Journal of Environmental Radioactivity 161 (2016) 35e41

Contents lists available at ScienceDirect

Journal of Environmental Radioactivity
journal homepage: www.elsevier.com/locate/jenvrad

Spatial variation in the 137Cs inventory in soils in a mixed deciduous
forest in Fukushima, Japan
Momo Takada a, *, Toshihiro Yamada a, Teruhiko Takahara a, b, Toshinori Okuda aab

Graduate School of Integrated Arts and Sciences, Hiroshima University, 1-7-1 Kagamiyama, Higashi-Hiroshima, 739-8521, Japan
Faculty of Life and Environmental Science, Shimane University, 1060 Nishikawatsu, Matsue, Shimane, 690-8504, Japana r t i c l e i n f o

a b s t r a c t

Article history:
Received 11 March 2016
Received in revised form
22 April 2016
Accepted 25 April 2016
Available online 9 May 2016

The spatial variation of the radiocesium inventory in forest soil was studied c.a. 44 km northwest of the
Fukushima Daiichi Nuclear Power Plant, Japan. This study focuses on the effects of canopy interception
and downward transfer from the forest canopy to the forest ﬂoor via stemﬂow and throughfall. We
established a study plot (400 m2) in the canopy layer of a secondary mixed deciduous forest dominatedby Japanese oak (Quercus crispula) and Japanese ﬁr (Abies ﬁrma), in August and November 2014. Soil was
sampled from 0 to 5 cm depth and 137Cs was measured under the canopy using a 2-m grid and also at thetree trunk bases. We divided the study plot into the ﬁve different types of subplot according to the
canopy projection areas and the tree species for the analysis. The geometric mean and coefﬁcient ofvariation of the 137Cs inventory were 202 kBq m2 and 0.11 (0.52 in the arithmetic coefﬁcient of variation), respectively. Within the forest, the variation in the 137Cs inventory under trees was larger than incrown gap areas. The large spatial variation may be attributed to canopy interception of the initialdeposition and downward transfer of radiocesium via stemﬂow and throughfall.
© 2016 Elsevier Ltd. All rights reserved.

Keywords:
Spatial heterogeneity
Radiocesium
Stemﬂow
Throughfall
Canopy interception

1. Introduction
The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident inJapan in 2011 released various types of radionuclide into theenvironment (Endo et al., 2012; Yoshida and Kanda, 2012). Morethan 70% of the contaminated areas in Fukushima and the neighboring prefectures are forests, and a large amount of radiocesium(speciﬁcally, 137Cs) will remain in these areas for a considerable
time because of its long half-life of 30.17 years (Kuroda et al., 2013;
Loffredo et al., 2014). Studies into the dynamics of radiocesium
within a forest ecosystem will be important over the coming decades in applications such as forest management and radiation
protection when resuming the use of the forest resources(Hashimoto et al., 2013; Fujii et al., 2014).
According to previous studies, radiocesium released into theatmosphere after the FDNPP accident was initially deposited not
only on forest ﬂoors but also intercepted by tree bodies (treecrowns, branches, and stems; Hashimoto et al., 2013; Kato et al.,2015). At the time of the FDNPP accident in Fukushima Prefecture, canopy interception was expected to be approximately 70%* Corresponding author. Tel.: þ81 82 424 6513; fax: þ81 84 424 0758.
E-mail address: taka-momo@hiroshima-u.ac.jp (M. Takada).
http://dx.doi.org/10.1016/j.jenvrad.2016.04.033
0265-931X/© 2016 Elsevier Ltd. All rights reserved.

