodically measured, the decrease rate slowed after the initial rapid drop (Fig. 8.12). The initial drop was ascribed to CsSM dissolution and the contribution of CsMPs to the total radioactivity was easily estimated by measuring the residual radioactivity after the acid treatment.

Fig. 8.11 IP read-out images (a–c) and optical micrograph (d) of a fragment of nonwoven fabric cloth. (a) Before acid treatment. (b) After immersion in 1 mM hydrochloric acid. (c, d) After immersion in 100 mM hydrochloric acid. The 137 Cs radioactivity corresponding to the entire area of the IP read-out images is noted below each image and the residual radioactivity after immersion is noted in brackets. (e) 137 Cs radioactivity determined from the hot spots in the IP read-out image shown in b. (Reprinted from Okumura et al. 2020a)

#### 8.5 Concluding Remarks

Knowledge of the composition and structure of CsMPs has increased considerably since their discovery in 2013 and it is generally agreed that CsMPs were formed in Unit 2. Fuel debris is recovered and investigated to understand the current conditions of the damaged reactors and guide their decommissioning processes. Debris investigations may provide further information regarding the CsMP formation processes and accident progression. The physicochemical properties of CsMPs, including their thermal and dissolution behaviors, have also been characterized to some extent, which will contribute significantly to the prediction of their dynamics in the environment and influence on human health. However, information regarding the abundance and distribution of CsMPs in Fukushima Prefecture and Kanto area is necessary to fully understand radioactive pollution by 137 Cs. For this, an approach to estimate the contribution of CsMPs to the radioactivity of environmental specimens was developed. In future research, this method should be improved to be more efficient and practical for the recovery of Fukushima and its surrounding regions.

Acknowledgements We are grateful to Prof. Y. Takahashi (University of Tokyo) for supporting the STXM measurements. This work was supported by the Japan Society for the Promotion of Science Grants-in-Aid for Scientific Research (19H01145, 20K19954), a research contract with the Japan Atomic Energy Agency for Fukushima Environmental Recovery, the University of Tokyo Advanced Characterization Nanotechnology Platform in the Nanotechnology Platform Project sponsored by the Ministry of Education, Culture, Sports, Science and Technology, Japan, and the Ministry of Agriculture, Forestry, and Fisheries contract research "Radioactivity surveys in Japan."

#### References

Open Access This chapter is licensed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made.

the copyright holder. The images or other third party material in this chapter are included in the chapter's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the chapter's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from

# Chapter 9 Verification of Effects on Crops and Surrounding Environment in Agriculture Using Radioactively Contaminated Grass Silage Compost Made by Aerobic Ultrahigh Temperature Fermentation

Takahiro Yoshii, Yoshihiro Furui, Tairo Oshima, and Noboru Manabe

# 9.1 Compost Derived from Grass Silage Contaminated with Radioactive Cesium

The Fukushima Daiichi Nuclear Power Plant (Tokyo Electric Power Co. Ltd.) accident in 11 March, 2011, released enormous amounts of radionuclides, causing radioactive contamination of many plants, animals, soil, rivers, oceans, and other environments. More than 10 years have passed since that accident, but grass silage contaminated with Cs-134 (134 Cs) and Cs-137 (137 Cs) was left untouched in pastures and farmlands immediately after the accident. While some of the low-containing grass silage (less than 400 Bq/kg) has been plowed into the soil, the highly contaminated grass silage (approximately 8000 Bq/kg or less) is still left untreated.

In our previous studies, we have attempted to utilize microbial methods of organic waste reduction to convert highly contaminated grass silage into compost (Manabe et al. 2016; Yoshii et al. 2019) through aerobic ultrahigh temperature (more than 110 °C) compost fermentation techniques (Oshima and Moriya 2008; Moriya et al. 2011). The contaminated grass of about 2700 Bq/kg was converted to 297 Bq/kg of

T. Yoshii · Y. Furui

T. Oshima

Faculty of Bioscience and Biotechnology, Tokyo Institute of Technology, Tokyo, Japan

N. Manabe (✉) Faculty of Agriculture, The University of Tokyo, Tokyo, Japan

Institute of Environmental Microbiology, Kyowa Kako Co. Ltd., Tokyo, Japan

Institute of Environmental Microbiology, Kyowa Kako Co. Ltd., Tokyo, Japan

Faculty of Human Sciences, Osaka International University, Osaka, Japan e-mail<EMAIL_ADDRESS>
compost. The compost made by aerobic ultrahigh temperature fermentation was applied to crops grown in the laboratory, but no radioactive cesium was transferred to the plants.

In the present study, we investigated the transfer of radioactive cesium to pasture grasses and its effects on the environment, including soil, space, and rivers, when compost is used on actual pastures.

# 9.2 Pasture Cultivation Using Compost Derived from Radioactively Contaminated Grass Silage

The experimental farmland in Kurihara-city, Miyagi-prefecture, Japan, located 150 km north of the Fukushima Daiichi Nuclear Power Plant, averaged 320 Bq/kg of radioactive cesium in June 2017 after radioactive decontamination (Fig. 9.1). This farmland originally was used for rice cultivation but was converted to pasture after the Fukushima Daiichi Nuclear Power Plant accident. The 1200 m2 farmland was divided into 6 plots of 200 m2 each, and 4 of these plots were applied with 2, 5, 10, and 30 kg of radioactively contaminated grass silage compost (297 Bq/kg, made by aerobic ultra-high temperature fermentation techniques) per m2 using manual spreader or excavator, and the remaining 2 plots were fertilized with chemical fertilizers or cattle manure as control (Fig. 9.2). After those plots were plowed, pasture Orchard grass (Dactylis glomerata) was sown and grown for about 2 months starting in June 2017. No fertilizer or pesticide was applied. In the first 3 weeks after sowing, the 30 kg/m2 compost application caused the growth disturbance in the

Fig. 9.1 The testing site (1200 m2 pastureland, average 320 Bq/kg) in Kurihara-city, Miyagiprefecture, Japan (150 km away from Fukushima Daiichi nuclear power plant) https://earth.google. com/web/@38.86985053,140.98021213,135.46996306a,211.35840595d,35y,358.53244018h,0t,0r

Fig. 9.2 Overview of test plots. Control plots were fertilized with chemical fertilizers (a) or cattle manure (b). The amounts of fertilizer applied were 2 kg (c), 5 kg (d), 10 kg (e), and 30 kg (f) per m2 of radioactively contaminated grass compost (297 Bq/kg). Compost application by manure spreader (b–d) or excavator (e, f). Location of measurement of air dose rate (①–④) and river water radioactivity (① upstream, ③ outlet of drainage) during the cultivation experiment

Fig. 9.3 The pastures at 2 months after radioactively contaminated grass compost (made by aerobic ultra-high temperature fermentation) application. (a–f) Soil pH and exchangeable potassium increased with increasing compost usage. Grass growth was best in plot (e) where 10 kg/m2 of grass compost was applied

pasture, but the pasture recovered after that. The other test plots showed good growth throughout the test period (Fig. 9.3). Soil pH and exchangeable potassium increased with increasing compost application. Sufficient potassium in the soil is thought to

Table 9.1 Radioactive cesium concentrations in soil before and after application of contaminated grass compost made by aerobic ultrahigh temperature fermentation

Table 9.2 The pH, radiocesium, and total nitrogen concentrations in river water upstream and the outlet of the test plots during the application of contaminated grass compost made by aerobic ultrahigh temperature fermentation

compete with crop absorption of radiocesium (134 Cs + 137 Cs). The pastures were cut at 37, 49, and 68 days after sowing at 5 cm above the ground surface. The concentrations of radioactive cesium in the grasses were measured with a germanium semiconductor detector (Mirion Technologies Canberra, Tokyo, Japan). As a result, radioactive cesium levels in the leaves and stems of the grasses were not detected in any of the test plots (total levels of 134 Cs and 137 Cs were less than 20 Bq/ kg). Genetic analyses of the cut grasses by polymerase chain reaction targeting the 5.8S ribosomal ITS region were performed and confirmed to be Japanese millet (Echinochloa esculenta) or Orchard grass. These weeds such as Japanese millet were thought to be species that were originally established in the test plots.

Each soil radioactivity did not change significantly before and after compost application (Table 9.1). Air dose rates at 1 m from the ground in four corners around the test sites before and after application of the radioactively contaminated grass silage compost were measured using a scintillation instrument (PA1000, Horiba, Kyoto, Japan). The results showed that the air dose rates before and after compost application ranged from 0.06 to 0.12 μSv per hour, with no significant changes due to the application of radioactively contaminated silage compost (Table 9.2).

Fig. 9.4 Air dose rate around the test plots in 2017 (see Fig. 9.2) during pasture cultivation test using grass compost contaminated with radioactive cesium

Water samples were collected from upstream of the river adjacent to the test site and directly below the test site, just six times. No radioactive cesium was detected in any of the river waters. There was also no significant change in the total nitrogen content of river water before and after application of radiocesium contaminated grass silage compost (Fig. 9.4).

#### 9.3 Conclusion

The pastures were grown with 2, 5, 10, and 30 kg of compost produced from contaminated grass silage applied per square meter, and the grass and weeds were undetectable for radioactivity. As the amount of contaminated grass compost applied increased, the pH in the soil increased and the amount of potassium also increased. Therefore, it may be expected to have a soil improvement effect to inhibit the absorption of radioactive cesium. No effect was observed on air dose rate or river radioactivity in the surrounding area where contaminated grass compost was applied. The results of this study confirm that crop cultivation using contaminated grass compost is safe in open field cultivation.

#### References

Open Access This chapter is licensed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made.

The images or other third party material in this chapter are included in the chapter's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the chapter's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.

# Chapter 10 Transport of 137 Cs into Fruits After External Deposition onto Japanese Persimmon Trees

Mamoru Sato

# 10.1 Introduction

The Fukushima Daiichi Nuclear Power Plant (FDNPP) is in Fukushima Prefecture, which is a major area of deciduous fruit production in Japan (Ministry of Agriculture, Forestry and Fisheries 2022). During the FDNPP accident, there was a large release of radiocesium (RCs) into the environment especially in the period 12–14 March 2011.

The transfer factor via soil (CRf-s) is one of the parameters used to quantify the extent of transfer of RCs from soil into the edible parts of crops, defined as an activity concentration ratio on a weight basis. Prior to the Fukushima accident, several studies have been conducted to determine the CRf-s for fruit trees (IAEA (International Atomic Energy Agency) 2020). Another most commonly used parameter to quantify transfer is the aggregated transfer factor (Tag), which is defined as the ratio of RCs activity concentration in edible parts (Bq kg-1 , FW or DW) to RCs deposition amount per unit area (Bq m-2 ). Tag does not necessarily imply that the soil is the source of contamination, as it quantifies the contribution via all migration processes leading to the radioactivity concentration of the fruit. However, there have been no previous reports of Tag quantifying migration when the source is bark deposition.

At the time of the FDNPP accident, most deciduous fruit trees, except Japanese apricot [Prunus mume (Sieb.) Sieb. et Zucc.], were in the dormant stage prior to bud burst. Since most deciduous fruit trees had not developed leaves, only the bark of the plant was directly contaminated. Of the released radionuclides, 137 Cs and 134 Cs were the most important contributors to plant contamination.

M. Sato (✉)

Faculty of Food and Agricultural Sciences, Fukushima University, Fukushima, Fukushima, Japan

e-mail<EMAIL_ADDRESS>
© The Author(s) 2023

T. M. Nakanishi, K. Tanoi (eds.), Agricultural Implications of Fukushima Nuclear Accident (IV), https://doi.org/10.1007/978-981-19-9361-9\_10

Initial studies in May 2011 reported elevated RCs activity concentrations in leaves and young fruits of peach, of 449 Bq kg-1 FW and 157 Bq kg-1 FW, respectively, at the Fruit Tree Research Center, Fukushima Agricultural Technology Center (FTRC), approximately 65 km northwest of the FDNPP. Absorption of RCs from the roots was initially unlikely as deposited RCs was deposited onto and retained in the upper soil layers of orchard soil where there are few tree roots. Therefore, the data indicated that RCs attached to the surface of the bark may have contributed to fruit contamination (Sato et al. 2019a). Before the FDNPP accident, a small study by Katana et al. (1988) reported that RCs migrate directly into tree tissues from contaminated bark. After the FDNPP accident, several studies also found that 137 Cs deposited on the aboveground part of deciduous trees, including fruit trees, had migrated into the tree mainly via bark (Tagami et al. 2012; Takata 2013; Kuroda et al. 2013; Sato et al. 2014; Aoki et al. 2017). Consistent with these observations, bark-washing of Japanese persimmon and peach was effective in decreasing the 137 Cs activity concentration ([137 Cs]) in fruit in the subsequent year (2012) (Sato et al. 2015a).

Furthermore, the [137 Cs] in fruits of the Japanese persimmon, which did not have bark washed, was significantly higher than that of trees with washed bark at least for 7 years after the FDNPP accident (Sato et al. 2019b). Decontamination by external washing resulted in a reduction in RCs activity concentration of 29.1% in fruits and 33.2% in leaves in the Japanese persimmon (Sato et al. 2015a). These data indicated that residual 137 Cs on the bark continued to move to other parts of the unwashed tree for several years after 137 Cs was deposited. However, prior to this study, there had been no reports that have quantitatively verified the transfer of RCs from externally contaminated bark of dormant branches to fruits.

