Differences in the distribution of radiocesium in deciduous and evergreen fruit trees: A case study

Abstract

We investigated the distribution of radiocesium (¹³⁴Cs and ¹³⁷Cs) in three orchards in Tsukuba, 170 km southwest from the Tokyo Electric Power Company’s Fukushima Daiichi Nuclear Power Plant 9 months after the accident. The radiocesium was distributed mainly in the surface soil. The distribution of radiocesium differed between deciduous and evergreen plants. In deciduous blueberry (Vaccinium virgatum Aiton), the concentration was high in the old branches because the bushes had no leaves at the time of the accident. Therefore, the concentration per bush was greater in unpruned than in pruned bushes. More radiocesium was present in the trunk and rootstock of each bush, although the concentration was low. In evergreen Satsuma mandarin (Citrus unshiu Marcow), the concentration was high in the leaves, and it was higher in old leaves that expanded before the accident than in new leaves that expanded after the accident, because the old leaves were contaminated by direct deposition of the fallout. However, the total radiocesium was higher in the new leaves than in the old leaves because of the greater amount of new leaves. The radiocesium concentration in fruits was higher in trees with fewer fruits than in trees with many fruits, but the total radiocesium in fruits was higher in trees with many fruits.

Key words:

• blueberry
• citrus
• Fukushima Daiichi Nuclear Power Plant accident
• orchard
• radiocesium

1. INTRODUCTION

Radionuclides released from the Fukushima Daiichi Nuclear Power Plant (FDNPP) were deposited in Tsukuba, 170 km southwest of the FDNPP, after the accident on March 11, 2011 (Hirose 2012). The major radionuclides observed in Tsukuba were radioiodine and radiocesium (Doi et al. 2013). The air dose rate peaked on March 15–16 and 20–22, 2011, after the radioactive plume traveled from the FDNPP to Tsukuba (Doi et al. 2013). At the time of the accident, evergreen trees had leaves, but deciduous trees had no leaves. In deciduous persimmon (Diospyros kaki Thunb.) fruit trees in Inage-ku, Chiba, 220 km southwest of the FDNPP, 1.5 months after the accident, the radiocesium concentration was 424 Bq kg^–1 fresh weight (FW) in leaves that expanded after the accident (Tagami et al. 2012). Four months after the accident, in 5-year-old peach (Prunus persica (L.) Batsch cv. ‘Akatsuki’) trees growing in Tokyo, the concentration of radiocesium in the bark was 1800 Bq kg^–1 dry weight (DW), while that in the skin of mature fruits was 83 Bq kg^–1 DW and that in the roots was below the detection limit (Takata 2013). In evergreen loquat (Eriobotrya japonica Lindl.) fruit trees growing in Inage-ku, Chiba, 3 months after the accident, it was 1803 Bq kg^–1 FW in leaves and 4688 Bq kg^–1 FW in branches, but 207 Bq kg^–1 FW in fruits (Tagami et al. 2012).

The differences between deciduous and evergreen trees were similar in forest trees. In early August, 5 months after the accident, it ranged from 180 to 25,000 Bq kg^–1 DW in the leaves of both kinds of trees in Abiko, Chiba, 200 km southwest of the FDNPP (Yoshihara et al. 2013). The mean concentration in the leaves of evergreen trees was 7.7 times higher than that in the leaves of deciduous trees (Yoshihara et al. 2013). Among evergreen conifers, the concentration in the leaves that expanded before the accident was 2.4 times higher than that in the leaves that expanded after the accident (Yoshihara et al. 2013). These results suggest that the accumulation pattern of the radiocesium differs between evergreen and deciduous trees, and that the radiocesium is transported to new organs produced after the accident from organs that were produced before the accident, at a rate that depends on the species.