and 20% of the total deposition in evergreen and deciduous forests,
respectively (Kato et al., 2015). Canopy interception of radiocesium
derived from the Chernobyl nuclear power plant accident is reported to account for 70% of the total deposition, and the radiocesium remains on canopies for several years (Bunzl et al., 1989).
The radiocesium intercepted by the tree bodies is transferred onto
the forest ﬂoor via stemﬂow, throughfall by precipitation, anddirectly from litterfall (Rafferty et al., 2000). Most of the radiocesium on the tree canopies was transferred on to forest ﬂoors andsoils through these processes in the Fukushima region (Hisadome
et al., 2013; Endo et al., 2015; Kato et al., 2012). Radiocesium
within the litter on the forest ﬂoor is released into the forest soil
following litter decomposition (Rafferty et al., 1997, 2000), and thedecomposition process in the Fukushima region is completed
within several years (Ono et al., 2013). As a result, most of theradiocesium present within the forest several years after the FDNPP
accident is now present in soil (Hashimoto et al., 2013), and is expected to remain in the soil for a long time as radiocesium adsorbed
on soil particles has a low mobility (Schimmack et al., 1994;
Matsunaga et al., 2013). Therefore, studies on the radiocesium dynamics in forest soils are important when considering forest
management and radiation protection for resuming the use of theforest resources.

36

M. Takada et al. / Journal of Environmental Radioactivity 161 (2016) 35e41

In a forest ecosystem, the spatial heterogeneity of the soilradiocesium inventory represents a major difﬁculty for studying
the dynamics. This limitation has been discussed for some years
and various studies have been conducted that aim toward a moreaccurate and precise evaluation (Guillitte et al., 1990; Khomutinin
et al., 2004; Korobova and Romanov, 2009, 2011). However, to
our knowledge, no studies have examined the characteristics of thespatial heterogeneity by comparing with other ecosystems such as
open areas, grasslands and croplands. As described above, spatial
distribution of radiocesium in the forest soils of Fukushima Prefecture three years after the FDNPP accident is expected to beaffected by canopy interception of initial deposition and translocation from the canopy to the forest ﬂoor via throughfall, stemﬂow and litterfall. Consequently, the spatial distribution in forest
soil may differ with those in other ecosystems.
Therefore, in the present study, we conducted ﬁeld surveys in amixed deciduous forest in Fukushima to evaluate the spatial variation of the soil radiocesium inventory. To clarify effects of canopy
interception, precipitation (throughfall and stemﬂow) and litterfall,
we divided the study plot into ﬁve subplot types according tocanopy projection areas and the tree species, and then examined
the spatial variation of radiocesium inventory in each of these
subplot types.
2. Materials and methods
2.1. Study site
The study site is a secondary mixed deciduous forest, located in
Soma in the northern part of Japan, 44 km northwest of the FDNPP(37450 N, 140 470 E; 570 m asl, Fig. 1). The study site was positioned
in a ﬂat area on a mountainside, and the surrounding environment
was same as the study plot. At the time of the FDNPP accident, theradioactive plume was estimated to arrive at the present study sitefrom the southeast side (Katata et al., 2012), and the ratio of wet
deposition to the total 137Cs deposition was estimated at 0.6e0.8(Terada et al., 2012). The total atmospheric deposition of 137Cs after
the accident was 100e300 kBq m2, based on the third airborne
monitoring survey by the Japanese Government (MEXT, 2011). The
mean annual air temperature and precipitation measured at themeteorological station located in Iitate village, 8.6 km southwest of
our study site were 10.0  C and 1361.6 mm, respectively (Japan

