The Transition of Radiocesium in Peach Trees After the Fukushima Nuclear Accident
In this chapter, we introduce the effects of radiocesium released by the Fukushima Daiichi nuclear accident on fruit trees, especially the change of radiocesium in fruit during the past 6 years. We investigated radiocesium and 40K in peach during the maturity of its fruit chronologically for 6 years. In the investigation during one crop period, the concentration of radiocesium in young fruit 15 days after the full bloom was the highest, and this result was common in all the investigated years. After that, the concentration of radiocesium decreased as the fruit became bigger; the decrease until 60 days after the full bloom was considerable. This tendency was also common among all investigations conducted until 2016. Though the concentration of 40K during the same period also decreased in the same way as radiocesium, the rate of the decrease from 15 to 30 days after the full bloom was different. When looking at the chronological transition, the concentration of radiocesium in harvested fruit decreased by one third every year from 2011 to 2013. However, such decrease could not be seen from 2014 to 2016. While the concentration in the harvested fruit tended to stop decreasing, the concentration in fruit 15 days after the full bloom tended to decrease over the years from 2012 to 2016. During the past 6 years, there was no year-over-year decrease in the concentration of 40K in fruit. The reason why the transition of radiocesium in fruit varied according to their stage of maturity was because the difference in timing to use the tree’s nutrient reserves.
To understand the year-over-year transition of radiocesium in peach, the amount of 137Cs in every part of the tree was measured. When comparing the distribution of 137Cs and 40K in the peach trees, it was found that 137Cs was existing in the body of the tree, which was contaminated by fallout, while 40K was distributed more in the leaves and fruit. Also, while the weight of the trees and the amount of 40K in the tree body increased with time, the amount of 137Cs decreased over the years. It is considered that radiocesium stored in the mature woody parts such as stem and branches had been transferred to fruit, leaves, or young branches.
KeywordsPeach Cs Fruit tree Fukushima Daiichi Nuclear Power Plant accident
Fukushima Prefecture has 6820 ha of orchards in 2016 and it is one of the top fruit producing areas in Japan (National Statistics Center 2016). When looking at each kind of fruit, the area of the land under cultivation with peaches is the second largest in Japan (1810 ha), and this means that the prefecture is one of the major producers of fruit in Japan even after the Fukushima Daiichi nuclear accident. With regards to changes to the retail price of fruit, Komatsu (2014) and the author’s previous study (Takata 2016), show that most fruits have returned to their pre-accident prices, yet the situation is not the same as before.
Six years after the accident, the concentration of 134Cs is thought to be in low in fruit because its half-life is about 2 years. On the other hand, 137Cs, whose half-life is longer than 134Cs can still be detected. However, previous studies found that radiocesium in fruit decreased more quickly than its actual half-life; for example, the concentration of 137Cs decreased by one third in the second year, and by another one third in the third year after the accident (Takata et al. 2014, 2016). These results correspond to what was found in several types of fruit after the Chernobyl nuclear power plant accident (Antonopoulos-Domis et al. 1990; Madoz-Escande et al. 2004). However, there are few reports which traced and investigated products in the same region to examine a long-term transition of the concentration of radiocesium in fruit trees, and the future transition is uncertain. To understand this point, it is necessary to investigate the concentration of radiocesium not only in fruit continuously but also in trees which are the source of Cs in fruit. The author’s previous study found that radiocesium in soil has almost no relationship with Cs in fruit in most cases. This is because (1) the radiocesium transfer factor (TF) ratio for fruit is lower than annual crops in Japan, (2) trees are no longer affected by fallout which fell directly on bark, and largely contributed to the concentration of radiocesium in the fruit (Takata et al. 2012b, 2013a), (3) since radiocesium in the trees remain in situ year after year, this has become a source of radiocesium in fruit (Takata et al. 2012d, 2013c), and (4) most kinds of fruit in Fukushima Prefecture (except a few kinds such as blueberries) do not form their rooting zones within 5 cm from surface horizon and thus they do not absorb radiocesium, which was distributed only on the surface horizon unevenly (Takata et al. 2013b). The fourth reason is also related to the fact that potassium fertilizer has no effect in most cases. Therefore, it is necessary to investigate trees continuously to understand the source of radiocesium in fruit.