Dissolved 137 Cs (termed here as dis-137 Cs solution) was detected in stemflow more than 4 years after the FDNPP accident (Sato et al. 2017a, b, 2021). An alternative possible pathway for continuing contamination of fruit of the transfer was via stemflow of 137 Cs from external bark to leaves, which had developed after the FDNPP accident. The observation raised the need to quantify the relative importance of the externally deposited RCs compared with that from contaminated soil uptake over several years after deposition. The leaf-to-fruit transfer rate of RCs has been previously examined in the UK, France, Italy, and Germany after the Chernobyl accident. The reported leaf-to-fruit transfer of 134 Cs expressed as fraction of the applied or intercepted activity has been highly variable at 1.8–9.6% in grapes (Zehnder et al. 1995; Carini et al. 1996, 1999; Carini and Lombi 1997) and 1–47% in apples (Katana et al. 1988; Carini et al. 1999; Pröhl et al. 2003). The transport of RCs to fruits via leaves in these studies was affected by the following factors: stage of contamination (seasonality), precipitation intensity, age of branch and tree, cultivar and tree species, distance between fruit and the contaminated leaf, dilution effect from growth, and yield.

The translocation of assimilate in fruit trees depends on the relationship between source (leaves) and sink (fruits) (Warren 1972). RCs is an analogue of K and, therefore, behaves in a similar manner in plants. The direction of translocation and distribution of K with various assimilates such as organic acids and sugars (Van Goor and Van Lune 1980; Possner and Kliewer 1985; Lang 1983; Westwood 1993) is controlled by the sink strength defined by Warren (1972) as: "sink strength = sink size × sink activity," which is represented by the growth rate of fruit (Faust 1989). Pröhl et al. (2003) found that the transfer to fruits via leaves was higher in apples after spraying 137 Cs solution during the fruit growing stage than at the flowering stage. Sink activity in fruit increases after fruit growing stage, so the transfer of 137 Cs is probably also affected by the relationship of source and sink between fruits and leaves.

Fruit load affects the sink-source relationship, and is often modified by the horticultural practice of fruit thinning that adjusts the fruit load using the leaf-fruit ratio as a criterion. Horii et al. (2019) revealed that there was no clear relationship between the leaf-fruit ratio and the [137 Cs] in the fruit during the defoliation study. However, there have been no previous studies that have examined the [137 Cs] in fruit due to fruit thinning.

Japanese persimmon has been cultivated and consumed for a long time in Japan. Ampo-gaki is a product of astringent persimmon fruit semidried in the sun or in the open air and is an important processed foodstuff produced largely in the northern part of Fukushima Prefecture. The production volume of ampo-gaki in 2010 was the highest recorded in Japan of 1423 t before the FDNPP accident (Ministry of Agriculture, Forestry and Fisheries 2010). Because drying fruit can lead to higher [ 137 Cs], ampo-gaki production has been under voluntary suspension in specific areas designated by Fukushima Prefecture because of the likelihood that [137 Cs] will exceed designated standard limits in Japan. Since 2013, an inspection system for [ 137 Cs] in ampo-gaki has been established, and production has resumed in areas designated by Fukushima Prefecture. Japanese persimmon has been, and remains, the most important economically damaged fruit product produced in Fukushima Prefecture after the FDNPP accident.

Japanese persimmon fruits are true fruits with enlarged ovaries.  After sprouting, the growing fruiting shoots bear fruit at several leaf axils.  A group of fruiting mother shoots is formed mainly on the lateral branches that are 2–5 years of age.  In an experiment conducted after the Chernobyl accident by Katana et al. (1988), the positional relationship between fruit and contaminated leaves within the same branch was not considered. It was reported that the [137 Cs] in fruits of Japanese persimmon varies widely even within the same lateral branch, but the cause of this variation was not clarified (Horii et al. 2019; Sato et al. 2021).

The surface of the peel of Japanese persimmon fruit lacks aerenchyma, and the respiration of fruit is carried out through a calyx. The calyx of the Japanese persimmon is a homologous organ comprising of leaves and has plenty of stomata and spongy tissues (Watanabe and Hoshi 1981). Since RCs is transported to pulp via the calyx in the Japanese persimmon (Sekizawa et al. 2016a, b), evaluation of the transfer rate of RCs to fruit via both the leaf and calyx is needed.

The aims of this work were to (1) quantify the transfer rate of 137 Cs applied to leaves and calyx into fruit; (2) examine effects of leaf position and fruit load on the transport of 137 Cs into fruit; and (3) quantify the transfer rates and aggregated transfer factors of 137 Cs applied to apical fruiting mother shoot and 2-year-old lateral branch into fruits before bud burst.

#### 10.2 Materials and Methods

An outline of experiments conducted from 2016 to 2021 is shown in Table 10.1.

# 10.2.1 Preparation of 137 Cs Solution

For Experiment 1 (EXP. 1), moss samples of Thuidium cymbifolium (Dozy & Molk.) Dozy & Molk., collected from the Yamakiya area in the town of Kawamata approximately 50 km northwest of the FDNPP in May 2015, were used for preparing a 137 Cs solution. After boiling approximately 200 g of moss with 1.5 L of distilled water for 15 min, the moss solution was filtrated through a pulp sheet (JK Wiper, NIPPON PAPER CRECIA CO., Ltd., Tokyo) laid onto a stainless-steel net, and centrifuged at 3500 rpm. The supernatant was filtered through a 0.45 μm membrane filter (cellulose ester mixed type, ADNANTEC A045A047A, Toyo Roshi Kaisha, Ltd., Tokyo) under suction. The 137 Cs solution was reduced in volume to approximately 350 mL by heating and then subjected to high-pressure sterilization treatment in an autoclave (SX-700, TOMY SEIKO CO., Ltd., Tokyo) at 120 °C for 20 min to obtain a final solution containing 2.2 Bq mL-1 of dis-137 Cs. For EXP. 2 and EXP. 3, moss samples of Plagiothecium nemorale (Brid.) Z. Iwats.; P. roseanum Schimp. in the Nagadoro area of Iitate village located about 40 km northwest from the FDNPP and of Thuidium cymbifolium (Dozy & Molk.) Dozy & Molk. in the same site as EXP. 1 were collected in July 2016 and May 2017. The moss was used to prepare the experimental dis-137 Cs solution as described above producing 1.8 Bq mL-1 of dis-137 Cs solution. For EXP. 4, moss samples of Pylaisiadelpha tenuirostris (Buch et Schimp.) W.R. Buch. in the Nagadoro area of Iitate village were collected in May 2017. The moss was used to prepare the experimental dis-137 Cs solution as described above producing 7.0 for 2020 and 7.5 for 2021 Bq mL-1 of dis-137 Cs solution. In Experiments 1, 2, and 3, epiphytic moss growing on rock was used, whereas epiphytic moss on bark was used in Experiment 4.

Table 10.1 Outline of the experimental plan and the results related to the TRf and the Tag f-b into mature fruit

a Arithmetic mean ± standard deviation

# 10.2.2 Measurement of the Amount of 137 Cs Applied to Various Organs

The [137 Cs] in several treated organs (i.e., calyx, leaf, shoot) collected immediately after treatment was measured, and the retained ratio (RR) was calculated using Eq. (10.1). The amount of 137 Cs (Bq) in individual treated organ was calculated using Eq. (10.2):

$$RR = \boldsymbol{\omega}\_{\rm soo} \times \mathbf{C}\_{\rm soo} \times \left(\mathbf{s}\_{\rm g} \times \boldsymbol{\nu}\_{\rm s} \times \mathbf{C}\_{\rm s}\right)^{-1},\tag{10.1}$$

where wso (kg DW) is total weight of the contaminated organs, Cso (Bq kg-1 DW) is the [137 Cs] in the contaminated organs, sg (= 1.00 g mL-1 ) is the specific gravity of the applied dis-137 Cs solution, vs (mL) is the volume of the applied dis-137 Cs solution, Cs (Bq g-1 ) is the [137 Cs] in the dis-137 Cs solution.

$$\rm{AS}\_{\rm{io}} = \rm{RR} \times \rm{s}\_{\rm{g}} \times \nu\_{\rm{s}} \times C\_{\rm{s}},\tag{10.2}$$

where ASio is the amount of 137 Cs in individual contaminated organs (Bq organ-1 ), RR is the retained ratio, vs (mL) is the volume of the applied dis-137 Cs solution, Cs (Bq g-1 ) is the [137 Cs] in the dis-137 Cs solution.

# 10.2.3 Transport into Japanese Persimmon Fruits of 137 Cs Applied on Calyx and Leaves (EXP. 1)

#### <IP_ADDRESS> Plant Material and Treatments

The experiments were conducted using two 12-year-old "Hachiya" Japanese persimmon (Diospyros kaki Thunb.) trees planted at the FTRC. Fruit bearing shoots less than about 20 cm in length were selected for the 137 Cs application treatment. The experiments were performed both at the young and the growing stage of fruits. In 2016, the fruit young stage was June 24–July 1 (23–31 days after full blossom) and the fruit growing stage was August 1–15 (61–75 days after full blossom). To compare the amount of 137 Cs transported to fruit from the calyx or from the leaves, the dis-137 Cs solution was applied onto either the front side of the calyx or onto two adjacent leaves using a bottle with a plastic brush (Fig. 10.1). A cellulose cloth was placed onto the calyx to prevent contamination by 137 Cs during application onto the adjacent leaves, which was repeated 1–3 times per day. After the treatment, the applied solution was left to dry naturally, and the fruits were then wrapped within a net bag. To calculate the RR, the [137 Cs] was measured in three sets of contaminated leaves collected immediately after each treatment and in 5 and 6 contaminated calyxes on June 27 and July 1, respectively, for the fruit young treatment, and in 3 contaminated calyxes on August 1 for the fruit growing stage treatment.

Fig. 10.1 Procedure to contaminate the plants by applying the dis-137 Cs solution to the calyx or leaves of the Japanese persimmon "Hachiya" in EXP. 1 conducted in 2016. A sheet of cellulose cloth was placed beneath the calyx to prevent adhesion of the treatment solution to the calyx during the leaf contamination process. The dis-137 Cs solution was applied on the surface of the front side of calyx and onto two leaves adjacent to the fruit using a bottle with a plastic brush

There were losses of contaminated fruit, commonly called June drop, caused by factors such as pollination and lack of sunshine (Sobashima and Takagi 1968; Kitajima et al. 1987, 1990; Chauhan 2014). During the period from the start of July to mid-August, the young unripe fruits, which had dropped into the net, were collected (on July 4 and 11) and the [137 Cs] was measured for both treatments (calyx and leaf contamination).

#### <IP_ADDRESS> Sample Preparation

Contaminated leaves and mature fruits were collected on October 26 and the [137 Cs] in leaves and fruit was measured. Fruit was separated into calyx, peel, and pulp. On July 27 and October 31, respectively, 7 and 6 uncontaminated fruits were also sampled to measure the [137 Cs] for control of the young fruit and the mature fruit. The [137 Cs] was measured for each part of the combined control fruit sample in a similar manner to that for contaminated fruits.

# 10.2.4 Effects of Leaf Position on the Transport into Fruit of 137 Cs Applied on Leaves (EXP. 2)

#### <IP_ADDRESS> Plant Material and Treatments

The experiments were conducted using three 13-year-old "Hiratanenashi" Japanese persimmon trees planted at FTRC. The effects of the distance between leaves and fruit on leaf to fruit transport of 137 Cs were examined. An outline of the treatments is shown in Table 10.1 and Fig 10.2. From June 26th to June 30th during the juvenile period, 20.3 Bq of 137 Cs was applied to 2 leaves as explained below. Twelve fruiting shoots that were longer than 20 cm (long-fruiting shoots) and bearing one fruit were selected. Two leaves were contaminated from each of the three positions: adjacent to

Fig. 10.2 The treatment of Japanese persimmon "Hiratanenashi" cultivar in EXP. 2 conducted in 2017. The dis-137 Cs solution was applied on the surface of the front side of two leaves from the adjacent to fruit, middle and top part of the long fruiting shoots (>25 cm length) and two leaves adjacent to fruit on the short fruiting shoot (<15 cm length) using a bottle with a plastic brush

the fruit, at the middle node, and at the apical node. Similarly, six short-fruiting shoots (less than 20 cm in length) were selected, where two leaves adjacent to the fruit were contaminated. The 137 Cs solution was applied over the surface of the front side of leaves using the same method as in EXP. 1. The [137 Cs] in six treated leaves collected within a few seconds after treatment was measured, and the RR was calculated.

#### <IP_ADDRESS> Sample Preparation

Samples of the Japanese persimmon were collected on October 18. Each replicate of fruit and contaminated leaves were prepared individually for each treatment position group. Leaves that had not been contaminated from each long fruiting shoot were collected and subdivided into the upper and the lower leaf from the node of treated leaves.