Meteorological Agency database from http://www.jma.go.jp/jma/index.html). The study site is a typical naturally regenerating forest (a secondary forest), which is dominated by deciduous broadleaf trees (e.g., Japanese oak, Quercus crispula) with sporadic
evergreen coniferous trees (e.g., Japanese ﬁr, Abies ﬁrma). The soil
type at the study site is brown forest soil (Kanno et al., 2008).
A 20  20 m plot was established in the study site (tilted at lessthan 2 towards the northeast). No herbaceous plants were present
in the study plot; only litter constituted ground cover. The thicknessof the litter layer was approximately 2 cm and the layer was
generally homogeneous with no bare areas. Ninety-one trees were
present in the plot with a stand density of 2275 ha1 (only trees
with a diameter at breast height (DBH) of 5 cm were counted).
The largest tree in the plot was a Japanese ﬁr (Abies ﬁrma) with a
DBH of 46 cm. Sixteen species were recorded; Japanese oak(Quercus crispula) and Japanese ﬁr (Abies ﬁrma) were dominant,accounting for 35% and 25% of total tree number, respectively.
Other species were equal to or less than 5% of the total tree number.
The crown projection diagram of the study plot given in Fig. 2a wasdrawn on July 28, 2015.
To compare the effects of canopy interception, wash-out with
precipitation (throughfall, and stemﬂow) on soil radiocesium inventory and its spatial variation, we divided the study plot into ﬁve
subplot types as shown in Fig. 2b: tree trunk base areas of evergreen coniferous trees (hereafter evergreen tree base areas, EB),
tree trunk base areas of deciduous broadleaf trees (hereafter deciduous tree base areas, DB), areas under evergreen coniferous
crowns excluding tree trunk base areas (hereafter, under evergreen
crown areas, EC), areas under deciduous broadleaf crowns
excluding tree trunk base areas (hereafter under deciduous crownareas, DC), and crown gap areas (CG). The EB and DB subplots in thepresent study included an area of less than 0.5 m radius from eachtree, which was expected to be strongly affected by stemﬂow(Matsubayashi et al., 1994; Nakajima and Kaneko, 2012). The EC andDC subplots were determined according to the canopy projection
diagram (Fig. 2a). To clarify effects of stemﬂow and throughfall
separately, EC and DC did not include the areas around the tree
trunk base. As shown in Fig. 2a, some areas of EB and EC wereoverlapped with those of DB and DC. In that case, these areas wereconsidered as EB and EC because the canopy interception rates andtranslocation amounts of radiocesium with stemﬂow, throughfall
and litterfall of evergreen coniferous trees were larger than those of

Fig. 1. Location of the study site and the Fukushima Daiichi Nuclear Power Plant.

M. Takada et al. / Journal of Environmental Radioactivity 161 (2016) 35e41

37

Fig. 2. Locations of soil sampling and crown projection diagram in the study plot (a), and the ﬁve subplot types for comparison of the soil 137Cs inventory (b): evergreen tree baseareas (EB), deciduous tree base areas (DB), areas under evergreen coniferous crowns excluding tree trunk base areas (EC), areas under deciduous broadleaf crowns excluding tree
trunk base areas (DC), and crown gap areas (CG).

deciduous broadleaf trees (Kato et al., 2015; Endo et al., 2015). The
CG was considered areas other than under trees including trees
outside the plot, and the areas was 0.9e32 m2. As the crown projection diagram at the time of the FDNPP accident is not expected toshow large differences with this crown projection diagram, weused it for the analysis. The DBH growth of the trees in the present
plot is estimated to be only a few centimeters and the consequent
increase of the canopy projection areas is estimated to be very small
after the accident (Hashizume, 1989; Shimano, 1997).
2.2. Collecting samples and radiocesium measurements
We decided to focus on the examination of spatial variation at
the soil surface (0e5 cm soil depth excluding the litter layer)
because we consider this layer to be more important than litter anddeeper soil for the following reasons. Currently, most radiocesium
within forest ecosystems in the Fukushima region is present within
the upper 5 cm of soil (Takahashi et al., 2015). In addition, we foundthat the 137Cs inventory at the soil surface (0e5 cm in depth) at thepresent study site accounted for more than 72% of that from litter
layer to 10 cm of soil as on 31 July 2014 (Table 1). In addition, we
have found that spatial variation of the 137Cs inventory in the litter

Table 1
Depth distributions of 137Cs inventory as on 31 July 2014.