A large proportion of radiocesium in soil was present in the top of a few-centimeter layer (Kato et al. 2012; Shiozawa 2013). These studies also indicated strong fixation of radiocesium to clay particles during the initial 2–3 months, although the degree of fixation might differ among soil types. Therefore, the rate of radiocesium uptake by plant roots may have decreased along with a decrease of radiocesium mobility in the soil.

In the present study, to clarify the distribution of radiocesium in the deciduous blueberry and evergreen Satsuma mandarin, we collected tree and soil samples in December 2011, 9 months after the accident, in three orchards in Tsukuba, and measured their Cesium 134(¹³⁴Cs) and Cesium 137 (¹³⁷Cs) radioactivity.

2. MATERIALS AND METHODS

2.1. Orchards and tree samples

The orchards were located at the National Agriculture and Food Research Organization (NARO) Institute of Fruit Tree Science (Tsukuba, Ibaraki, Japan), 170 km southwest of the FDNPP. Two rabbit-eye blueberry (Vaccinium virgatum) orchards (one unmulched and one with mulch made of woodchips from deciduous fruit trees) and one Satsuma mandarin (Citrus unshiu) orchard (unmulched) were selected. The soil in each orchard was a low-humic Andosol. The blueberry bushes were ≥ 20 years old. The unmulched orchard had been pruned in January before the accident, whereas the mulched orchard was unpruned. The Satsuma mandarin trees were 5 years old (planted 2 years before the accident) and had fruits at the time of sampling. The number of fruits differed among the trees, although the tree sizes were similar. Both the blueberry bushes and the citrus trees were established as rooted cuttings, and the rootstock could be clearly distinguished from the shoot scion.

2.2. Sample preparation

Whole-tree samples were collected in December from one bush in each blueberry orchard and from two trees in the Satsuma mandarin orchard. We collected all leaves and fruits first, then collected new branches that had expanded after the accident, old branches (≥ 2 years old) and trunks. Roots were collected starting at a distance of 50 cm from the tree from soil at depths of 0–5, 5–15 and 15–30 cm within a radius of 50 cm around the trunk of each tree. The root samples were washed with tap water to remove the soil. Next, the remaining rootstocks were collected and washed. All samples were cut to approximately 1 cm in length. The whole sample from each part of the tree was used for direct measurement of radioactivity.

Whole soil samples were collected within a radius of 50 cm around the trunk of each bush and tree. Samples from depths of 0–5, 5–15 and 15–30 cm were cleared of roots and bulked for each layer, and then were homogenized to provide a homogeneous sample. Samples of about 1 L were dried at 105°C until constant weight before analysis and used for measurement of radioactivity.

2.3. Measurement of radiocesium concentrations

Radiocesium concentrations were measured by means of gamma-ray spectrometry using a germanium semiconductor detector (GEM20P4-70 ORTEC, MCA7600 Seiko EG&G). The detector was calibrated against standard sources. Detection times were 10,000 s for plant samples and 2000 s for soil samples. Concentrations were corrected for physical decay to a baseline value on February 14, 2012, using values in Bq kg^–1 FW for plant samples and in Bq kg^–1 DW for soil samples.

2.4. Estimation of radiocesium amount and distribution in the orchards

The total radiocesium content of each plant part was calculated by multiplying the concentration by the fresh weight. That of the soil was estimated by multiplying the concentration by the bulk density, which we assumed to be 0.6 g cm⁻³. The distribution of radiocesium in each orchard was compared using these values.

3. RESULTS

3.1. Radiocesium concentrations in plants

In blueberry bushes, the radiocesium concentrations were highest in the old branches (Table 1). The concentrations in the old branches and leaves of the unpruned bush were more than double those in the pruned bush, but the concentrations in the trunk and rootstock were similar between the bushes. Concentrations in the roots were highest in the top 5 cm of the soil in both pruned and unpruned bushes, and decreased greatly in lower layers.