# 10.2.5 Effects of Fruit Load on the Transport into Fruit of 137 Cs Applied on Leaves (EXP. 3)

#### <IP_ADDRESS> Plant Material and Treatments

The experiments were conducted using a 15-year-old "Hiratanenashi" Japanese persimmon tree planted at FTRC. Eight fruiting mother shoots with 5 consecutive fruiting shoots longer than 20 cm (long fruiting shoots) bearing one fruit were selected, and comparative treatments by fruit load were applied as follows:

Four "No fruit thinning" group where each of five consecutive fruiting shoots bears one fruit (Fig. 10.3a).

Four "Fruit thinning" group where fruits on the adjacent shoots of the central one were picked off just before the contamination treatment took place on July 31 (Fig. 10.3b).

Three short fruiting-shoots (less than 20 cm) developed on other lateral branches were selected to compare with the above-mentioned long fruiting shoot groups. Replicates were made on four fruiting mother shoots.

Dis-137 Cs solution was applied to all leaf surfaces of the central (among five) fruiting shoots and of the short fruiting shoots during the period from July 31 to August 2. The total volume of applied dis-137 Cs solution was 12 mL. The [137 Cs] in leaves of three treated fruiting shoots collected within a few seconds after treatment was measured, and the RR was calculated.

Fig. 10.3 The treatment of Japanese persimmon "Hiratanenashi" cultivar in EXP. 3 conducted in 2018. Eight fruiting mother shoots with 5 "No fruit thinning" shoots longer than 20 cm (long fruiting shoots) that bear one fruit were selected. Prior to applying dis-137 Cs solution, the comparative group was set as follows. "No fruit thinning" group (a): One fruit was set on each of the five "No fruit thinning" shoots. "Fruit thinning" group (b): Fruits on the upper and lower adjacent shoots of the central shoot among the five consecutive shoots were thinned. Dis-137 Cs solution was applied to all leaf surfaces of the central fruiting shoots among five fruiting shoots and the short fruiting shoots

#### <IP_ADDRESS> Sample Preparation

Fruits, leaves, and wood with bark of fruiting shoots and fruiting mother shoots were collected on October 22. Same organs were also collected from uncontaminated lateral branches in the same tree to use as a control.

# 10.2.6 The Transfer Rate and Aggregated Transfer Factors into Fruits of 137 Cs Applied on Fruiting Mother Shoots Before Sprouting (EXP. 4)

#### <IP_ADDRESS> Plant Material and Treatments

The experiments were conducted using a 17-year-old "Hiratanenashi" Japanese persimmon tree planted at FTRC. Eight 2-year-old lateral branches, which had several fruiting mother shoots, were selected before sprouting for treatment. In 2020, the apical fruiting mother shoots and their previous year's branches (2-yearold lateral branch) were selected for application of the dis-137 Cs solution. In 2021, only apical fruiting mother shoots were selected. The dis-137 Cs solution was applied to the front side of the apical fruiting mother shoots and of the 2-year-old branches using a polyamide brush (Fig. 10.4). The [137 Cs] was measured in two treated shoots or branches that had been collected a few seconds after treatment, and the RR was calculated. In 2020, 30 days after full blossom the number of fruit set was adjusted to provide a leaf-fruit ratio of 25 leaves per fruit on July 2. No fruit thinning was carried out in 2021.

#### <IP_ADDRESS> Calculation of the [137 Cs] per Unit Area of Contaminated Fruiting Mother Shoot

Before application of the dis-137 Cs solution, the length and thickness of the fruiting mother shoot or 2-year-old lateral branch for experiment were measured, and the [ 137 Cs] per unit area of shoots and branches (Ab) was calculated as follows:

$$A\_b = A S\_{\rm iso} \times \left( 0.5 \times \pi \times W \times L \right)^{-1},\tag{10.3}$$

where Ab is the amount of 137 Cs per unit area of the contaminated shoots and branches (Bq m-2 ), ASio is the amount of 137 Cs in the contaminated shoots and branches (Bq shoot-1 ), W is the thickness of the contaminated shoots and branches (m), L is the length of the contaminated shoots and branches (m).

Fig. 10.4 The treatment (a) and schematic of the mature stage (b) of Japanese persimmon "Hiratanenashi" cultivar in EXP. 4 conducted in 2020 and 2021. Before sprouting in the year of 2020 and 2021, dis-137 Cs solution was applied to the front side of the apical fruiting mother shoot and 2-year-old branches (only in 2021) using a polyamide brush (a), whereas only apical fruiting mother shoots were applied in 2021

#### <IP_ADDRESS> Sample Preparation

In 2020, thinned young fruits and mature fruits were collected from the contaminated or uncontaminated apical fruiting mother shoot and the other fruiting mother shoots on 2-year-old branches on July 2 and October 14, respectively. Mature fruits were subdivided into calyx and pulp with skin.

In 2021, on June 25, at the young fruit stage, the contaminated apical fruiting mother shoots and the uncontaminated fruiting mother shoot adjacent to the apical one were collected and divided into wood with bark, shoots, leaves, and young fruits. The contaminated fruiting mother shoots were further subdivided into bark and wood. During the maturity period, the contaminated fruiting mother shoots were collected on October 11 and divided into organs in the same way as in 2020. The replicated samples were combined and then the [137 Cs] was measured.

#### 10.2.7 Radiocesium Measurements

After freeze-drying for at least 72 h, all samples from EXP. 1 to EXP. 4 were placed into a 100-mL polypropylene container (U-8 pots) to measure [137 Cs]. The [137 Cs] in the samples of EXP1, EXP2, EXP4 was measured using a germanium detector (GEM40-76, Seiko EG & G ORTEC, Tokyo, Japan) at Fukushima University. Gamma-ray emissions at 662 keV were measured for 43,200–200,000 s. For the pulse-height analysis, a multichannel analyzer (MCA7, Seiko EG & G ORTEC) was used together with the spectrum analysis software (Gamma Studio, Seiko EG & G ORTEC). The [137 Cs] and [40 K] in the samples of EXP3 were measured with a germanium detector at the Foundation for Promotion of Material Science and Technology of Japan. Decay correction was not applied as the 137 Cs measurement was conducted within 2 weeks of sampling.

# 10.2.8 Transfer of 137 Cs Applied on the Dormant Fruiting Mother Shoot into Organs and Measurement of the Aggregated Transfer Factors of 137 Cs into Fruit

The contaminated 137 Cs amount (AS, Bq) in each sample due to the application of dis-137 Cs solution was calculated using the following equations:

$$AS\_i = w\_i \times (C\_i - C\_0) \div 1000,\tag{10.4}$$

where the subscript i represents the type of the organ (e.g., fruit, leaf, calyx, peel, pulp, bark and wood) to be examined, wi (g DW) is weight of i, Ci (Bq kg-1 DW) is the [137 Cs] in i, C0 is the [137 Cs] in control of i.

The transfer rate of 137 Cs to fruit via contaminated organs (TRf, %) was calculated using the following equations:

$$TR\_f = 100 \times AS\_f \div (RR \times AP),\tag{10.5}$$

where ASf is the amount of 137 Cs in fruit (Bq fruit-1 ), RR is the retained ratio, AP is the amount of applied 137 Cs (Bq organ-1 ).

Based on the equations above, TRf represents the percentage of 137 Cs transported into fruit from the leaves, calyx, or dormant fruiting mother shoots (on which it was deposited).

The aggregated transfer factors of 137 Cs applied on the dormant fruiting mother shoot to fruit were calculated using the following equations:

$$T\_{ag}{}^{f-b} = \left(\mathbf{C}\_f - \mathbf{C}\_0\right) \mathbf{A}\_b{}^{-1},\tag{10.6}$$

where Tag f-b (m2 kg-1 ) is the aggregated transfer factors into fruit of 137 Cs via the dormant fruiting mother shoot, Cf (Bq kg-1 DW) is the [137 Cs] in the mature fruit on the contaminated dormant fruiting mother shoot, C0 is the [137 Cs] in control of mature fruit, Ab is the amount of 137 Cs per unit area of the contaminated shoots and branches (Bq m-2 ).

#### 10.2.9 Data Analysis

Analysis of variance (ANOVA) with Tukey pairwise comparison and t-tests were used to test the significance of the differences between comparable [137 Cs], TRf and Tag f-b values. Log-transformed data were used in both the ANOVAs and t-tests.

#### 10.3 Results

# 10.3.1 Transfer of 137 Cs Applied on Calyx and Leaves at the Young Fruit Stage and the Fruit Growing Stage into Dropped Fruit and Mature Fruit of Japanese Persimmon "Hachiya" (EXP. 1)

During the period from the start of July to mid-August, some young unripe fruit had dropped from the tree, so only 2–4 fruits in each treatment remained on the tree up to the harvest time. The TRf of 137 Cs into fruit that had naturally dropped after contamination at the young fruit period was significantly higher due to contamination of the calyx than that for contamination of leaves for both sampling occasions (Table 10.2).

The TRf of 137 Cs derived from EXP. 1 for mature fruit are shown in Table 10.3. The [137 Cs] (Bq kg-1 DW) measured in the control collected on October 26 were as follows: pulp 5.92, peel 7.20, calyx 10.1 and leaf 10.3 Bq kg-1 . The TRf via calyx was higher after contamination at the young fruit stage, whereas the TRf via leaves showed the opposite trend. Nevertheless, the TRf were not significantly different (by ANOVA) between organs, or the stage of contamination. However, ANOVA indicated that there was a possibility of the interaction between contaminated period and organ (0.069 of P value by ANOVA).

Table10.3Transferof137CsadministeredviacalyxandleavestomatureJapanesepersimmonfruitattwodifferentgrowthstages

 bArithmetic mean ± standard deviation

aCollected

 on 26

October

# 10.3.2 Effects of the Position Where 137 Cs Was Applied onto Leaves on the Transfer into Japanese Persimmon Fruit (EXP. 2)

The result of EXP. 2 is shown in Table 10.4. 137 Cs was not detected in the uncontaminated leaves of each treatment. The TRf of the pulp with peel resulting from contamination of the leaves adjacent to fruit both on the long and on the short fruiting shoot showed the same trend as the [137 Cs]. They were significantly higher than those from contamination of the leaves at the middle and top part on a long fruiting shoot (0.01 of P value by ANOVA).

# 10.3.3 Effects of Fruit Load on the Transfer of 137 Cs Applied onto Leaves into Persimmon Fruit (EXP. 3)

The leaf to fruit ratio of "No fruit thinning" was significantly lower than that of the "Fruit thinning" (Table 10.5). The [137 Cs] in fruit and the TRf into fruit of the "Fruit thinning" were higher than that of the "No fruit thinning" (0.02 and 0.055 of P value by t-test, respectively), and the TRf in short fruiting shoots was an intermediate value. There was no significant difference in fruit weight between both groups. The TRf of 137 Cs into the fruit on the uncontaminated fruiting shoots was less than 2% (Table 10.6). Values of the TRf of the fruit two nodes below and of the sum of TRf of the five fruiting shoots in "Fruit thinning" were, respectively, higher than the corresponding TRf in "No fruit thinning." There was a significant correlation between the amount of 137 Cs and 40 K as well as of their concentration in the fruit on the contaminated fruiting shoot. The amount of 137 Cs as well as 40 K in fruits tended to be higher in the "Fruit thinning" (Fig. 10.5). The TRf of the 137 Cs into wood with bark of 2-year-old lateral branch in the "No fruit thinning" group was 1.54 ± 0.15% (mean ± standard deviation, SD), which was significantly higher than that in the "Fruit thinning" group of 0.30 ± 0.06%. Conversely there was no significant difference in the TRf into wood with bark of the five fruiting shoots in the "No fruit thinning" and the "Fruit thinning" group, which were 1.18 ± 0.46% and 1.36 ± 0.41%, respectively.