Geometric mean (kBq/m2)
Range
CVg (CVa)an

Litter layer

Soil (0e5 cm)

Soil (5e10 cm)

13
5e22
0.47 (0.04)816443e3760.50 (0.03)84520e2301.03 (0.07)
8a
CVg indicates ratio of the standard deviation of log-transformed 137Cs inventory
to the geometric mean. CVa indicates ratio of the standard deviation of 137Cs inventory to the arithmetic mean.

layer was smaller than that in soil in mixed deciduous forests inFukushima (Table 1; M. Takada, unpublished data). Finally, radiocesium in soil shows low mobility (Schimmack et al., 1994;
Matsunaga et al., 2013), while the mobility in the litter layer isassumed to be relatively high because of the movement of litteraround the forest ﬂoor under processes such as wind (Yamamoto
and Bunzl, 1993). Accordingly, spatial variation of radiocesium inlitter and soil should be examined separately.
We collected surface soil (0e5 cm depth) with a 100 mL-soil
sampler (20 cm2 and 5 cm depth) at 121 points in the 2-m grid plot
on 31 July 2014 (Fig. 2a). In addition to the sample collection on thegrids, we also collected soil samples in tree trunk base areas (EB andDB) of all identiﬁed trees in the plot from 129 points on 7
November 2014 (Fig. 2a). The soil samples were collected from thenorth- and south-facing sides of each tree within a distance of
10 cm from the tree trunk, and the radicoesium activities were
measured separately. Some sampling points from the north andsouth sides of trees overlapped when trees were closely positioned.
As some Japanese oak trees in the plot had been coppiced, soil
samples from these coppices were collected from the north andsouth sides of the stocks.
The collected soil samples were dried at 100  C for 48 h. The
samples were ﬁlled into 100-mL plastic polypropylene containers(U-8) and analyzed for 137Cs and 134Cs using a low-background Ge
spectrometer (GEM-110225, Seiko EG&G). The measurement times
were between 600 and 10,000 s, depending on the radioactivity ofthe samples. The associated errors were composed of 5% from thedetection efﬁciency and 1e5% from peak counting error. Activity of137
Cs and 134Cs in all samples showed a similar pattern, and theratio of 137Cse134Cs activity was almost constant (1.0e1.1 inMarch 2011). In the present study, we only employed 137Cs data forthe determination of the spatial distribution of radionuclides. The
134
Cs data were not used because the relatively short half-life (2.06

38

M. Takada et al. / Journal of Environmental Radioactivity 161 (2016) 35e41

Table 2
Descriptive statistics for soil 137Cs inventory (kBq m2) in the entire study plot and the ﬁve subplot types.

Total
Evergreen tree base areas (EB)
Deciduous tree base areas (DB)Evergreen crown areas (EC)
Deciduous crown areas (DC)
Crown gap areas (CG)

Geometric mean (kBq m2)aRange

CVg (CVa)bN

Area (m2)

202
204ab
248a
189ab
170b
156b

22e69722e62457e69757e43924e57456e4380.11 (0.52)
0.12 (0.51)
0.09 (0.47)
0.11 (0.48)
0.11 (0.53)
0.09 (0.46)

250
57
85
27
49
32

400
18
30
117
147
89

a

Different letters indicate statistically signiﬁcant differences between these areas (ANOVA test with multiple comparisons).
CVg indicates ratio of the standard deviation of log-transformed 137Cs inventory to the geometric mean. CVa indicates ratio of the standard deviation of 137Cs inventory to
the arithmetic mean.
b

years) is unsuitable for this analysis more than three years after the
FDNPP accident. Activity of 137Cs was corrected for radioactive
decay to the ﬁrst sampling day of this survey: 31 July 2014. In thepresent study, we discussed the spatial variation of soil 137Cs byinventory (Bq m2). The radiocesium activity of whole the soil
samples collected with the 100 mL-soil sampler was measured andthe inventory was calculated by dividing the 137Cs activity by thearea of the sampler (20 cm2).
2.3. Statistical analysis
Application of the ShapiroeWilk normality test showed thatsome datasets in the present study were logarithmically normal
distributions (P > 0.05), although the remaining datasets were
marginally not (P < 0.05). Many studies after the Chernobyl

accident showed that approximation of soil 137Cs inventory withlogarithmically normal distribution is suitable for statistical analysis (Khomutinin et al., 2004; Shcheglov et al., 2001). Therefore, all
the datasets in the present study were log-transformed and aparametric test was used for statistical analysis.
Geometric means of the 137Cs inventory (kBq m2), the coefﬁcient of variation (ratio of the standard deviation of logtransformed 137Cs inventory to the geometric mean, hereafterCVg), and the arithmetic coefﬁcient of variation for comparisonswith those in previous research (ratio of the standard deviation to
the arithmetic mean, hereafter CVa) were calculated. The one-way
analysis of variance (ANOVA) test with multiple comparisons(Tukey's test) was used to compare 137Cs inventory between theﬁve subplot types (EB, DB, EC, DC and CG).

