Changes in radiocesium concentration in a Japanese chestnut (Castanea crenata Sieold & Zucc.) orchard following radioactive fallout

Abstract

We investigated changes in radiocesium concentrations in a Japanese chestnut (Castanea crenata Sieold & Zucc.) orchard in Ibaraki prefecture for 3 years after the Tokyo Electric Power Company’s Fukushima Daiichi nuclear power plant accident in March 2011. The radiocesium concentrations in the aboveground organs of Japanese chestnut trees were almost the same, while the concentration in the roots was the lowest among all the organs investigated. The concentration of radiocesium decreased exponentially for 3 years in nuts, leaves and current shoots. The radiocesium concentrations in soils were higher in the surface layer, and the trend of an annual decrease in radiocesium in the soils was similar to that of the natural decay of radiocesium. The transfer factor of radiocesium from soils to nuts of Japanese chestnut decreased annually. These results suggest that radiocesium adhered directly to the aboveground organs of Japanese chestnut trees in March 2011, and that the accumulation of radiocesium in nuts is mainly due to radiocesium transfer from the branches and trunk to nuts several years after the nuclear power plant accident.

Key words:

• annual change
• Japanese chestnut
• nuclear power plant accident
• radiocesium
• soil

INTRODUCTION

After the Tokyo Electric Power Company’s Fukushima Daiichi nuclear power plant (FDNPP) accident in March 2011, radionuclides diffused over a large area. As a consequence, many orchards were contaminated, mainly by radiocesium. At the time of the accident, many kinds of deciduous fruit trees had not yet sprouted, including the Japanese chestnut (Castanea crenata Sieold & Zucc.). Therefore, the diffused radiocesium fell onto tree bark, branches and the soil surface in orchards. Sato et al. (2013) reported that the radiocesium in new organs of deciduous fruit trees contaminated during the dormant period might be transferred mainly from the tree bark and branches. A previous study found that radiocesium had a relatively high transfer rate from the soil to the nuts of the chestnut (Monte et al. 1990). After the 2011 accident, nuts of the Japanese chestnut in several contaminated orchards had radiocesium concentrations exceeding the official tolerance level, which was 500 Bq kg fresh weight (FW)⁻¹ until April 1, 2012 (when it was changed to 100 Bq kg FW⁻¹). To reduce the radiocesium concentration in nuts, the actual situation of the contamination in Japanese chestnut orchards should be clarified. In this report, we describe the changes over 3 years in the radiocesium contamination of a Japanese chestnut orchard following the FDNPP accident. Additionally, we have investigated radiocesium concentrations in a blueberry orchard (Vaccinium virgatum Aiton) (Kusaba et al. accepted), and here we discuss differences between the radiocesium concentrations in Japanese chestnut and blueberry trees.

MATERIALS AND METHODS

Plant and soil samples

Samples from Japanese chestnut trees (Castanea crenata Sieold & Zucc.) and the soil in which they grew were collected in an experimental field of the NARO Institute of Fruit Tree Science (Tsukuba, Ibaraki, Japan), 200 km south of FDNPP, in 2011–2013. The soil of the field is a low humic Andosol (Haplic Andosol). Three Japanese chestnut trees were used in this study, a 20-year-old tree of the cultivar ‘Porotan’, a 12-year-old tree of the cultivar ‘Porotan’ and a 12-year-old tree of the cultivar ‘Ishizuchi’ (ages as of 2011). All samples were collected at the optimum harvesting time of the nuts, September for ‘Porotan’ and October for ‘Ishizuchi’. Leaf samples were collected from the middle part of the current shoot. Current shoots (2011–2013), 2-year-old branches (2011–2012), and 3-year-old branches (2012) were collected as branch samples. Root samples were collected from the soil at a site 1–2 m from the trunk and at a depth of 0–15 cm. Using a core sampler (ϕ = 3.5 cm), soil samples were collected from four points under the canopy at sites approximately 2 m from the trunk and at depths of 0–5 cm and 0–15 cm. The bulk density of the soil was 690 kg m⁻³.