# 10.3.4 Transfer of 137 Cs Applied on the Dormant Fruiting Mother Shoot into Fruit (EXP. 4)

The leaf to fruit ratio for the 2-year-old branch was 28.6 ± 7.2 where the fruit thinning was applied in 2020 and for the apical fruiting mother shoot was 28.8 ± 8.1 in 2020 and 6.5 ± 3.4 in 2021. Ab was 5.2 ± 4.3 and 4.8 ± 2.3 kBq m-2 of branch in

Table 10.6 TRf of 137 Cs into fruit for "No fruit thinning" and "Fruit thinning" in contaminated shoots and into fruits borne by branches one or two nodes away from the contaminated central one

a Arithmetic mean ± standard deviation

Fig. 10.5 Relationship between [137 Cs] and [40 K] (a), and the amount of 137 Cs and 40 K (b) in fruit on the fruiting shoot contaminated dis-137 Cs. \* and \*\* indicate a significant correlation at P ≤ 0.05 and 0.01 by ANOVA

2020 and 2021, respectively. The TRf of 137 Cs for fruit on the contaminated apical fruiting mother shoot at young and mature stage was 0.64 ± 0.27% and 1.3 ± 0.6% in 2020, which was significantly higher than that of 137 Cs into fruit on the uncontaminated 2-year-old lateral branch, which was 0.30 ± 0.27% and 0.29 ± 0.22% (Fig. 10.6a). In contrast, there was no significant difference in the

Fig. 10.6 TRf of 137 Cs applied on the apical fruiting mother shoot (a) and the 2-year-old lateral branch (b), Tag via the fruiting mother shoot and 2-year-old lateral branch contaminated dis-137 Cs before sprouting in 2020 (c) and TR of 137 Cs into leaf, fruit and branch on and of the contaminated apical fruiting mother shoot in 2021 (d). The TR into branch in (d) was combined both TR of the shoot, of which was bark and wood, and of the wood of the contaminated apical fruiting mother shoot except for bark on which surface remains the contaminated 137 Cs.  In graph D, "branch" represents the TR except for bark of contaminated branch/shoot. \* and \*\* indicate a significant correlation at P ≤ 0.05 and 0.01 by ANOVA. Vertical bar represents arithmetic standard deviation

TRf of the fruit from the contaminated 2-year-old lateral branch between the uncontaminated apical fruiting mother shoot and the other fruiting mother shoot at young and mature period in 2020 (Fig. 10.6b). The Tag f-b of the mature fruit on the contaminated apical fruiting mother shoot was significantly higher than that in fruit on the corresponding uncontaminated fruiting mother shoots, whereas there was no significant difference in the Tag f-b of the mature fruit on the apical fruiting mother shoot and other fruiting mother shoots on the contaminated 2-year-old branch (Fig. 10.6c). In 2021, the TRf of 137 Cs for mature fruit on the contaminated apical fruiting mother shoot was 3.16 ± 1.75%, which was higher than 1.05 ± 0.56% for young fruits (Fig. 10.6d). The Tag f-b of the mature fruit was 8.0 ± 4.5 × 10-4 m2 kg-1 DW. The TRf of 137 Cs for the mature fruit via the contaminated apical fruiting mother shoot was significantly higher in 2021 than in 2020 (0.025 P value by t-test).

#### 10.4 Discussion

Because RCs was deposited directly onto fruit tree surfaces (Sato et al. 2015b), it was important to evaluate how much RCs was transferred via leaves to fruits after bud burst after the FDNPP accident.

The calyx of Japanese persimmon is larger than that of other fruit trees and plays an important role in fruit enlargement as the respiratory organ of fruit. Calyx lobes fall during the young fruit stage. The shape of the calyx resembles a dish after the fruit growing stage, because the calyx lobes become erect, so RCs in raindrops intercepted by leaves and branches would tend to accumulate in/on the calyx (Fig. 10.7). Previous studies on the transfer of dis-137 Cs from cotton contaminated with 137 Cs solution onto the calyx of mature Japanese persimmon fruit (cultivar 'Hiratanenashi') reported that TRf were 2.3% in the "Immature period" during 7–40 days after full blossom and 16.9% in the "Mature period" during 41 days after full blossom to harvest (Sekizawa et al. 2016a, b). The data derived from this study are not consistent with these findings (Table 10.1) because of differences in the method applied and the period during which the contamination occurred. In this study, the contamination period was 23–31 days after full blossom in the young fruit stage and 61–75 days after full blossom in the fruit growing stage. These time periods were shorter than that of Sekizawa et al. and the contamination method involved direct application of 137 Cs, whereas Sekizawa et al. used an indirect treatment via cotton. Before 20 days after full blossom most calyx lobes fall, during which dis-137 Cs from cotton may have been lost from the calyx. Such losses may explain why TRf in the "Immature period" on the experiment of Sekizawa et al. was

Fig. 10.7 Stemflow stayed on the calyx of "Hachiya" persimmon

lower than that reported in this study. Anyway, further studies are needed to clarify the transfer of 137 Cs into fruit via calyx.

The TRf of 137 Cs from contaminated calyx and leaves in the young fruit stage into fruit that had dropped naturally in EXP. 1 was less than 3% (Table 10.2), whereas the TRf into mature fruit was three fold higher at >10% (Tables 10.1 and 10.3). In EXP. 4. the TRf of 137 Cs from contaminated apical fruiting mother shoots into fruit before sprouting increased from the young fruit to the mature fruit, whereas for leaves, it decreased during the same period (Fig. 10.6d). Therefore, the data indicates that translocation of 137 Cs to fruit via the leaves and bark consistently increases after the fruit growing stage. The TRf of 137 Cs from contaminated leaves in the young fruit stage in Japanese persimmon was lower than that reported for fruit after the fruit growing stage (Table 10.1).

These results are corroborated by studies of Pröhl et al. (2003) who found that the transfer rate to fruits via leaves in apples was higher after spraying a 137 Cs solution during the fruit growing stage than at the young fruit stage. Other previous data also shows that for grape, the translocation of 137 Cs absorbed from the aboveground part of the tree to fruits increased after the fruit growing period (Madoz-Escande et al. 2002). Furthermore, Davies et al. (2006) showed that two potassium transporters in grape are expressed most highly in the berry skin during the pre-véraison period (a stage at which berries begin to soften and change color). Overall, these studies indicate that the TRf of 137 Cs in fruit due to deposition onto leaves is affected by the time at which contamination occurs, and that the extent of accumulation (sink strength) of 137 Cs increases after the fruit growing stage. These results are consistent with other relevant data, which showed that translocation of photosynthetic products from leaves to fruits accelerated after the fruit growing stage (Pavel and DeJong 1995; Yamamoto 2001; Zhang et al. 2005; Beshir et al. 2017).

There was a significant correlation between 40 K and 137 Cs in the mature fruit in EXP. 3. Moreover, the correlation was higher for the total Bq amount than for activity concentration. Amount represents sink strength and activity concentration represents sink activity (Fig. 10.5). K, an analogue of Cs, promotes the translocation of photosynthetic products from leaves to fruit (Eplon et al. 2015), which explains why the TRf of 137 Cs in fruit via leaf also increased after the fruit growing period.

The effect of the distance between fruit and the leaves as the source of 137 Cs was shown in the experiment with Japanese persimmon "Hiratanenashi," where the TRf after contamination of the leaves adjacent to the fruit was much higher than that after contamination of leaves located further away at the top of the long shoot (Table 10.4). These observations indicate that the relationship between supply and demand of photosynthetic products on the fruiting shoot influenced the sink and source relationship of 137 Cs between leaf and fruit. Katana et al. (1988) placed a 134 Cs solution onto leaves at the top of the apple branch. Consequently, 19–42% of 134 Cs administered was transferred to fruit on the same contaminated branch, whereas a small amount of 134 Cs (at 0.01–0.4%) was detected in fruit at the base of the 25 cm length lower branch. They concluded that the relationship between the contaminated leaves and fruit had a tendency that the transport of radiocesium deposited on the leaves to the fruits is restricted within the branches on which contaminated leaves are growing, and it is difficult to migrate into the fruits on the different branches. Carini et al. (1999) compared the transfer of 134 Cs from contaminated leaves to fruits between contaminated branches and uncontaminated branches in grapes, apples, and pears. The transfer factor of 134 Cs in fruit of the contaminated branch, which was defined by fruit concentration (Bq kg-1 DW) divided by Bq administered per plant, was 1.33, 5.11 and 0.69; this was significantly higher than that of the uncontaminated branch of 0.098, 0.1 and not detected for grapes, apples, and pears, respectively. These data showed that there was a close relationship between source and sink for 134 Cs on the contaminated branch.

At the time of the FDNPP accident, the Japanese persimmon was in the dormant stage prior to bud burst and 137 Cs on the contaminated bark of the aboveground part of the fruit tree migrated directly into the tree. After bud burst, Sato et al. (2021) reported that leaves were also contaminated with 137 Cs via raindrops in the canopy during or after rainfall. K+ is readily absorbed by leaves (Franke 1967; Schönherr and Luber 2001) and Mcfarlane (1974) reported that Cs+ penetrates through the cuticle more easily than K+ . K+ penetrates through the fine pores in the cuticle on the leaf surface, thereby reaching parenchyma cells and is then transported via the vascular system. K plays a key role in elevating the translocation of assimilates from leaves to fruit after the fruit growing period (Eplon et al. 2015). Conversely, K leaches through the cuticle easily from leaves due to rainfall (Tukey and Mecklenburg 1964; Mecklenburg and Tukey 1964; Potter et al. 1991), which probably also occurs for radiocesium.

In this study, the effect of the fruit load on TRf differed depending on which organ was contaminated. In EXP. 3, 137 Cs administration onto leaves of fruiting shoots that had been subjected to fruit thinning just before the contamination treatment on July 31 resulted in a higher value of the TRf than when thinning was not applied. Similarly, a higher amount of 40 K was translocated into fruits on the contaminated long fruiting shoot in the "Fruit thinning" scenario (showing that the sink activity and strength of these fruits works effectively) compared with the "No fruit thinning" shoot. The difference indicates that the accumulation by these fruits was suppressed possibly by competition with other fruits on the uncontaminated shoots that had developed on the same mother shoot.

In EXP. 4, the administration of 137 Cs to a dormant apical fruiting mother shoot without fruit thinning resulted in a higher TRf than that in the year with fruit thinning. The number of fruits per shoot in the experiment in 2021 was approximately four times higher than that of the experiment in 2020. The difference in fruit load between the experimental years in EXP. 4 is larger than that between the treatments in EXP. 3. This suggests that the higher TRf in the experiment without fruit thinning was due to the different mass of the fruits. Conversely, there was a significantly higher [ 137 Cs] in EXP. 3 (Table 10.5) and Tag f-b in EXP. 4 (Fig. 10.6c, d) in the experiment with fruit thinning. These initial data indicate that refraining from heavy fruit thinning may be a useful remediation option that could reduce the [137 Cs] in Japanese persimmon fruits.

In EXP. 4 conducted using the same leaf to fruit ratio, the transfer of contaminated 137 Cs was compared between fruits on the apical fruiting mother shoot and the other fruiting mother shoots developed from the same 2-year-old lateral branch. When dis-137 Cs solution was applied to the apical fruiting mother shoot, the TRf of the combined mature fruits collected from the other fruiting mother shoots was less than a quarter of that for the fruit on the contaminated apical mother shoot (Fig. 10.6a). In contrast, when dis-137 Cs solution was applied to the 2-year-old lateral branches, the TRf and the Tag f-b into fruit were similar between the fruit on the apical fruiting mother shoot and the other fruiting mother shoots (Fig. 10.6b, c).

These results indicate that the transfer of 137 Cs to fruits grown on fruiting mother shoots developed from the same contaminated 2-year-old lateral branch is similar, whereas the transfer of 137 Cs from a contaminated apical fruiting mother shoot to fruits grown on the other shoots developed from the same 2-year-old lateral branch is limited. Furthermore, [137 Cs] of fruit on the contaminated fruiting shoot in the fruit thinning was higher than that with no fruit thinning in EXP. 3 (Table 10.6), and Tag f-b in 2020 when the fruit was thinned was higher than that in 2021 where the fruit was not thinned in EXP. 4. These data are consistent with the experimental results assuming that the transfer of 137 Cs is controlled by the sink activity of fruits.

Tag f-b values derived in EXP. 4 were of the order of magnitude of the 10-4 m2 kg-1 DW. Unfortunately, there are no previous data reported for Japanese persimmon after the Chernobyl accident, and only a few data for Tag in fruit after the FDNPP accident. Renaud and Gonze (2014) reported Tag (m2 kg-1 FW), which vary in a range of 10-4 to 10-3 m2 kg-1 FW. For persimmon Tag values of 9.7 × 10-4 m2 kg-1 FW was previously reported by Tagami and Uchida (2014). The value of 15.2 × 10-4 m2 kg-1 DW of Tag f-b in EXP. 4 is the equivalent of 2.4 × 10-4 m2 kg-1 FW of Tag f-b . Although the leaf area of the contaminated leaves was not measured in EXP. 3, the Tag into fruit via leaves calculated assuming a leaf area of 72 cm2 (Fujimoto and Tomita 2000) was 10-3 m2 kg-1 FW. Therefore, Tag into fruit via dormant branches can be tentatively estimated to be one order of magnitude lower than Tag into fruit via leaves.

This study has shown that Tag f-b for the fruit on the apical mother shoot were approximately twice as high as for fruits on other fruiting mother shoots developed on the same 2-year-old branch contaminated during dormancy. However, similar experiments on fruiting mother shoots on 2-year-old branches other than apical fruiting mother shoots have not yet been conducted. The currently available data suggest that fruit load is likely to be one of the main factors affecting variation in the extent of [137 Cs] transfer into fruits within the same lateral branch.

To our knowledge, this was the first reported study that has quantified the transport of 137 Cs into fruit via the dormant lateral branch. Further studies are needed to clarify the dynamics of 137 Cs inward migration into deciduous fruit trees that have been externally contaminated during dormancy and other growth phases.

#### 10.5 Limitations

The dis-137 Cs solution used in EXP. 4 contained up to approximately 0.06 Bq mL-1 of 40 K (0.2% as natural K), but no 40 K was detected in the dis-137 Cs solution used epiphytic moss growing on rock. Natural K is present on leaves and bark surfaces by leaching, and inward-migration of RCs from the tissue surface is a physical permeation phenomenon, so inward-migration of dis-137 Cs likely is not affected by K in the applied solution unlike absorption via the roots. No significant differences in 40 K activity concentrations in organs between of the control group and the experimental group were found on each sampling date, whereas a considerable amount of applied 137 Cs was detected in each experiment of this study. While K in the applied dis-137 Cs seems to have negligible relevance to the result in each experiment, utilizing the parameter calculated in EXP. 4 needs to take into account the precondition which is the experiment using the dis-137 Cs solution contained up to 0.2% as natural K.