Fig. 3. Spatial distribution of soil 137Cs inventory (kBq m2) in the study plot. Activity of 137Cs was corrected for radioactive decay to the ﬁrst sampling day of this survey: 31 July2014.

M. Takada et al. / Journal of Environmental Radioactivity 161 (2016) 35e41

3. Results
The geometric mean of the 137Cs inventory in the study plot andthe CVg were 202 kBq m2 and 0.11 (0.52 in CVa), respectively. The
range of the inventory was from 22 to 697 kBq m2, showing a
difference of approximately 30 times and large spatial heterogeneity in the 20  20 m area (Table 2, Fig. 3). The 137Cs inventory in
the DB subplots showed the highest value (248 kBq m2 in geometric mean) and lowest was in the CG areas (156 kBq m2). The
CVg (CVa) values in the EB and DC subplots were relatively large at
0.12 (0.51) and 0.11 (0.53), respectively; however, the values for CG
were small at 0.09 (0.46). The 137Cs inventories showed statisticallysigniﬁcant differences between the ﬁve subplot types (one-way
ANOVA; F4, 239 ¼ 5.7, P < 0.001). The 137Cs inventory in the DB
subplot was higher than those in the DC and CG subplots.
Fig. 4 shows frequency distributions of the 137Cs inventories inthe study plot and the ﬁve subplots. The magnitudes of all thehistograms showed signiﬁcant spreads in values and right-handed
asymmetry (skewness, 0.4e1.3). In CG, over 56% of the inventory39

was in the 100e200 kBq m2. In contrast, the frequent inventory
varied in the under tree areas (EB, DB, EC and DC), and a higher
number of the lowest and highest inventory samples (0e100
and > 200 kBq m2) were observed under evergreen trees (EB andEC) than those under deciduous trees (DB and DC) and CG.

4. Discussion
The 137Cs inventory in the present study plot showed large
spatial heterogeneity and had a CVa value of 0.52. The CVa ofChernobyl-derived radiocesium inventory in a pine and oak forest
soil was from 0.31 to 0.58, showing large spatial variation that issimilar to our results (0e4 cm depth; Korobova and Romanov,2009). These CVa values in forests were larger than those estimated for open areas (excluding forests) in Fukushima and theneighboring prefectures soon after the FDNPP accident (0.36 on
average; Onda et al., 2015; Saito et al., 2015) collected soil samples
in areas under homogeneous conditions (e.g. ﬂat, no obstacles, novegetation coverage).

Fig. 4. Frequency distributions of soil 137Cs inventory (kBq m2) and the skewness in the entire study plot and the ﬁve subplot types. The ﬁve subplot types correspond to those
identiﬁed in Fig. 2b.