Sample preparation

Nuts were washed with tap water. The edible parts of the nuts were prepared by peeling the shells, and then minced after measuring the fresh weights. Branch samples were cut to approximately 1 cm in length, weighed to determine their fresh weight, then dried at 70°C to a constant weight. Leaf samples were weighed and dried at 70°C to a constant weight before mincing. Root samples were washed with tap water until soil was no longer visible on the root surface, and wiped with paper towels to remove residual water before cutting to approximately 1 cm in length. After measuring the fresh weight, the samples were dried at 70°C to a constant weight.

Stones (> 2 cm) and coarse organic matter were removed from the soil. Part of each soil sample was dried at 110°C to a constant weight to calculate the water content of the sample.

Measurement of radiocesium radioactivity

Radiocesium concentrations were measured by gamma-ray spectrometry with a high-purity germanium detector. Radiocesium concentrations were calculated in Bq kg FW⁻¹ for tree samples, and in Bq kg dry weight (DW)⁻¹ for soil samples. The radiocesium concentrations were corrected for physical decay to the date of sample collection and were expressed as sum of cesium-134 (¹³⁴Cs) and cesium-137 (¹³⁷Cs).

RESULTS AND DISCUSSION

Radiocesium concentrations in organs of Japanese chestnuts

The radiocesium concentrations were obtained from three trees as replications. We investigated the differences in radiocesium concentrations in selected parts of Japanese chestnut trees (Table 1). In 2011, the radiocesium concentrations in leaves, burs and 2-year-old branches were relatively high in the samples investigated. However, the concentration in the roots was the lowest. The trend was almost the same in 2012. In apricot (Prunus armeniaca L.) trees (Antonopoulos-Domis and Clouvas 1990), peach (Prunus persica (L.) Batsch) trees (Takata 2013) and blueberry bushes (Kusaba et al. accepted) the highest concentration of radiocesium, with one or two orders of magnitude difference, was detected in branches that were directly affected by radioactive fallout. In contrast to these reports, the radiocesium concentrations in different aboveground parts of Japanese chestnut trees were almost the same. This may indicate that the translocation of radiocesium from the organs affected by the fallout directly to new organs produced after the accident might be different in chestnut trees than in other kinds of fruit trees.

Table 1 Radiocesium concentration (¹³⁴Cs + ¹³⁷Cs Bq kg FW⁻¹) in each organ of chestnuts (Castanea crenata Sieold & Zucc.)

Organ year^–1	2011	2012
Nuts	29^† ± 5.7	7.3 ± 1.0
Shells	31 ± 18	NS
Burs	46 ± 27	NS
Leaves	45 ± 6.1	13 ± 1.5
Current shoots	21 ± 1.2	5.0 ± 0.6
2-year-old branches	38 ± 12	4.7 ± 0.3
3-year-old branches	NS	14 ± 4.1
Roots ( < 7 mm)	8.4 ± 2.4	6.5 ± 2.2
Roots ( > 7 mm)	NS	3.4 ± 0.7

^† Values are means of three trees, a 20-year-old tree of the cultivar ‘Porotan’, a 12-year-old tree of the cultivar ‘Porotan’, and a 12-year-old tree of the cultivar ‘Ishizuchi’ (ages as of 2011), with standard error of the mean. FW: fresh weight; NS: not sampled.

Additionally, the trends in the changes of radiocesium concentrations in current shoots in 2011 and 2-year-old branches in 2012 (these were the same organs) were different between chestnut and blueberry. The radiocesium concentrations in the 2-year-old branches of blueberry bushes in 2012 were almost the same as those in the current shoots in 2011 (Kusaba et al. accepted), whereas those in the 2-year-old branches of chestnut trees in 2012 had decreased in comparison with those in current shoots in 2011. These results might be attributed to the difference in non-uniformity of the radiocesium concentration in the aboveground organs of blueberry bushes and chestnut trees. In blueberry bushes, the highest concentration of radiocesium was detected in branches that were directly affected by radioactive fallout. On the other hand, in Japanese chestnut trees, the distribution of radiocesium in the aboveground organs was relatively uniform compared with that in blueberry bushes; therefore, it is possible that the trends in the changes of the radiocesium concentration in the same organ were different between chestnut trees and blueberry bushes. The mechanism underlying this observation, however, remains unclear.