Table 1 Radiocesium concentration in fruit trees

	Blueberry		Satsuma mandarin
	Bush 1 (pruned^x)	Bush 2 (unpruned)	Tree 1 (few fruits^y)	Tree 2 (many fruits^y)
	Radiocesium concentration (^134Cs + ^137Cs, Bq kg^−1 FW)^z
Tree part
Fruits	–	–	51	28
New leaves	27	61	373	411
Old leaves	–	–	417	614
New branches (1 year old)	21	42	211	304
Old branches (≧ 2 years old)	161	355	134	146
Trunk	143	154	124	138
Rootstock	140	104	101	108
Roots (0–5 cm)	136	54	78	78
Roots (5–15 cm)	5	21	65	64
Roots (15–30 cm)	7	9	69	71

^xSome branches were pruned in January 2011 before the accident.

^yTotal fruit fresh weights were 0.62 kg (few fruits) and 1.56 kg (many fruits).

^zConcentrations were corrected for physical decay to a baseline value on February 14, 2012.

In Satsuma mandarin trees, the concentration was highest in the old leaves, followed by the new leaves and new branches. The concentrations in these organs were higher in the tree with many fruits than in the tree with few fruits. However, the concentration in the fruits was lower in the tree with many fruits. The concentrations were similar between trees in the old branches, trunks, rootstocks and roots.

3.2. Radiocesium concentrations in the soil

The radiocesium concentrations in all three orchards were highest in the surface 5 cm of the soil (Table 2). The concentrations at 0–5 cm were higher in the blueberry orchards than in the Satsuma mandarin orchard, but those at 15–30 cm were lower in the blueberry orchards. In the unmulched blueberry orchard, the concentration was higher in the surface 5 cm of the soil than in the same layer in the mulched blueberry orchard, because the wood chips used for mulching adsorbed some of the radiocesium, but concentrations were higher in the unmulched soil at 5–30 cm. The concentrations varied little horizontally at each site in the unmulched Satsuma mandarin orchard.

Table 2 Radiocesium concentration in orchard soils

	Blueberry orchard		Satsuma mandarin orchard
Mulch (wood chips)	Orchard 1–	Orchard 2+	Orchard 3–	–
Radiocesium concentration (^134Cs + ^137Cs: Bq kg^−1 DW)^z
Mulch		3140
Soil depth (cm)
0–5	1893	1204	831	769
5–15	61	229	109	137
15–30	20	48	79	93

^zConcentrations were corrected for physical decay to a baseline value on February 14, 2012.

3.3. Radiocesium distribution in the orchards

The total radiocesium per tree ranged from 407 to 1744 Bq (Table 3). In the pruned blueberry bush, 54% of the radiocesium was present in the old branches and trunk, versus 66% in the unpruned bush. These organs were contaminated by direct deposition after the accident. In the Satsuma mandarin trees, 50% of the total was present in the new and old leaves, versus 8–9% in the fruits. In the soils, the estimated total radiocesium ranged from 38,580 to 62,250 Bq m^–2 (Table 3). In the unmulched blueberry orchard, 91% of this was present in the surface 5 cm; in the mulched orchard, only 67% was present in the surface 5 cm, followed by 25% at 5–15 cm and 8% at 15–30 cm. In the unmulched Satsuma mandarin orchard, 58–65% of the radiocesium was present in the top 5 cm of the soil, versus 18–21% at 15–30 cm and 17–21% at 5–15 cm.