40

M. Takada et al. / Journal of Environmental Radioactivity 161 (2016) 35e41

The large spatial heterogeneity in the present study plot isassumed to be strongly affected by initial transprocesses after theaccident (from initial deposition to wash-out with precipitation).
Even in the present study plot (20  20 m), the spatial variationdiffered among the ﬁve subplots; the spatial variation in the CG
subplots was smaller than those under trees. The spatial variationin CG is thought to mainly reﬂect initial deposition and migration
with litter decomposition. In other subplots that were located under trees (EB, DB, EC and DC), more samples of lower and higher
inventory were observed. This result indicates that the presence oftrees increases the spatial variation of soil radiocesium inventory(increases heterogeneity) as of three years after the FDNPP accident. This larger spatial variation in EB, DB, EC and DC may be due
to several processes including canopy interception of initial deposition and wash-out of radiocesium on tree bodies with precipitation. Kato et al. (2015) reported that canopy interception was 70%
and 20% of the total deposition in evergreen and deciduous forests,
respectively, at the time of the accident in Fukushima Prefecture.
The initial deposition under trees was also expected to be smaller
than that in crown gap areas in our study site, leading to some soil
samples with low radocesium inventory. This also supports theﬁnding that more of the lower inventory samples were observed
under evergreen trees (EB and EC) than under deciduous trees (DBand DC). In addition, radiocesium intercepted by trees is transferredon and into forest ﬂoors and soils by precipitation through theprocesses of stemﬂow and throughfall (Endo et al., 2015; Kato et al.,2015). Accordingly, some high inventory samples may be observed
where stemﬂow and throughfall were likely to have seeped.
The geometric mean of the inventory around tree areas washigher than that under tree crowns in some distance from the trees
although the spatial variation under trees was very large.
Yamamoto and Bunzl (1993) reported the similar spatial gradientsin a German forest after the Chernobyl accident and attributed theirﬁndings to the process of radiocesium on trees washing out andseeping around tree trunk bases. The spatial gradients in the present study site differed between evergreen coniferous trees anddeciduous broadleaf trees; the inventory gradually decreased fromthe tree trunk bases of evergreen coniferous trees although there
were no statistically signiﬁcant differences (EB  EC  CG). In
contrast for the deciduous broadleaf trees, the inventory in DB washigh and suddenly decreased under DC, showing almost the same
inventory values to CG (DB > DC  CG). A similar difference wasobserved by Yamamoto and Bunzl (1993) between beech trees(deciduous) and spruce trees (evergreen). This may be attributed tothe phenological difference between evergreen and deciduoustrees (only evergreen trees were in leaf at the time of the respective
incidents).

5. Conclusion
Our result showed that the spatial variation of soil radiocesium
inventory under trees (around tree trunk bases and under tree
crowns) was larger than those in crown gap areas at our study site.
The soils under the trees were thought to be strongly affected bysome processes such as canopy interception and translocation by
precipitation (throughfall and stemﬂow) soon after the accident,
leading to areas of low and high radiocesium inventory values in
the forest soil.
In the emergency soil sampling protocol, ﬁve soil samples
within a 3  3 m area are recommended as the minimum number
for reducing measurement uncertainty (Onda et al., 2015). Our
result implies that more than ﬁve soil samples may be required inforest ecosystems to enhance the accurate evaluation of the radiocesium inventory.

