1 Objective

The magnitude 9.0 earthquake and the subsequent large tsunami that occurred on March 11, 2011, caused extensive damage to coastal areas in Tohoku, Japan. In particular, the cooling system of the Fukushima Daiichi Nuclear Power Plant [Tokyo Electric Power Company (TEPCO) FNPP] collapsed from the tsunami in excess of 10 m, which resulted in several explosions in the four reactors of the plant. Large amounts of radioactive materials, mainly noble gas, 131I, 134Cs, and 137Cs, were released into the atmosphere, and consequently agricultural land and forests in Eastern Japan were contaminated. Radiocesium (134Cs + 137Cs) is an important radionuclide that can be used for the assessment of radiation exposure to the public because it has a long half-life (134Cs, 2.06 years; 137Cs, 30.2 years), high transferability, and wide distribution in the environment. Because of their long half-lives, there is concern that radiocesium (134Cs + 137Cs) will remain on the surface of agricultural land and persist for a long time [1, 2]. Therefore, we have started monitoring of radiocesium in soil collected from agricultural land in Fukushima Prefecture from March 2011. Based on these data, Nuclear Emergency Response Headquarters showed planting areas of rice in all regions, except a 20-km exclusion zone and the deliberate evacuation zone (DEZ) in Fukushima Prefecture in 2011. However, the brown rice produced in some areas in northern part of Fukushima prefecture exceeded the provisional regulation value for agricultural crops at the times (>500 Bq kg−1). Consequently, the planting of rice crops for the year 2012 had been restricted in that area. The present studies show investigation of radiocesium transfer in the agricultural plants.

2 Effect of Potassium Application on Root Uptake of Radiocesium in Rice

To decrease radiocesium uptake in brown rice from the contaminated fields, we examined the effect of using potassium fertilizer on the radiocesium uptake in brown rice.

Rice plants (Oryza sativa) were cultivated in the five experimental paddy fields in the northern area in Fukushima Prefecture, which was contaminated with radiocesium, in 2011. Soils and rice plants were collected from five points in each experimental field at harvest time. The mean concentration of radiocesium at soil depths of 0–5, 5–10, and 10–15 cm was 5,800, 3,200 and 1,800 Bq kg−1 dry weight (DW), which was not uniformly distributed, even during plowing before cultivation. The concentration of radiocesium in each field at the depth of 0–5 cm showed approximately threefold variation.

The concentration of radiocesium in brown rice from five rice paddy fields was 231 ± 135 (52–485) Bq kg−1 fresh weight (FW), and the values were different in each collecting point. The soil-to-plant transfer factor is a simple but important parameter that can be used to estimate the concentrations of radionuclides in plants. The transfer factor generally shows a very wide range of variation. The transfer factor of soil-to-brown rice collected from a pair of soil and brown rice samples at each point was in the range of 0.0075–0.11, which was more than one order of magnitude different. Tsukada et al. and Komamura and Tsumura reported that the geometric mean of the soil-to-plant transfer factor of polished rice in rice paddy fields, which were determined by the fallout depositions derived from the nuclear weapons tests, as 0.0016 and 0.0030, respectively [3, 4]. The observed values, which were determined in the same or nearby fields, were higher than previously reported values. The difference may be attributed to nonuniform distribution of radiocesium in the available fractions because of the early stage of the aging periods after deposition onto the soil.

Potassium is an important essential element in plant physiology, and it is supplemented by the application of fertilizers to agricultural soils. There was a high correlation (r = 0.88) between the soil-to-brown-rice transfer factor of radiocesium and the exchangeable K2O in the soil (Fig. 13.1). Other researchers have reported that the transfer factor of 137Cs decreased with increasing concentrations of potassium in soils [5–7]. Kato [8] also reported that the soil-to-plant transfer factor of radiocesium decreased with increasing concentrations of exchangeable K2O in soils. Further, the soil-to-brown-rice transfer factor of radiocesium also decreased from 0.074 to 0.024 with the application of potassium fertilizer through top dressing. Hence, it is clear that the application of potassium fertilizer reduces the concentration of radiocesium in brown rice.

Fig. 13.1
figure 1

Relationship between transfer factor of soil to brown rice and exchangeable K2O in the soil

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3 Phytoremediation of Radiocesium in Different Soils Using Cultivated Plants

Following the nuclear power plant disaster, more than 90 % of the radionuclides were distributed in the upper 6 cm of the soil column in wheat fields, and within 4 cm of the surface in rice paddies, orchards, and cedar forests [9]. It is well known that radiocesium is adsorbed into the soil and binds strongly to clay. As a result, it is difficult to reduce the contamination level in the soil. It was reported that phytoremediation using rice plants in a paddy field was also difficult [10]. We examined the possibility of decontamination by means of phytoremediation using four agricultural plants such as sunflower (Helianthus annuus L.), amaranth (Amaranthus L.), sorghum (Sorghum bicolor), and buckwheat (Fagopyrum esculentum) in upland fields.

The total yields of biomass cultivated in the light-colored Andosol and gray lowland soil is shown in Table 13.1. The biomass of the sorghum in the gray lowland soil (2.91 kg dry weight m−2) was five times higher than that of buckwheat cultivated in the light-colored Andosol, and the gray lowland soil was within a factor of 2. Sorghum had the highest biomass in both the light-colored Andosol and the gray lowland soil.

Table 13.1 Yield of four plants cultivated in light-colored andosol and gray lowland soil
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The concentration of 137Cs in the soil among the fields was 1,300–2,000 Bq kg−1 dry weight. The concentration of 137Cs in the plant components is indicated in Table 13.1. Among the components, the leaves exhibited the highest concentration of 137Cs, except sorghum cultivated in the light-colored Andosol. The concentration of 137Cs in the roots, including adhered soil particles, was relatively similar among the plants cultivated in each soil. However, the concentration of 137Cs in the stem differed approximately fivefold among the plants. The 137Cs concentration in the aboveground part of the plant was 36.7–78.9 Bq kg−1 dry weight in the light-colored Andosol and 18.0–69.1 Bq kg−1 dry weight in the gray lowland soil (Table 13.2).

Table 13.2 Concentration of 137Cs in plant components (Bq kg−1 dry weight)
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The total content of 137Cs in the biomass among the four plants was 19.8–132 Bq m−2 cultivated in the light-colored Andosol and 17.6–79.8 Bq m−2 cultivated in the gray lowland soil. The content in amaranth and sunflower was the highest in the light-colored Andosol and the gray lowland soil, respectively.

The removal percentage of 137Cs, which is defined as the ratio of the total content of 137Cs in the plant biomass (20–154 Bq m−2) to that in the cultivated soil of 0–15 cm depth (154,000–247,000 Bq m−2), was 0.015–0.109 % for the light-colored Andosol and 0.008–0.039 % for the gray lowland soil. The removal percentage of 137Cs for aboveground parts, excluding the root part, was 0.013–0.093 % for the light-colored Andosol and 0.007–0.038 % for the gray lowland soil. The highest values of the aboveground parts were obtained in amaranth (0.093 %) and sunflower (0.038 %) in the light-colored Andosol and the gray lowland soil, respectively (Table 13.3). The ratio of the removal of radiocesium from the surface soil to that of the cultivated biomass, that is, sunflower, amaranth, sorghum, and buckwheat, was negligible. This result indicates that it is difficult to remove radiocesium from contaminated soil by means of phytoremediation.

Table 13.3 Removal percentage of 137Cs by cultivated plant
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