In this chapter, new research findings, after the author’s previous two reports stated above, are reported. Especially, understanding the dynamics of radiocesium in fruit and redistribution of 137Cs in trees 6 years after the accident is the focus.
9.2 The Year-Over-Year Transition of Radiocesium in Fruit
After the nuclear power plant accident, one variety of peach “Akatsuki” was investigated continuously regarding the concentration of radiocesium in the fruit in Shimo-oguni district, Ryozen-machi, Date City from harvest in 2001 to 2016. In 2011, the concentration of radiocesium in the harvested fruit was measured. From 2012 to 2016, the concentration of radiocesium and 40K was measured throughout fruit development: from Day 15 after the full bloom to harvest (approximately Day 103).
9.3 The Year-Over-Year Transition of Radiocesium in Trees
Radiocesium was detected mainly in organs exposed to the atmosphere such as bark, because fruits trees are perennial plants and grew in the fields even at the time of the accident (in winter) (Takata et al. 2012a, d). Of course, fruit also absorb radiocesium from the soil through their roots, but it has already been confirmed that when considering the source of Cs in fruit, the ratio of radiocesium due to the fallout that adhered to the bark and was transferred to fruits is much higher than absorption through the roots. Therefore, it is important to grasp the transition of the concentration of radiocesium not only in the soil but also in the trees to understand the concentration in fruits. As stated in an earlier section, the concentration of radiocesium in peach fruit decreased as they matured and decreased over 3 years after the accident. It is important to understand the amount of radiocesium in trees to consider the cause of rapid decrease of the concentration over the 3 years after the accident; there is no report which investigated the amount of radiocesium in trees year-over-year. We introduce a study (Takata et al. 2016) which dug up trees after leaf fall to investigate the distribution of 137Cs and compare the dynamic of 137Cs in trees with that of 40K.
We used another variety of peach “Yuzora” planted in the fruit research center of Fukushima Agricultural Technology Center. The individual parts of peach trees of a similar age and planted in the same year were separated for the investigation from 2012 to 2014. Stems and branches were collected, and the branches were sorted according to their age. The roots were collected and sorted according to their thickness. The concentration of 137Cs and 40K in each part were measured and calculated using the dry mass of each part to convert the measured values into the quantities (expressed by Bq) of each organ.
Figure 9.4a shows that the total amount of 137Cs in trees was highest after 1 year and decreased in subsequent years. When looking at the year-over-year transition of each organ, 137Cs in five and seven-year-old branches and stems decreased. The five-year-old branches were the youngest branches which received fallout, and the seven-year-old branches were the main branches at the time of the accident. There was no year-over-year transition of the distribution of 137Cs in organs which did not receive the fallout: that includes branches younger than 4 years old, fruit and leaves (Fig. 9.4b), though the percentage of 137Cs which was transferred to fruit and leaves was 13.1% in the first year and decreased to 7.5% and 3.9% over the subsequent years. On the other hand, the distribution of 137Cs increased year-over-year in the four-year-old branches, which developed in the year of the accident. This could be because branches accumulated radiocesium as they grew, while new leaves and fruits were produced every year. Also, it is considered that the source of Cs in new-grown organs were the stems, five-year-old and seven-year-old branches which decreased the amount of 137Cs in them. However, five-year-old branches were pruned during winter and the cause of the decrease could be either the pruning or redistribution. Therefore, it is estimated that the source of 137Cs in new-grown organs were the stems and seven-year-old branches, which were not pruned.