Acknowledgments
We sincerely thank the landowners for permitting us to conduct
the ﬁeld survey. We also thank Prof. Y. Onda at the Center for
Research in Isotopes and Environmental Dynamics, University ofTsukuba for the many useful suggestions, and Profs. K. Shizuma and
S. Endo at the Graduate School of Engineering, Hiroshima University for their support for sample measurements. This study was
supported by the Japan Society for the Promotion of Science
KAKENHI Grant Number 15J03548, and the Hiroshima University
Phoenix Leader Education Program (Hiroshima Initiative) for the“Renaissance from Radiation Disaster”, funded by the Ministry of
Education, Culture, Sports, Science and Technology.
References
Bunzl, K., Schimmack, W., Kreutzer, K., Schierl, R., 1989. Interception and retentionof Chernobyl-derived 134Cs, 137Cs and 106Ru in a spruce stand. Sci. Total Environ.
78, 77e87.
Endo, I., Ohte, N., Iseda, K., Tanoi, K., Hirose, A., Kobayashi, N., Murakami, M.,
Tokuchi, N., Ohashi, M., 2015. Estimation of radioactive 137-cesium transportation by litterfall, stemﬂow and throughfall in the forests of Fukushima.
J. Environ. Radioact. 149, 176e185.
Endo, S., Kimura, S., Takatsuji, T., Nanasawa, K., Imanaka, T., Shizuma, K., 2012.
Measurement of soil contamination by radionuclides due to the Fukushima Daiichi nuclear power plant accident and associated estimated cumulative external
dose estimation. J. Environ. Radioact. 111, 18e27.
Fujii, K., Ikeda, S., Akama, A., Komatsu, M., Takahashi, M., Kaneko, S., 2014. Verticalmigration of radiocesium and clay mineral composition in ﬁve forest soils
contaminated by the Fukushima nuclear accident. Soil Sci. Plant Nutr. 60,751e764.
Guillitte, O., Andolina, J., Koziol, M., Debauche, A., 1990. Plant-cover inﬂuence on thespatial distribution of radiocaesium deposits in forest ecosystems. In:
Desmet, G., Nassimbeni, P., Belli, M. (Eds.), Transfer of Radionuclides in Naturaland Semi-natural Environments. Elsevier Applied Science, London, UK,
pp. 441e449.
Hashimoto, S., Matsuura, T., Nanko, K., Linkov, I., Shaw, G., Kaneko, S., 2013. Predicted spatio-temporal dynamics of radiocesium deposited onto forests
following the Fukushima nuclear accident. Sci. Rep. 3, 2564.
Hashizume, H., 1989. On the growth and timber price of some useful broad-leaved
trees. Hardwood Res. 5, 13e20.
Hisadome, K., Onda, Y., Kawamori, A., Kato, H., 2013. Migration of radiocaesium withlitterfall in hardwood-Japanese red pine mixed forest and Sugi plantation. J. Jpn.
For. Soc. 95, 267e274.
Kanno, H., Hirai, H., Takahashi, T., Nanzyo, M., 2008. Soil regions map of Japan basedon a reclassiﬁcation of the 1:1 million soil map of Japan (1990) according to theuniﬁed soil classiﬁcation system of Japan: 2nd approximation (2002). Pedologist 52, 129e133.
Katata, G., Ota, M., Terada, H., Chino, M., Nagai, H., 2012. Atmospheric discharge anddispersion of radionuclides during the Fukushima Dai-ichi nuclear power plant
accident. Part I: source term estimation and local-scale atmospheric dispersion
in early phase of the accident. J. Environ. Radioact. 109, 103e113.
Kato, H., Onda, Y., Gomi, T., 2012. Interception of the Fukushima reactor accidentderived 137Cs, 134Cs and 131I by coniferous forest canopies. Geophys. Res. Lett.
39, L20403.
Kato, H., Onda, Y., Hisadome, K., Loffredo, N., Kawamori, A., 2015. Temporal changes
in radiocesium deposition in various forest stands following the Fukushima
Dai-ichi nuclear power plant accident. J. Environ. Radioact. http://dx.doi.org/
10.1016/j.jenvrad.2015.04.016.
Khomutinin, Y.V., Kashparov, V.A., Zhebrovska, K.I., 2004. Sampling Optimisation
when Radioecological Monitoring. Ukraine Institute for Agricultural Radiology,
Kiev, Ukraine, ISBN 966-646-034-3.
Korobova, E., Romanov, S., 2009. A Chernobyl 137Cs contamination study as an
example for the spatial structure of geochemical ﬁelds and modeling of thegeochemical ﬁeld structure. Chemom. Intell. Lab. Syst. 99, 1e8.
Korobova, E., Romanov, S., 2011. Experience of mapping spatial structure of Cs-137
in natural landscape and patterns of its distribution in soil top sequence.
J. Geochem. Explor. 109, 139e145.
Kuroda, K., Kagawa, A., Tonosaki, M., 2013. Radiocesium concentrations in the bark,sapwood and hardwood of three species collected at Fukushima forests half ayear after the Fukushima Dai-ichi nuclear accident. J. Environ. Radioact. 122,
37e42.
Loffredo, N., Onda, Y., Kawamori, A., Kato, H., 2014. Modeling of leachable 137Cs inthroughfall and stemﬂow for Japanese forest canopies after Fukushima Daiichi
nuclear power plant accident. Sci. Total Environ. 493, 701e707.
Matsubayashi, U., Takagi, F., Velasquez, G.T., Sasuga, H., Sumi, T., 1994. On the effects
of vegetation on inﬁltration and the ﬂow path in mountainous slopes. Proc.
Hydraul. Eng. 38, 185e190.
Matsunaga, T., Koarashi, J., Andoh, A.M., Nagao, S., Sato, T., Nagai, H., 2013. Comparison of the vertical distributions of Fukushima nuclear accident radiocesium