Table 3 Radiocesium distribution in orchards

	Orchard 1	Orchard 2	Orchard 3
	Blueberry		Satsuma mandarin
	Bush 1^w (pruned^x)	Bush 2^w (unpruned)	Tree 1^w (few fruits^y)	Tree 2^w (many fruits^y)
	Radiocesium (Bq tree^−1)
Tree part
Fruits	–	–	32	43
New leaves	40	69	172	150
Old leaves	–	–	32	95
New branches (1 year old)	26	19	27	28
Old branches (≧ 2 years old)	203	732	31	25
Trunk	334	413	46	74
Rootstock	360	471	21	30
Roots (0–5 cm)	23	19	6	5
Roots (5–15 cm)	3	10	11	13
Roots (15–30 cm)	2	10	30	26
Tree total	991	1744	407	490
	Radiocesium (Bq m^−2)^z
Mulch (wood chips)	–	+4440	–	–
Soil depth (cm)
0–5	56,790	36,120	24,930	23,070
5–15	3660	13,740	6540	8220
15–30	1800	4320	7110	8370
Soil total	62,250	54,180	38,580	39,660

^wTotal fresh weight of bushes and trees: 9.95 kg (bush 1), 12.81 kg (bush 2), 2.76 kg (tree 1), 3.80 kg (tree 2).

^xSome branches were pruned in January 2011 before the accident.

^yTotal fruit fresh weights were 0.62 kg (few fruits) and 1.56 kg (many fruits).

^zRadiocesium in the soil was estimated by multiplying the concentration by the bulk density, 0.6 g cm⁻³.

4. DISCUSSION

The distribution of radiocesium differed between the deciduous blueberry bushes and the evergreen Satsuma mandarin trees. Blueberry did not have a leaf but Satsuma mandarin had many leaves(old leaf) at the accident (Table 1), on which radiocesium was directly deposited at the time of the accident.

In new leaves and the new and old branches of blueberry, the concentrations in the unpruned bush were double those in the pruned bush. The unpruned bush had many small, old branches (≥ 2 years old) with a large surface area per unit weight, and a large proportion of the radiocesium appears to have been deposited on their surface. The deposited radiocesium might have been transported into the new leaves and branches from these small, old branches, leading to the greater accumulation per unpruned tree. Although the concentration was moderate, the total radiocesium accumulation per tree was also high in the trunk and rootstock because of the large volume of these organs.

The radiocesium concentration was much higher in new leaves of Satsuma mandarin than in those of blueberry. In a previous study, new leaves of tea Camellia sinensis (L.) Kuntze accumulated ¹³⁷Cs transported directly from the old branches during the growing season after the Chernobyl accident (Topcuoğlu et al. 1997). Thus, in the present study, the radiocesium might have been transported from the old leaves of Satsuma mandarin into the new branches, and from the new branches into the new leaves, because the concentration was highest in the old leaves. However, the concentration was lower in the fruits than in any other tissues. Similarly, the radiocesium concentration in the leaves of winter wheat (Triticum aestivum L. ‘Kinuazuma’) growing in Tokyo at the time of the FDNPP accident was more than 1000 times that in the panicles 2.5 months after the accident (Tanoi 2013). The translocation of radiocesium from new branches into fruits might therefore be lower than the translocation from new branches into new leaves.

The distribution of radiocesium in soil also differed among the three orchard fields. In the present study, we estimated that 91% of the radiocesium was retained in the surface 5 cm in the unmulched blueberry orchard (Table 3). However, 33% of the radiocesium was present in the 5–30 cm layer in the mulched blueberry orchard, while 35–42% of the radiocesim was present in that layer in the unmulched Satsuma mandarin orchard. These differences in the distribution of radiocesium in the soils may be related to the wood chip mulch or disturbance of the soil, because the solid–liquid distribution coefficient (the K_d value) of radiocesium depends on several soil parameters, including the organic matter content (Hormann and Fischer 2013). In the mulched blueberry orchard, wood chips have been added every year as mulch. In the Satsuma mandarin orchard, the trees had been transplanted only 2 years prior to the accident.

Thus, the distribution of radiocesium in orchards was influenced by the difference in tree species, and that in the soil was influenced by woodchip mulching.

ACKNOWLEDGMENTS

This study was supported by the research project “Development of practical technologies to implement new agricultural, forestry, and fishery policies” of Japan’s Ministry of Agriculture, Forestry and Fisheries (2011).