Changes in radiocesium concentrations of nuts, leaves and current shoots

The changes in radiocesium concentrations over 3 years in nuts, leaves and current shoots are shown in Fig. 1. The concentrations decreased exponentially for 3 years in each of the sampled organs, and the rates of decrease were faster than the natural decay of radiocesium. Although the initial concentrations were different, the trends of the decrease in concentration were similar in every organ. Exponential decreases in radiocesium have also been reported in the fruits of apricots, sweet cherries (Prunus avium (L.) L.), pears (Pyrus communis L.) and apples (Malus pumila Mill.) by Antonopoulos-Domis et al. (1991). The trend of the decrease in concentration in Japanese chestnut trees was the same as for these fruit trees.

Figure 1 Changes in radiocesium concentrations (¹³⁴Cs + ¹³⁷Cs) in nuts, leaves and current shoots of chestnuts (Castanea crenata Sieold & Zucc.). Values are means of samples collected from three trees. Bars indicate standard error of the mean.

Changes in soil radiocesium concentrations

The changes in radiocesium concentration over 3 years in soils under the canopy are shown in Fig. 2. Radiocesium concentrations in the surface soil (0–5 cm) were higher than those in the soil at 0–15 cm. In the undisturbed orchard, over 90% of the radiocesium was found within the top 6 cm of soil in Fukushima after the accident (Sato 2014). The radiocesium concentration in the surface soil of the Japanese chestnut field in this study might have been high in comparison with that in the soil at 0–15 cm because the field was also undisturbed after the accident. The trend of the decrease of radiocesium in the soil was similar to that of the natural decay of radiocesium and slower than that in the nuts.

Figure 2 Changes in the soil radiocesium concentration (¹³⁴Cs + ¹³⁷Cs). Values are means of samples collected from sites under the canopy of three trees. Bars indicate standard error of the mean.

Accumulation of radiocesium in Japanese chestnut trees

It has been reported that 15 and 35% of thick roots and 32 and 65% of fine roots exist within the top 10 and 20 cm of soil, respectively, in chestnut trees (Karizumi 2010). Although the radiocesium concentration was 280–570 Bq kg DW⁻¹ in the soil (0–15 cm depth), roots had the lowest radiocesium concentrations of the organs sampled in this study (Table 1). Sato (2014) reported that over 95% of radiocesium was in the aboveground parts in apple, peach, and persimmon (Diospyros kaki Thunb.) trees. Antonopoulos-Domis et al. (1991) demonstrated that the root uptake of radiocesium contributed a small portion of the total contamination of the leaves and fruits of trees planted before the Chernobyl accident. Furthermore, the trend of the annual decrease of the radiocesium concentration in the nuts was different from that in the soil. The transfer factor (TF) shown as [radiocesium concentration of nut (Bq kg FW⁻¹)/radiocesium concentration of soil (Bq kg DW⁻¹)] decreased annually, suggesting that the radiocesium TF value in this study might have been affected by the direct contamination of the aboveground parts of chestnut trees. Therefore, the contribution of roots to radiocesium uptake from the soil might be small in nuts of the Japanese chestnut. In peach trees, Takata (2013) suggested that radiocesium that fell on the bark might be translocated inward through lenticels and transported to the fruits after the FDNPP accident. These previous findings and the results in this report indicate that the accumulation of radiocesium in chestnut trees might be caused mainly by the direct adhesion of radiocesium onto the tree. Further, the accumulation of radiocesium in nuts might be mainly due to radiocesium translocation from branches and the trunk to nuts for several years after the nuclear power plant accident.

ACKNOWLEDGMENTS

We thank Sogo Nishio and Noriyuki Onoue (NARO Institute of Fruit Tree Science) for their technical assistance. This work was supported by the research project “Development of Radioactive Materials Removal Technologies for Agricultural Soils” (2011) in Strategic Funds for the Promotion of Science & Technology conducted by the Ministry of Education, Culture, Sports, Science and Technology of Japan, and by the research projects “Development of Radioactive Materials Removal and Reduction Technology for Forests and Farmland” (2012), and “Development of Decontamination Technologies for Radioactive Substances in Agricultural Land” (2013) conducted by the Ministry of Agriculture, Forestry and Fisheries of Japan.