Acknowledgements The author wishes sincerely to thank Prof. Brenda Howard and Prof. Franca Carini for their useful comments and for improving the English in the manuscript. Part of the research results was obtained from projects conducted at the FTRC.

#### References

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# Chapter 11 Progress Toward Managing Radiocesium Contamination in Orchards

Daisuke Takata

# 11.1 Introduction

Prior to the major earthquake in Fukushima Prefecture in 2011, the prefecture had 7177 ha of land under fruit tree cultivation and was the prefecture with the largest area used for fruit trees in Japan (Ministry of Agriculture, Forestry and Fisheries (Japan) 2018). Fukushima Prefecture was ranked second in Japan with respect to the hectarage used for peach cultivation. According to the Special Fruit Production Survey, 2010 (Ministry of Agriculture, Forestry and Fisheries (Japan) 2012), it ranked first among Japanese prefectures for the largest production of dried persimmons, much of which was "anpo-gaki," a product of especially high value. The sales of anpo-gaki in 2010 by Date Mirai, a former branch of the Japan Agricultural Cooperatives centered around dates, reached ¥1.9 billion (Komatsu 2014a). Although the retail prices of most types of fruit are gradually returning to pre-earthquake levels (Komatsu 2014a, b; Takata 2016), they have still not completely recovered. Nevertheless, Fukushima Prefecture's position as one of the most important fruit-producing prefectures in Japan has not changed since the Tokyo Electric Power Co.'s Fukushima Daiichi Nuclear Power Plant (FDNPP) 2011 disaster.

The effects of radiocesium from the FDNPP disaster on agricultural products as reference materials for the time of the disaster have been estimated. Numerous studies have reported on the effects of radiocesium on annual crops. These studies include (1) surveys of the effects of radioactive fallout from atmospheric nuclear tests; (2) dynamic surveys after the Chernobyl disaster; and (3) studies in experimental environments such as application-using reagents. Although the transfer

D. Takata (✉)

Faculty of Food and Agricultural Sciences, Fukushima University, Fukushima City, Fukushima, Japan

e-mail<EMAIL_ADDRESS>
T. M. Nakanishi, K. Tanoi (eds.), Agricultural Implications of Fukushima Nuclear Accident (IV), https://doi.org/10.1007/978-981-19-9361-9\_11

coefficient of radiocesium concentration in the soil and perennial fruit crops has been studied (IAEA 2003, 2010), overall, fewer radiocesium-related studies have been conducted on perennial crops than on other crops. One major difference between the FDNPP and Chernobyl disasters was the season in which they occurred; the FDNPP disaster occurred while deciduous fruit trees had no leaves, suggesting that radiocesium fallout would have landed on the tree bodies (i.e., trunks and main branches). With respect to the transfer of radiocesium to inside the fruit-tree bodies, the effects of direct contamination of the trunk and transfer of radiocesium from the soil must be investigated. As fruit trees are long-lived, they have many similarities with timber trees in terms of life cycle and growth. More studies on radiocesium have been reported in forestry than in fruit trees; therefore, forestry studies had to be referred to when clarifying the dynamics of radiocesium in fruit trees. However, in timber plantations, the surface layers of the soil form the litter layer, which contains large quantities of fallen leaves and other organic matter, and this layer plays important roles in root growth and nutrient uptake and cycling. Grass grows on the soil surface in orchards, and, indeed, leaves that fall from trees are often removed by the workers to prevent tree diseases. Considering that orchards are maintained with far more human effort than timber plantations, fruit trees may exhibit vastly different radiocesium dynamics.

In summary, numerous issues related to radiocesium dynamics in orchards were not elucidated before the FDNPP disaster, and there may be numerous differences between perennial crops and timber trees. The reference information available offers starting points to elucidate radiocesium dynamics in fruit trees and develop technologies relevant to this issue.

# 11.2 FDNPP Disaster and Radioactive Contamination of Orchards

# 11.2.1 Radiocesium Contamination of Fruit and Leaves in the First Year After the FDNPP Disaster

High concentrations of radiocesium, exceeding the predictions based on coefficients of transfer from the soil at the immature fruit stage (IAEA 2003, 2010), were found in deciduous fruit trees in Fukushima Prefecture after the FDNPP disaster. Radiocesium accumulates readily in immature stages of fruit such as ume (Prunus mume: a small, plum-like fruit, in which the flowers were directly contaminated), and chestnuts (which has edible seeds) (Hamada et al. 2012). However, the FDNPP disaster occurred before budbreak, and radiocesium was detected at above the detection limit in the usual deciduous-tree fruits in Fukushima Prefecture that do not belong to the above groups (Suzuki et al. 2018). Radiocesium was detected in all five of the most common fruit species in Fukushima Prefecture in 2011, that is, peaches, grapes, Japanese pears, apples, and persimmons (Abe et al. 2014; Yuda et al. 2014).

With respect to the effects of tree age on radiocesium concentration in fruit, a study was performed in which peach trees of the "akatsuki" variety, 12- and 18-year old, were compared with peach trees of the "yuzora" variety, 6- and 18-year old, and the radiocesium concentration in mature fruit was found to be substantially higher in the older trees (Abe et al. 2014). In addition, differences were found between radiocesium concentrations in the following seven species of fruit: ume, cherry, peach, grape, pear, apple, and persimmon. The radiocesium concentrations in mature fruit differed according to tree age and species, suggesting that above-ground tree morphological factors, such as the spread of the crown, may have affected fruit radiocesium concentration in the first year after contamination.

# 11.2.2 Radiocesium Contamination of the Bark and Tree in the First Year After the Disaster

Radiocesium concentrations were compared in (1) soil below peach trees planted in pots, with the soil surface covered before the FDNPP disaster; (2) soil below uncovered trees; and (3) the fruit (Takata et al. 2012a). The radiocesium concentration in covered soil decreased by one-sixth, but, with or without covering, the radiocesium concentration in the roots was below the limit of detection. The radiocesium concentrations in fruit with and without covering were 26.6 and 27.8 Bq kg-1 fresh weight, respectively, the difference between these values being within the range of measurement error. In addition, the radiocesium content was mostly above ground, and there was no difference with and without covering.

In peach trees, the transfer of radiocesium from the above-ground part of the tree to inside the tree has been investigated (Takata et al. 2012b; Takata 2013; duplicated figure: Fig. 11.1). Samples of the trunks were collected in January 2012, and images of the species of radioactive nuclei were prepared using an imaging plate. Figure 11.1 shows a superimposed photograph and the original image, and the black spots are the loci where radioactive nuclei were detected. Photographs were only taken of the outermost layer of the peach tree bark, and no photographs were taken for loci immediately below that. Radiocesium concentrations were measured using germanium semiconductor detectors in the epidermis and directly under the epidermis, and it was found that extremely high concentrations of radiocesium were present in the epidermis. Therefore, the radiocesium deposited on the tree may be heterogeneously distributed in the bark, with a very high concentration in the thin, outermost layer of the bark.

# 11.2.3 Radiocesium Contamination of Soil and Grass in the First Year After the Disaster

In orchards in Fukushima Prefecture, grass is grown using the usual ground management methods. In orchards, unlike other cultivated land, branches are at least 3 m

Fig. 11.1 Imaging plate of trunk in 'Akatsuki' peach. From Takata (2013)

high. Consequently, before radioactive fallout reaches the ground in an orchard, it may be affected by stagnant air, owing to the trees, and radiocesium capture, which may result in differences in the amount of radiocesium deposited on the ground surface. In this context, an investigation of the distribution of mean radiocesium concentrations in the top 5 cm of soil (Sato et al. 2015) revealed that the radiocesium concentration in the 5-cm layer in orchards showed two- to fivefold differences between tree crowns. This difference was greater in higher-density apple orchards than in peach orchards, indicating that horizontal differences in the distribution of radiocesium concentration in soil are affected by planting density.

# 11.3 Elucidation of Radiocesium Uptake by Tree Bodies and Fruit, and Transfer Routes

#### 11.3.1 Radiocesium Transfer from the Bark to Inside

The route of penetration of the tree through the bark by radiocesium has been hypothesized to involve some mode of transfer from the lenticels on the bark to the phloem and/or xylem (Takata 2013). Lenticels on peach trees are often split, and radiocesium can penetrate the body from these sites. Physical splitting on branches not only occurs at lenticels, and intense images of contamination were also obtained at sites with pruning scars, that is, cavities. Physical irregularities are marked at such sites, and dust containing radiocesium tends to accumulate, such that lenticel tissues are likely to be one of the penetration routes for radiocesium into the trees. The lenticels on the outer bark are continuous with the ray tissue in the inner bark. The ray tissue is composed of parenchyma cells that grow in a radiating pattern and pass through the phloem, cambium, and wood. In addition to having an aeration function for the sapwood (Mio and Matsumoto 1979), in recent years, the ray tissue has been acknowledged to have nutrient storage functions (Islam and Begum 2012). Studies on nutrient and water exchange functions with the wood and phloem are in progress to elucidate these mechanisms (Fromm 2010; Pfautsch et al. 2015; Van Bell 1990). In studies with sugi (Cryptomeria japonica) and konara (Quercus serrata), 133 Cs applied to the bark was detected in the wood (Mahara et al. 2017). Visualization assays revealed that 133 Cs localized in the ray tissue of sugi, and 133 Cs in the bark was transported into the wood via the ray tissue (Aoki et al. 2017). In the FDNPP disaster, it is probable that radiocesium passed through the bark of deciduous trees during the dormant period and was then transferred directly into the body of the fruit trees.

# 11.3.2 Distribution of Radiocesium in Peach Tree Bodies Five Months After the Disaster

A series of surveys was performed on peach trees to study the distribution of radiocesium in fruit-tree bodies in the first year after the disaster (Takata et al. 2012a, b, c, d). In akatsuki peach trees in orchards that received fallout, at least 20% of the radiocesium, as becquerel equivalents, was in the leaves and fruit (Fig. 11.2). Leaves that fell in November had lower radiocesium concentrations than those that fell in August (Takata 2013), and fresh growth after harvest showed secondary elongation, such that 15–20% of the radiocesium was eliminated from the tree. It has been reported that, in the standard pruning methods for peach trees, 56.6% of the length is removed in the winter pruning season (Takata et al. 2008), but in the first year after the disaster, approximately 30% of the radiocesium in the branches (becquerel equivalents) was eliminated from the tree. By calculating the amount of material collected from the ground (fallen leaves), the amount pruned, and the increase in wood thickness, the amount eliminated from the tree each year can be estimated. The bark of the trunk, which is vertical, has a lower radiocesium concentration than the ground and the main branches, which grow approximately horizontally (Takata et al. 2012b, d). Therefore, when such branches were removed in the winter pruning season, the vigorous renewal of the lateral branches from adventitious buds had a positive effect on tree decontamination. High-pressure washing of the trees to artificially remove radiocesium from the tree bodies in Fukushima Prefecture, from December 2011 to February 2012, considerably reduced the radiocesium concentration.

Fig. 11.2 Dry matter weight and Cs content for every organ in 'Akatsuki' peach tree. From Takata (2013). \*Under detection limit. Value means detection limit × dry weight

In a study performed with the above group the year after the disaster, in which contaminated peach trees were transplanted in uncontaminated soil (Takata et al. 2014; Takata 2016), only approximately 2% of the radiocesium in the tree was transferred to fruit and other newly grown organs, clearly showing there were major differences in radiocesium transfer and distribution between the year of the disaster and subsequent years. This shows that the efficacy of decontamination of the tree is greatest during the first year after the disaster. It is therefore imperative to perform decontamination, such as washing of trees, promptly after a disaster.

Sato et al. (2014) performed a study in which 6-year-old persimmon trees, grown before the FDNPP disaster, were replanted in pots filled with radiocesiumcontaminated soil. Of the 137 Cs absorbed from the soil after 3 years, 38.9 ± 6.9% (mean ± standard deviation) was above ground, and 61.1 ± 6.9% was below ground, which was approximately the opposite of the distribution of 70.0% above ground and 30.0% below ground for trees cultivated outdoors in the first year after the disaster. In a damage survey performed for 3 years after the disaster in yuzora peach trees planted before the disaster in experimental plots at the National Institute of Fruit Tree Science, distribution to underground parts was <20% (Takata 2019). In a study with 2-year-old Pinot Blanc grapevines in pots, in which contamination was restricted to the soil or leaves (Carini and Lombi 1997), 42% of uptake from the soil only was distributed to underground parts, whereas proportional distribution to underground parts was 10% when uptake was from the leaves only, clearly indicating that the distribution of radiocesium above and below ground reflects the site of origin of the radiocesium. In the case of fruit trees contaminated by the FDNPP disaster, it is highly probable that radiocesium uptake was directly into the tree above ground, and the amount of radiocesium storage in the tree is therefore greater above ground.