Although there was no general tendency in the transition of the amount of 137Cs in the roots; the roots thinner than 5 mm and rootlets had higher amounts of 137Cs year- over-year (Fig. 9.4c). Two reasons can be considered to explain this phenomenon: (1) the absorption from the soil and (2) translocation from thick roots and terrestrial organs. However, as explained in an earlier section, radiocesium existed mainly in the surface horizon, especially to a depth of 5 cm rather than the lower horizons. Because the main root area of peach trees is deeper than the horizon, it is unrealistic that 137Cs can be absorbed by the roots. Therefore, it is considered that translocation from thicker roots contributes to the increase in the amount of 137Cs in medium and fine roots. The amount of 137Cs in the trees decreased more than the estimated quantity removed by fruit, leaves and pruned branches (Fig. 9.3a). It is necessary to consider other possibilities to explain this result such as 137Cs exudation from the surface of branches or roots and decay of rootlets. As mentioned earlier, a proportion of 137Cs is translocated to newly grown fine roots, and these new roots often decay in a single season. Therefore, 137Cs transferred to the soil by falling out or decay is not possibly negligible.
On the other hand, the total 40K in trees was smallest in the first year and increased with years; the increase in the fruit and leaves were considerable (Fig. 9.5). This result is closely related to the increase of weight of the trees (Fig. 9.3), and completely opposite to the year-over-year transition of 137Cs which was mentioned earlier.
During the 4 years of the investigation after the accident, it was found that the absorption of radiocesium from soil was much smaller than derived from the tree. The source of 137Cs in the fruits could be the stems and main branches, and the results show that 137Cs transferred from the stems or branches to fruits was not supplemented by the soil. On the other hand, 40K is transported from the soil through the stems and main branches. It suggests that the dynamics of Cs and K in fruits trees is different.
9.4 The Current Investigation
It is important to investigate the movement of 137Cs in trees, but its concentration will become too low to investigate like 134Cs when it reaches its half-life in the future. The dynamics of Cs need to be clarified in case a similar nuclear accident was to occur elsewhere in the world in the future. Although the dynamics of K, which is in the same group as Cs, is sometimes compared to understand the dynamic of Cs, it has been found that those dynamics are partially different in fruit trees. Therefore, the difference in the distribution of Cs and K in soil, absorption through roots, the transition from roots to terrestrial parts, and the transition rate from old to new organs can be obstacles to understanding the dynamics of Cs. Though comparing stable isotope 133Cs is another method, there are several problems to its use as an alternative to 137Cs. The analysis using high precision equipment is necessary to measure 133Cs in the natural environment though it is more abundant than 137Cs. Even if 133Cs is added to the environment to make the measurement easier, its dynamics and concentration may be different than 137Cs. Also, even if 133Cs was not added, the natural 133Cs may show a different dynamic from 137Cs because the natural 133Cs concentration is much higher than 137Cs. A simultaneous measurement of 133Cs and 137Cs is essential to solve these problems and it should be performed before the concentration of 137Cs becomes too low to measure. Now we found several results on the similarity and difference of dynamics and abundance between 133Cs and 137Cs. For example, the transfer factor ratio of 133Cs and 137Cs to fruit is likely to be similar when they are applied at the same time to blueberries. Other experiments such as spraying with 133Cs have been performed together with investigations using 137Cs. Also, the difference of dynamics of Cs which stored in terrestrial and subterranean tree parts has been analyzed.
This article is based on the collaborative research with Dr. Eriko Yasunaga, Dr. Haruto Sasaki, Dr. Keitaro Tanoi, Dr. Natsuko I. Kobayashi, Dr. Seiichi Oshita (the University of Tokyo) and Mamoru Sato, Kazuhiro Abe (Fukushima Prefecture Agricultural Technology Centre), with the assistance of Mr. Kengo Izumi, Mr. Kenichiro Ichikawa (the University of Tokyo). This study was partly supported by of Ministry of agriculture, forestry and fisheries (MAFF, Japan).
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