M. Takada et al. / Journal of Environmental Radioactivity 161 (2016) 35e41
in soil before and after the ﬁrst rainy season, with physicochemical andmineralogical interpretations. Sci. Total Environ. 447, 301e314.
MEXT, 2011. Results of the Third Airborne Monitoring Survey by MEXT. http://
radioactivity.nsr.go.jp/en/list/307/list-1.html (accessed 25.10.01).
Nakajima, G., Kaneko, N., 2012. The effect of Japanese cypress plantation on soil
biochemical characteristics converted from deciduous forests. J. Jpn. For. Soc.
94, 112e119.
Onda, Y., Kato, H., Hoshi, M., Takahashi, Y., Nguyen, M.L., 2015. Soil sampling andanalytical strategies for mapping fallout in nuclear emergencies based on the
Fukushima Dai-ichi nuclear power plant accident. J. Environ. Radioact. 139,
300e307.
Ono, K., Hiradate, S., Morita, S., Hirai, K., 2013. Fate of organic carbon duringdecomposition of different litter types in Japan. Biogeochemistry 11, 7e21.
Rafferty, B., Dawsoni, D., Kliashtorjn, A., 1997. Decomposition in two pine forests:
the mobilisation of 137Cs and K from forest litter. Soil Biol. Biochem. 29,
1673e1681.
Rafferty, B., Brennan, M., Dawson, D., Dowding, D., 2000. Mechanisms of 137Cs
migration in coniferous forest soils. J. Environ. Radioact. 48, 131e143.
Saito, K., Tanihata, I., Fujiwara, M., Saito, T., Shimoura, S., Otsuka, T., Onda, Y.,
Hoshi, M., Ikeuchi, Y., Takahashi, F., Kinouchi, N., Saegusa, J., Seki, A.,
Takemiya, H., Shibata, T., 2015. Detailed deposition density maps constructed bylarge-scale soil sampling for gamma-ray emitting radioactive nuclides from the41

Fukushima Dai-ichi nuclear power plant accident. J. Environ. Radioact. 139,308e319.
Schimmack, W., Bunzl, K., Dietl, F., Klotz, D., 1994. Inﬁltration of radionuclides withlow mobility (137Cs and 60Co) into a forest soil: effect of the irrigation intensity.
J. Environ. Radioact. 24, 53e63.
Shcheglov, A.I., Tsvetnova, O.B., Klyashtorin, A.I., 2001. Biogeochemical Migration ofTechnogenic Radionuclides in Forest Ecosystems. Nauka, Moscow.
Shimano, K., 1997. Analysis of relationship between DBH and crown projection area
using a new model. J. For. Res. 2, 237e242.
Takahashi, J., Tamura, K., Suda, T., Matsumura, R., Onda, Y., 2015. Vertical distribution and temporal changes of 137Cs in soil proﬁles under various land uses after
the Fukushima Dai-ichi nuclear power plant accident. J. Environ. Radioact. 139,351e361.
Terada, H., Katata, G., Chino, M., Nagi, H., 2012. Atmospheric discharge anddispersion of radionuclides during the Fukushima Dai-ichi nuclear power plant
accident. Part II: veriﬁcation of the source term and analysis of regional-scaleatmospheric dispersion. J. Environ. Radioact. 112, 141e154.
Yamamoto, M., Bunzl, K., 1993. Environmental effect studies on a forest ecosystem
in Germany transport of chernobyl-derived radiocesium in a forest. Radioisotopes 42, 180e188.
Yoshida, N., Kanda, J., 2012. Tracking the Fukushima radionuclides. Science 336,
1115e1116.