#### 11.3.3 Transfer from Soil to Fruit

Comparison of the bodies of peach trees, in which the soil surface was either covered and uncovered at the time of radiocesium fallout, showed that the radiocesium concentration in soil was reduced by one-sixth by covering and the radiocesium concentrations of the roots and the above-ground parts were similar, whether covered or not (Takata et al. 2012a). When peach trees that received radiocesium fallout were replanted, there were no differences in the radiocesium concentration in the fruit, irrespective of whether the soil used was contaminated or uncontaminated with radiocesium. Horii et al. reported that the transfer coefficients over several years after planting uncontaminated unshu mandarin orange (Citrus unshiu), chestnut, and persimmon trees were in the range of 10-4 to 10-3 (2018). On the basis of a previous report (IAEA 2003), and the present results, it is considered that in fruit trees, the contamination of the soil surface layer has little effect on uptake by the roots.

There have been several reports on the transfer of radiocesium from the soil, including one on fig and grape trees, in which the soil concentration was changed locally (Takata et al. 2013b); a second, with blueberry bushes, on the relationship between the exchangeable potassium concentration and radiocesium concentration in fruit (Iwabuchi 2014); and a third on the cation concentration in the soil solution and the suppression of radiocesium uptake (Matsuoka et al. 2019). Radiocesium transfer dynamics within the soil are influenced by the proportions of clay, sand, and organic matter in the soil (Hiraoka et al. 2015), so there is a need for longitudinal studies on the dynamics of radiocesium transfer to the lower layers.

# 11.4 Development of Techniques to Reduce Radiocesium Content in Trees and Orchards

# 11.4.1 Time-Course of Radiocesium Content in Mature Peaches Using Fruit Thinning

From the second half of 2011 until 2012, various measures were taken (including high-pressure washing of trees), but many issues could not be clarified until the trees had grown. In a study of cherries contaminated in the Chernobyl disaster, the radiocesium concentration in the fruit was found to have fallen to approximately one-third by the year after the disaster (Antonopoulos-Domis et al. 1996), and it is thought that there was probably a similar tendency after the FDNPP disaster. However, the actual concentrations in fruit were uncertain. The year after the disaster, the provisional regulation value for radiocesium concentration in food was changed from 500 to 100 Bq kg-1 , and in connection with that change, there was a demand for establishment of a more precise method for tracking the concentration in fruit. In this context, a study was performed that focused primarily on the changes in concentration in fruit from year to year during the growing season (Takata et al. 2014).

Fig. 11.3 Relationship between thinning fruit and harvested fruits in 137 Cs (from Takata 2016) left: each tree, n = 70, right: each orchard, n = 24). Dashed line means correlation line. Solid line means Y = X line

The radiocesium concentration in fruit was generally found to reach a minimum level during the second growing season. The concentration in fruits during the growing season and at harvest was investigated and the variability in the measurement times and the feasibility of moving them forward were predicted (Fig. 11.3). This method was shown to be useful for the removal of contaminated trees. The peach fruit radiocesium concentrations were generally below the specified values in the fruit-producing region of Fukushima Prefecture after the FDNPP disaster, so this prediction system was not needed for peaches, whereas in persimmons (for making anpo-gaki), screening tests were performed to identify orchards where the harvested fruit had high radiocesium concentrations. This prediction method will be useful for radiocesium concentrations in fruit if any more nuclear disasters occur, either in Japan or overseas. In the case of deciduous fruit trees, the trees had no leaves and only the radiocesium fallout could be measured. In future, it will be necessary to elucidate transfer dynamics over a wide range of seasons.

# 11.4.2 Reducing Fruit Radiocesium Concentrations by Bark Washing

With fruit trees, bark peelability varies between species, with the bark being peeled readily from trees such as pear, grape, persimmon, and apple. By removing the rough bark, at least 80% of the radiocesium is removed (Sato et al. 2015). However, trees in which the seeds are hard stones, such as peaches and ume, have bark that is of low peelability. In this context, Abe et al. (2014) found that high-pressure washing of the bark of akatsuki peach trees, in July 2011, reduced the total radiation level by 55.9%. Washing was, however, not found to be effective for reducing radiocesium concentrations in fruit grown in 2011. This confirmed that most of the radiocesium was transferred internally within 60 days of contamination, suggesting that decontamination in July, approximately 4 months after the disaster, was already too late to restrict transfer of radiocesium into the tree.

In peaches and persimmons, washing the bark from winter 2011 to spring 2012 substantially reduced the radiocesium concentration of fruit harvested in 2012 in comparison with the fruit from unwashed trees (Abe et al. 2014; Sato 2014). This suggests that, in the case of uncontaminated persimmon trees, radiocesium transfer internally from the bark continues the year after contamination. After 3 years of persimmon bark formation, strip-shaped fissures form, and water droplets often collect in the scars left after the fissures, probably leading to the growth of moss and lichen on the bark (Abe et al. 2014). This could influence the radiocesium contamination.

# 11.4.3 Reducing Transfer to Fruit by Pruning and Cutting the Trunk

In the second year after radioactive contamination, radiocesium inside the fruit tree is the principal source of transfer to fruit (Antonopoulos-Domis et al. 1990), and pruning is considered to be effective for removing radiocesium that has been transferred internally into the tree. The effectiveness of pruning for removing radiocesium inside the tree is suggested to depend on not only the proportional distribution of radiocesium to the part that is removed, but also the amount of fruit collected (Hiraoka et al. 2015; Pröhl et al. 2003). However, in studies performed before the FDNPP disaster, the relationship between pruning and fruit radiocesium concentration had not been investigated.

In order to elucidate the relationship between pruning intensity and radiocesium concentration in chestnuts, Matsuoka et al. (2016) used 7-year-old trees and treatments of no-pruning, standard-pruning, and intense-pruning areas of the orchard in a March 2013 trial. In addition to measurement of the fruit radiocesium concentration over 2 years, in the second year, the trees were cut up, and the proportional radiocesium and radiopotassium distribution in each part was investigated. No significant differences in radiocesium concentration were found in the fruit that grew during the year of treatment, but the level of transfer of radiocesium to all fruit was substantially higher in the nonpruned area. The proportional distribution of radiopotassium to branches and roots that grew during the first to fourth years after the disaster was higher than that of radiocesium, suggesting that the radiocesium in the new growth had not come from the soil.

In March 2014, Kuwana et al. (2017), using persimmon trees of the hachiya variety (the fruit used to make anpo-gaki), included the following treatments: (1) no pruning; (2) standard pruning; (3) intense pruning; and (4) trunk-cutting, with the trunk cut at a height of 60–100 cm above the ground. The radiocesium concentrations were then measured in the fruit collected in each harvest season. The fruit radiocesium concentration decreased with time, except that in the no-pruning area in the fifth year, it was slightly higher than in the fourth year. No significant differences due to treatment mode were found in any of the study years, but the fruit radiocesium concentrations in the three pruned areas tended to show less variability than in the no-pruning areas.

In summary, although intense pruning is not clearly effective in reducing fruit radiocesium concentration, it is effective in restricting the variability of contamination strength in tree bodies and cultivation plots, and, in terms of ensuring yield, pruning operations are considered to be important for safe and stable fruit production.

# 11.4.4 Soil Radiocesium Elimination and Aerial Radiation Dose Reduction by Topsoil Stripping

Due to the deposition of radiocesium at high concentrations in the soil surface layer in orchards, the aerial radiation dose is higher in orchards than in surrounding areas, and there are concerns about workers suffering external exposure. In orchards, decontamination operations involving topsoil stripping have been performed, but many of the orchards in Fukushima Prefecture are on sloping land because it is a mountainous region, and there are limitations to the use of heavy machinery in such areas. Methods for topsoil stripping that do not involve use of machinery, using revegetation nets and other surface-covering materials, have been investigated (Sato et al. 2019). Approaches that make use of the ecological systems that exist in orchards are considered to be highly practical as orchard radiation decontamination methods.

# 11.5 Progress Toward Restarting Anpo-Gaki Shipment: Persimmons for Making Anpo-Gaki and Product Control

#### 11.5.1 Restarting Shipments of Persimmons for Anpo-Gaki

In the northern part of Fukushima Prefecture, which is an important area for anpo-gaki production, immature fruit was sampled and tested from all persimmon cultivation plots after the FDNPP disaster. Screening of processed anpo-gaki for transport was performed before shipment. However, several years after the disaster, some products were confirmed to exceed the screening level, albeit only slightly (Table 11.1; Fukushima Prefecture Anpo-gaki Production Area Promotion Association 2018a), but Good Agricultural Practice (GAP) had been introduced in

Table 11.1 Results of radioactive substance inspection of Anpo-gaki (sulfur-smoked semi-dry persimmon)

These data refers to the Fukushima prefecture anpo-gaki production area promotion association a The numbers in parentheses are the ratio to the total number of bags inspected b In 2017 and 2018, the number of bags is counted below 50 Bq kg-1

Table 11.2 Cs-137 concentration of the young persimmon fruit [reorganized the data of Sekizawa et al. (2019)]

a The numbers in parentheses are the ratio to the total trees

connection with processing (Fukushima Prefecture Anpo-gaki Production Area Promotion Association 2018b), so the adhesion potential to the external fruit during processing was reduced. It is highly likely, therefore, that the cause of the screening level being exceeded lay with the fruit used to make the anpo-gaki. The testing of immature fruit in the cultivation plots was on a sampling basis, so, if certain trees had higher contamination levels, the screening level could be exceeded in the fruit. Therefore, Sekizawa et al. (2019) investigated the intertree differences in fruit 137 Cs concentration in the same plots. In the anpo-gaki persimmons with excessive 137 Cs concentrations, the trees had high 137 Cs concentrations (Table 11.2).

#### 11.5.2 Anpo-Gaki Product Control

To enable stable shipments of anpo-gaki, controls are required in both the production process for the fruit used, and in anpo-gaki processing for concentration variability in the fruit used and fluctuations in radiocesium concentration during processing. The present authors have verified the variability in radiocesium concentration in the fruit used, the proportional changes in radiocesium concentration during processing, and the risk of radiocesium contamination in commercial premises for anpo-gaki to be considered in risk management during anpo-gaki processing. The nondestructive radiation measurement devices used in full-quantity tests of anpo-gaki before shipment were strictly calibrated using model samples with known concentrations of certified reference materials in brown rice.

To clarify the variability in radiocesium concentration in persimmons used for anpo-gaki from the same tree, Sekizawa et al. (2019) performed a follow-up study of the differences in radiocesium concentration between persimmons collected from the same branches of the same trees, in northern Fukushima Prefecture. In 2013, up to 3.6-fold difference in concentration between persimmons from the same main branch was observed, and in 2014, when the radiocesium concentration had halved from the 2013 level, the concentration difference in the same main branch was up to approximately twofold. Both the concentration and concentration difference had decreased in the same main branch over time.

The commercial anpo-gaki processing facilities where persimmons were dried for an extended period with natural air flow were inspected, and radiocesium adhesion to the shelves and floor inside the facilities was confirmed. Although it was shown that the contamination status in the processing facilities had no effect on the anpogaki radiocesium concentration, the contamination risk is considered to be higher when persimmons are in direct contact with the floor and/or pipes, and decontamination must therefore be performed every year. For this reason, in 2014, preuse cleaning was included in the Fukushima Prefecture Anpo-gaki Production Area Promotion Association's "Manual of Good Agricultural Practice for Safe Anpogaki Production" (referred to below as the "GAP Manual").

Screening via nondestructive, full-quantity testing was introduced when anpogaki shipments restarted. In the first year after restarting anpo-gaki shipments, tests were performed on the basis of the product packaging as one box containing eight trays, each with six anpo-gaki. Therefore, for the testing device (separate from the calibration source), a reference model sample was prepared in accordance with the preparation method for the brown rice certified reference materials for radiocesium analysis (U-8 containers packed with 81 g of brown rice sample; Hachinohe et al. 2016). These samples were placed in an anpo-gaki tray, with the containers laid on their sides with three next to each other. The mass of each container was approximately twice that of the anpo-gaki; the brown rice samples had a 40 K content of 70–100 Bq kg-1 , which was approximately 50% that of the edible part of an anpogaki (174 Bq kg-1 ). Using homogeneous brown rice samples with radiocesium concentrations of 0–140 Bq kg-1 , reference materials were prepared at five concentrations. These reference materials were introduced in the anpo-gaki full-quantity testing device and were used for verification of screening performance.

In summary, the radiocesium concentration in fruit used for anpo-gaki has a major effect on the concentration in the final product. Therefore, the risk of contamination in the commercial premises should be decreased by predicting the proportional increase in radiocesium concentration due to anpo-gaki processing. Radiocesium testing before and after anpo-gaki processing is effective for managing the risk in the processed product.

#### 11.6 Societal Implementation of Study Results

The radiocesium decontamination measures for orchards are reflected in the fundamental policies for decontamination of agricultural and forestry areas in Fukushima Prefecture and include washing trees, removing rough bark, and pruning. Data and statement methods supporting the effectiveness of such techniques were included in the "Policies for decontamination and technical measures relating to radiocesium in agricultural products" (Fukushima Prefectural Agriculture, Forestry and Fisheries Dept. 2012).

From December 2011 to March 2012, on the basis of the fundamental decontamination policies; in collaboration with prefectural, city, town, and village governments, and the Japan Agricultural Cooperatives, approximately 4300 ha of orchards of peaches, persimmons, apples, pears, and grapes included measures such as high-pressure washing of the trees (Fukushima Prefectural Agriculture, Forestry and Fisheries Dept. 2013). The outcome of these measures was that, since 2012, no restriction of shipment of the principal tree crops produced in Fukushima Prefecture, including peaches, apples, pears, and grapes, has been in effect.

In 2013, the third year of self-imposed control of anpo-gaki shipment, a model area for restarting processing was established, and a plan was established for shipment only of products confirmed to be safe using a nondestructive testing device. To establish this model area, the radiocesium concentration of the persimmons used for anpo-gaki had to meet specified values. Before using the nondestructive testing device, radiocesium reference samples were used in performance tests on the device for calibration.

The results of studies on the prevention of secondary contamination during cultivation and processing are reflected in the GAP Manual (Fukushima Prefecture Anpo-gaki Production Area Promotion Association 2018b), and all producers in the anpo-gaki production process comply with this. Cleaning of commercial premises is performed as a cooperative operation, maintaining a consistent operational level, on the basis of the GAP Manual.

It is expected that replacement of trees that still have fruit radiocesium concentrations exceeding the specified value will be performed after discussing and deciding upon appropriate compensation.

All data obtained in an agriculture, forestry, and fisheries survey, which covers fruit and is performed as emergency monitoring of environmental radiation, is made available on the homepage of the Fukushima Association for Securing Safety of Agricultural Products. In addition, all test data for anpo-gaki obtained using the nondestructive testing device are made available on the homepage of the Fukushima Division of the National Federation of Agricultural Cooperative Associations. In parallel with efforts to ensure safety at production sites, information disclosure and the expansion of public relations activities has ensured that there has been a recovery in demand. The value of prefectural fruit shipment, which in 2011 had fallen to ¥19.7 billion (63% of that in 2010), had recovered to ¥27.1 billion by 2016 (Ministry of Agriculture, Forestry and Fisheries (Japan), 2010, 2011, 2016). In the case of anpogaki, the shipments increased yearly since processing was restarted, so that in 2017, shipments were approximately 76% of the 2010 level.

The trade in fruit trees in Fukushima Prefecture is an important industry, and can be reflected in the status of peach trees. Figure 11.4a shows the mean price at the central wholesale market for peaches produced in Fukushima Prefecture. The mean price in the prefecture was ¥438 in 2010, before the disaster, but it decreased to ¥222 in the year of the disaster. Subsequently, as recovery continued, it reached ¥429 in 2015, which was equal to the 4-year predisaster mean. In 2018 and 2019, the mean sale price was approximately ¥500, which was even higher than before the disaster. However, this interpretation may be subjective, and in order to explain this, Fig. Y has been prepared as a follow-up of the national mean sale price shown in Fig. 11.4b. When the mean sale prices of peaches in Fukushima Prefecture and the whole of Japan are compared, the FDNPP disaster caused sales and economic problems. The price of peaches in Fukushima Prefecture before the disaster was ¥20–¥50 below the national mean. This price differential was linked to the production volume and marketing routes in Fukushima Prefecture, although no major fluctuation in that respect is shown here. In contrast to prices only within Fukushima Prefecture, in 2018 and 2019, the mean price of peaches produced in Fukushima Prefecture was nearly ¥100 less than the national mean. Over the past 10 years, Japanese peach production has been supported by the national policy of establishing high prices and high demand, especially in relation to overseas exports, and production can be seen to have increased markedly as a result. In the context of this expansion of the industry itself, production in Fukushima Prefecture has been unable to ride the tide of high-price sales, such that the difference from the national mean has widened as the latter has increased. The period from 2010 to 2020 was one of marked growth in Japanese peach production, but in Fukushima Prefecture, it was a period of recovery from the earthquake-related disaster, and the opportunity to achieve high-price sales was thus missed. This "lost decade" due to the FDNPP disaster means that the earthquake had a major negative impact on fruit production in Fukushima Prefecture.

#### 11.7 Conclusions and Future Issues

The radiocesium concentration in fruit in the second year after the disaster was found to be approximately one-third of that during the first year, and by the third year, it had again fallen to a third; this rate of decrease is greater than that due to the half-life predicted by physics (Kusaba et al. 2015a, b; Renaud and Gonze 2014; Takata 2016, 2019; Yuda et al. 2014). These findings were similar to those reported from several studies on fruit trees after the Chernobyl disaster (Antonopoulos-Domis et al. 1990, 1996; Madoz-Escande et al. 2002; Mück 1997; Pröhl et al. 2006). However, there are interspecies differences in the pattern of radiocesium concentration decrease in fruit. In yuzu (a small, sour citrus fruit; Citrus junos) and persimmons, certain trees have especially high concentrations in the same orchards, but the cause of this has not

Fig. 11.4 (a) Changes in the average unit price of peaches produced in Fukushima Prefecture at the Tokyo Central Wholesale Market (From Ministry of Agriculture, Forestry and Fisheries (Japan) (2021)). (b) Changes in the average unit price of peaches produced in Fukushima Prefecture and national average at the Tokyo Central Wholesale Market (From Ministry of Agriculture, Forestry and Fisheries (Japan) (2021))

been sufficiently determined (Sekizawa et al. 2019). With respect to long-term changes in radiocesium concentration in fruit trees, there have been few follow-up studies at the same sites (Tagami 2016), and it is uncertain what changes will occur in future. In this respect, it is important not only to perform follow-up studies on concentrations in fruit, but also to ascertain the level of radiocesium, in terms of both concentration and absolute quantity, in the fruit trees from which fruit is taken. In previous studies on fruit trees, it has been shown that radiocesium in the soil makes almost no contribution to radiocesium in the fruit, and the following explanations for this have been put forward: (1) in Japan, even in annual crops, the coefficient of transfer to the fruit is low; (2) uptake of radiocesium deposited directly on fruit trees into the tree via the bark is not complete, and radiocesium in the tree makes a major contribution to the radiocesium concentration in the fruit (Takata et al. 2012c, 2013a); (3) radiocesium in the tree still remains there the following year, so that is the origin of radiocesium in the fruit (Takata et al. 2013a, b); and (4) radiocesium is currently distributed heterogeneously in the upper layers of the soil only; most fruit trees grown in Fukushima Prefecture (excluding blueberries) do not form their main root zones in the top 5 cm of soil, which would be responsible for the uptake of radiocesium (Kusaba et al. 2016; Sato et al. 2019; Takata et al. 2013b). Suggestion (4) is linked to the fact that fertilization with potassium is usually ineffective. It is therefore necessary to perform a follow-up study on the trees that produce radiocesium in the fruit. It is important to note that the accumulation of high levels of radioactive cesium in the upper soil layers of orchards is not only related to the safety of the agricultural products produced, but also to the safety of the growers' operations. In relation to yuzu trees growing in the lower parts of the slopes, it is suspected that radiocesium was absorbed via the roots due to flooding caused by heavy rain (Sato et al. 2019). It is necessary to perform long-term monitoring of the potential for radiocesium uptake via the roots. Little progress has been made with measures to decontaminate orchards by topsoil-stripping, and there are numerous orchards where the established guidelines (Ministry of the Environment (Japan) 2011) have still not been met (Sato et al. 2019). There is therefore also a need for long-term, continuous measurement of the aerial radiation dose rate.

In Fukushima Prefecture, 8 years after the disaster, some fruit-growing regions are specified as evacuated areas, and measures to restart commercial agriculture in these areas have yet to be established. In areas where almost no radioactive nuclei have been detected, problems in terms of consumption behavior remain. With no direct relationships to radioactive nuclei, this has been linked to the phenomenon of "misinformation." A major factor involved in this phenomenon is considered to be the delays in updating information for consumers. As a result, even when accurate data is obtained, neither the environment of the sender nor the receiver of that information is satisfactory. In the Tokyo region, which is a major consumption area, opportunities to transmit information about fruit radiocesium concentrations are much fewer than in Fukushima Prefecture. The current situation is disheartening, as straightforward, positive information about areas where cultivation is currently possible and acceptable, which is gradually being accumulated and collated, does not catch people's attention as much as sensational information about specified safety levels having been exceeded. In addition, in terms of the positioning of fruit in the Japanese market, one difference from other products is the strong perception of fruit being sold as gifts. There are differences between consumption behavior relating to items for gifts and for use in one's own home. For example, once an alternative avenue for gifts has been identified, it is difficult for the original production site to win those consumers back. Long-term monitoring is as important as the safety survey soon after the disaster. Scientific issues pertaining to this problem must be investigated continuously in 10-year intervals, and the aspect of this being a scientific study must be emphasized. The study must also be accompanied by firm and clear statements that do not misconstrue concealed safety concerns.

The fruit tree industry does not depend solely on production. Making progress with a wide range of research, from studies performed close to the production site with the aim of reducing radiocesium levels to analysis of the consumption psychology of the actual purchasers, should provide a boost that will lead to sales price recovery.

#### References

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# Chapter 12 Overview of Radiocesium Dynamics in Forests: First Decade and Future Perspectives

Shoji Hashimoto

#### 12.1 Introduction

Fukushima Prefecture and the surrounding areas are among the most forested areas in Japan (Fig. 12.1); ~70% of the contaminated areas are covered with forests (Hashimoto et al. 2012). Accordingly, although the amount of total deposition on the land is still in debate, ~70% of it was deposited onto forest ecosystems (Kato et al. 2019a). Therefore, the forest is one of the key ecosystems in the radioactive contamination caused by the Fukushima Daiichi Nuclear Power Plant accident.

In Fukushima, there are 0.97 million ha of forests. The forests in Fukushima and adjacent regions are rich in tree species, like the forests in other areas of Japan. The most dominant species are the Japanese cedar (Cryptomeria japonica), the Japanese cypress (Chamaecyparis obtusa), the red pine (Pinus densiflora), and the Konara oak (Quercus serrata). These trees in artificial forests are used for building, paper, and mushroom cultivation. Furthermore, in forests, local inhabitants collect wild edible mushrooms and vegetables, and hunt wild game for meat (e.g., sika deer, Asian black bear, and wild boar). Therefore, the radioactive contamination by the Fukushima Daiichi Nuclear Power Plant accident seriously affected the lives of the local inhabitants.

Therefore, the radiocesium dynamics in the forest are of great concern to people and the authorities. The studies conducted after the Chernobyl (Chornobyl) Nuclear Power Plant accident have reported that the radiocesium deposited onto forests migrates within forest ecosystems (IAEA 2006; Shaw 2007). The radiocesium is

S. Hashimoto (✉)

Laboratory of Silviculture, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo, Japan

Department of Forest Soils/Center for Biodiversity and Climate Change, Forestry and Forest Products Research Institute (FFPRI), Tsukuba, Ibaraki, Japan e-mail<EMAIL_ADDRESS><EMAIL_ADDRESS>
T. M. Nakanishi, K. Tanoi (eds.), Agricultural Implications of Fukushima Nuclear Accident (IV), https://doi.org/10.1007/978-981-19-9361-9\_12

Fig. 12.1 Spatial distribution of forests in Fukushima area (Ministry of Land, Infrastructure, Transport and Tourism 2020). Black triangle indicates the location of the Fukushima Daiichi Nuclear Power Plant

first trapped by the tree canopy and migrates to the forest floor afterward. The mineral soil under the organic layer of the soil surface becomes the largest reservoir of radiocesium eventually. With time, spanning from days to years, migrations occur and are driven by various physical and biological processes in forest ecosystems. However, the overall picture described above is not necessarily inferred from the abundance of observation data (particularly limited data in the first few years).

In Fukushima, immediately after the fallout, many researchers and governmental authorities collected huge amounts of data that described the spatiotemporal radiocesium dynamics. This chapter describes the overall radiocesium dynamics in forest ecosystems that were revealed by various research studies conducted in the first decade after the accident. Also, it discusses the future status of forest radiocesium with predictions from modeling studies.

#### 12.2 Decrease in Radiation Dose in Forests

The important characteristic of radioactive contamination is its radioactive decay. Figure 12.2 shows the temporal changes in the air dose rates measured in forests in Fukushima Prefecture (Fukushima Prefecture Forestation Division 2021). The longterm monitoring clearly illustrates the decreasing trend of the radiation dose in forests, which is consistent with the theoretical changes caused by the physical

decay of 134 Cs and 137 Cs. As in other ecosystems, the decrement in the air dose rate is more rapid in the first several years due to the decay of 134 Cs. In the later phase of the first decade after the accident, the decrement slows down, because most of the remaining radiocesium is 137 Cs, which has a half-life of 30 years. The decrease in the air dose rate may occur faster than predicted based on the theoretical curve due to the further migration of radiocesium in the deeper layers of mineral soils. However, the decrease would basically follow the theoretical curve, because the migration of radiocesium within the soil is very slow, and most radiocesium will stay in the shallowest layer of forest soils. Besides, the forest retains most radiocesium without substantial outflow via stream flows. These characteristics of temporal changes in the air dose rate in forests are the fundamental fact for local people and authorities that take safe and effective countermeasures against contaminated forests.

#### 12.3 Intensively Investigated Parts of Forests

The obtained forest radioactivity data have been published in scientific journals, government reports, and the Web. Published data on activity concentration and inventory of trees, soils, and mushrooms observed in forests were comprehensively compiled (Hashimoto et al. 2020b). The compilation showed that trees were investigated more intensively, but soils and mushrooms were also moderately investigated (Table 12.1). The inventory data for the soil were actively obtained, whereas those for the tree part were less investigated.

As expected, the compilation found much data for the Japanese cedar, because it is the most widely distributed tree species in managed forests (Table 12.2). However, the Konara oak, an important tree species for mushroom cultivation in Japan, was less studied.

Concerning the parts of the Japanese cedar, the leaf (needle) was mostly investigated intensively (Table 12.2). However, the woods (bulk wood, heartwood, and sapwood) were well studied, probably because wood is commercially the most important forest product. Interestingly, the pollen of the Japanese cedar was also

Table 12.2 Counts for activity concentration data for trees by species and parts (Hashimoto et al. 2020b)

Total 8593 4105 3189 471 3521

well investigated, because it is the main cause of the hay fever in Japan, and the potential pollen contamination could be of great concern to the public.

#### 12.4 Radiocesium Dynamics in Forests

#### 12.4.1 Trees to Soils

Most of the radiocesium deposited onto forests was first trapped by the tree canopy (Kato et al. 2012, 2019a; Gonze et al. 2021). The reported interception rates vary between studies due to the different observation periods, methods, and forest types (Hashimoto et al. 2020c). Gonze et al. compiled the early data and back-calculated the interception rate for coniferous trees during deposition; the estimated value was

Fig. 12.3 Changes in percentages of radiocesium inventory (134 Cs + 137 Cs) in trees and soil (organic layer + mineral soil) to total inventory in the forest observed at long-term monitoring sites operated by FFPRI (Forestry Agency 2022). The red vertical line in 2011 is the tree interception ratio in March 2011 estimated by Gonze et al. (2021)

0.7–0.85 in March 2011 (Gonze et al. 2021). The interception ratio for the deciduous trees, which had no leaves in March, was lower than that for coniferous trees.

As reported in many studies, the radiocesium then rapidly moved from the tree canopy to the forest floor in time scales of days to years through litterfall and rainfall (Itoh et al. 2015; Kato et al. 2019b). Figure 12.3 shows the percentage of the radiocesium inventory in the tree canopy and soil (soil surface organic layer + mineral soil layer) relative to the total inventory observed in the forests in Fukushima Prefecture (Forestry Agency 2022). Note that the sampling was conducted during the summer yearly, and that the first data were taken 5 months after the fallout in 2011. Although the tree inventory ratio varies among sites and species, the tree inventory decreases with time, and accordingly, the soil inventory increases with time. Within a few years, the soil surface organic layer (A0 layer) and mineral soil retain more than 90% of the total inventory in the forests. The activity concentration of 137 Cs in needles and branches exponentially decreased with time from 2011 to 2015, with effective ecological half-lives of 0.45–1.55 and 0.83–1.69 years for needles and branches, respectively (Imamura et al. 2017).

#### 12.4.2 Soil

As shown above, the radiocesium trapped by the tree biomass moved to the forest floor (Fig. 12.3). However, most of the radiocesium deposited onto the forests is now in mineral soils, not in the soil surface organic layer (Fig. 12.4). The soil surface organic layer retains almost the same amount of inventory as in the mineral soil layer

Fig. 12.4 Time dependency of percentage of radiocesium inventory (rCs: 134 Cs + 137 Cs) in the soil surface organic layer (litter layer, A0) to total inventory in soil (soil surface organic layer + mineral soil layer) observed in long-term monitoring sites operated by the FFPRI (Forestry Agency 2022)

in the early stage, but the percentage continuously decreases over time (Fig. 12.4). Imamura et al. compiled the radiocesium distributions in the soil surface organic and mineral soil layers observed between 2011 and 2017, and found that the radiocesium fraction in the soil surface organic layer drastically decreased in the first 6 years from ~60% in 2011 to ~10% in 2017 (Imamura et al. 2017). Probably because the soil surface organic layer is thinner in Japan than in the forests in European countries affected by the Chernobyl accident, radiocesium did not remain long in the surface organic layer in Japan.

In the mineral soil layer, most radiocesium materials remain in the shallow layer (Fig. 12.5). Figure 12.5a shows the temporal changes in the activity concentration in each soil layer in a forest in Kawauchi village, Fukushima (Forestry Agency 2022). First, the activity concentration at depths of 0–5 cm increases from 2011 to 2012. Afterward, the highest concentration between layers is maintained. The vertical distribution of the radiocesium inventory observed in 2011 and 2015 is shown in Fig. 12.5b (Imamura et al. 2017). The radiocesium inventory in the surface layer has remained the largest since 2011.

#### 12.4.3 Wood

The activity concentrations of wood, which is the most important part of a tree in terms of commercial use, differ among tree species and forests. Figure 12.6 shows the temporal trends in the activity concentrations of the Japanese cedar, Japanese cypress, red pine, and Konara oak observed in forests in Fukushima. These trends

Fig. 12.5 Vertical distributions of radiocesium (rCs: 134 Cs + 137 Cs) in Japanese cedar forest in Kawauchi village (long-term monitoring sites of FFPRI). (a) Temporal changes in activity concentrations (Forestry Agency 2022), and (b) vertical distributions of radiocesium inventory observed in 2011 and 2015 (Imamura et al. 2017)

differ even for the same tree species. However, the Konara oak shows increasing trends, whereas the cedar shows stable or small increasing and decreasing trends. The normalized activity concentration, which is almost comparable to Tag, mainly ranges from 10-4 to 10-3 (m2 /kg, dry weight).

The Tag values for the wood observed in Fukushima were collated and compared with those in Chernobyl studies (Hashimoto et al. 2020c). The range and geometric mean of the Tag values for the conifer (pine excluded), pine, and oak are similar to those observed after the Chernobyl accident. Although the record number is small, those for the pine are probably lower than those reported for the Chernobyl accident, including the data measured for hydromorphic soils (Hashimoto et al. 2020c). The

Fig. 12.6 Activity concentration of 137 Cs for Japanese cedar, Japanese cypress, red pine, and Konara oak normalized with total inventory of 137 Cs in 2012. [The data was originally from Ohashi et al. (2017), and were normalized by the author]

similarity indicates that the overall magnitude of transfer to the wood was similar and comparable, although the number of samples and different species and timing since the fallout must be considered. The comparison suggests the mutual global applicability of radioecological parameters and the need for further collation of parameters. However, Fig. 12.6 shows that the transfer to wood and its time dependence vary among species and sites, thereby emphasizing careful monitoring of trees and soils at affected areas where accidents occurred is always essential. The cause of the variability remains unclear; thus, future studies should be conducted.

#### 12.4.4 Wild Food in Forests

The radiocesium deposited onto forests was also transferred to wild edible foods, such as wild mushrooms, wild edible plants, and game animals. The consumption of these contaminated foods in forests is an even more sensitive issue in affected areas. After the accident, the government and scientists have monitored the radioactivity levels in wild foods in the forests. The data revealed huge variations of radiocesium transfer among species (Tagami et al. 2016; Komatsu et al. 2019, 2021; Hashimoto et al. 2020c). For example, Komatsu analyzed open government monitoring data for wild mushrooms and found that the normalized concentration ranged from 1.1 × 10- 4 to 2.3 × 10-2 (m2 /kg, fresh weight) and that the mycorrhizal species tend to have higher radioactive concentrations than saprophytic species (Komatsu et al.

2019) (Fig. 12.7). It was also confirmed that wild plants generally had lower normalized concentrations than mushrooms, although some species of wild plants showed much higher transfer (e.g., koshiabura, Eleutherococcus (Chengiopanax) sciadophylloides) (Komatsu et al. 2019, 2021). A similar analysis was also conducted using the monitoring data for game meat (Tagami et al. 2016). The analysis of the data collected between 2011 and 2015 revealed that the geometric mean values of the Tag values of 137 Cs in 2015 for the Asian black bear, wild boar, sika deer, and copper pheasant were of similar values, ranging from 1.9 × 10-3 to 5.1 × 10-3 (m2 /kg, fresh weight), whereas those for the green pheasant and wild duck were about one order of magnitude lower than these species, ranging from 1.0 × 10-4 to 2.2 × 10-4 (m2 /kg, fresh weight). Also, different time dependencies for different game animals were observed.

The Japanese government sets and operates the criteria concerning radiation and radioactive concentrations to protect people from radiation exposure. However, the regulations have affected people's lives in contaminated areas through limited access to forests, restrictions on the use of timber, wild mushrooms and plants, and wildlife. More details are described in the study by Hashimoto et al. (2022).

#### 12.5 New Features Captured in Fukushima

The Fukushima Daiichi Nuclear Power Plant accident is the second-largest nuclear accident since the Chernobyl Nuclear Power Plant accident. There are several new features captured in the research on the Fukushima accident.

In the Chernobyl accident, few data captured the very early radiocesium dynamics in the forests (days to months after the accident). However, in the Fukushima accident, the preinstalled observation system, which had been operated for biogeochemically studying the forest before the accident, monitored the very early dynamics of the radiocesium, including deposition, trapping by the tree canopy, and migration from the canopy to the forest floor. Furthermore, the monitoring activity started early enough in many forests; thus, days to months and a few years of radiocesium in forests were well captured. These data are new and essential in areas of radioecology research, aiding the understanding of radiocesium dynamics in forests.

Another new feature is the transparency of various datasets. Various datasets about forest and forest products were released and opened by researchers and the government, and these datasets are available for researchers and the public. For example, the FFPRI and the Forestry Agency have monitored the forest radiocesium dynamics (i.e., contamination of tree organs and soils and radiation) since August 2011 and published the data annually. Fukushima Prefecture has also published similar monitoring data. The government has also released monitoring data for edible forest products, such as wild mushrooms, wild edible plants, and game meat. The airborne survey of radiation and radiocesium inventory was also demonstrated repeatedly and is available in a very easy-to-see Web interface and GIS format for specialists (e.g., https://emdb.jaea.go.jp/emdb/). These datasets have demonstrated forest contamination, as researchers have used them to study forest contamination. Furthermore, they provided the general situation of the forest contamination to the public with transparency.

#### 12.6 Future Prediction

More than a decade of research by scientists and authorities revealed the radiocesium dynamics and radiation changes in forests. What will happen to forests in the future? Because most of the 134 Cs, which contributed to the rapid decrease in the radiation dose, had decayed in the last 10 years, the decrease in radiation declines and occurs with the decay of 137 Cs (Fig. 12.8). Thus, 137 Cs will continue to migrate to forests. Modeling analysis is a good tool for understanding the transfer processes and predicting future radiocesium dynamics in forests (Ota et al. 2016; Nishina et al. 2018; Thiry et al. 2018; Kurikami et al. 2019). After the Fukushima nuclear accident, modeling studies were conducted to predict radiocesium dynamics in forests, revealing that, as already demonstrated by many observations, the mineral soil

Fig. 12.8 Predicted changes in air dose rate with time based on physical decay. The ratio of 134 Cs and 137 Cs emitted at the time of the accident was assumed to be 1:1. 134 Cs emits radiation ~2.7 times more intense than 137 Cs

Fig. 12.9 Prediction of soil inventory percentage in coniferous forests by six models (Hashimoto et al. 2021)

compartment will be the largest radiocesium reservoir (Fig. 12.9). However, a small portion of radiocesium in the forest ecosystem circulates continuously within forests, between trees and soils, probably approximately <1% of the total initial deposition (Hashimoto et al. 2020a) (Fig. 12.10). However, the six-model intercomparison revealed that the future of radiocesium concentration in wood, which is the most important part of a tree in terms of forest products, is quite uncertain. The intercomparison demonstrates that the state-of-the-art models worked well to predict the radiocesium concentration in trees within a decade, but the uncertainty will increase with time (Fig. 12.11). These patterns were also found in a preliminary simulation for the Konara oak. In the equilibrium stage, the key driver of forest radiocesium dynamics between trees and soils is root uptake; the uncertainty and

Fig. 12.10 Conceptual diagram of radiocesium circulation in forests; early phase after the fallout and later phases in the equilibrium state. In the equilibrium state, the emission from trees and the uptake from the soil are almost balanced

Fig. 12.11 Prediction of normalized activity concentration (NC) in conifer wood using six models. The activity concentration is normalized with the total initial deposition (Hashimoto et al. 2021)

variation in the model predictions are attributable to the different root uptake assumptions among the models. To reduce the future prediction uncertainty, it is essential to quantify the root uptake ratio and the biogeochemical processes of radiocesium in the soil. The radiocesium dynamics in forests will decrease with time, but will never stop. Thus, it is important to continue monitoring and understanding the transfer process of radiocesium within forests.

Another important perspective on the future of contaminated forests is the health of forest ecosystems. Contaminated forests, particularly artificial forests, are now abandoned or undermanaged, which risks the health of forest ecosystem services. Additionally, no-go areas and less used forests have changed the balance between humans and wild animals in the affected areas. Note that these changes in forest ecosystems are not direct effects of radiation. It is crucial to monitor the forest health in affected areas and promote healthy forest functioning.

#### 12.7 Conclusion