changes in the radioactive cesium concentrations of grasslands during the first year after the...

ORIGINAL ARTICLE

Changes in the radioactive cesium concentrations ofgrasslands during the first year after the FukushimaDaiichi Nuclear Power Plant accident in east JapanYoshito Yamamoto1, Takeshi Shibuya1, Kiyoshi Hirano1, Kazumasa Shindo1, Hideto Mashiyama2,Tamotsu Fujisawa3, Michinaga Nakamura3, Yoshiro Tozawa3, Hirotake Miyaji1, Seiji Nakao1 andYasuko Togamura1

1 National Agriculture and Food Research Organization (NARO) Institute of Livestock and Grassland Science, Nasushiobara, Tochigi,

Japan

2 Tochigi Prefectural Livestock and Dairy Experiment Center, Nasushiobara, Tochigi, Japan

3 National Livestock Breeding Center, Nishigo, Fukushima, Japan

Keywords

Grassland; monitoring; nuclear power plant

accident; radioactive cesium; soil.

Correspondence

Yoshito Yamamoto, NARO Institute of

Livestock and Grassland Science, 768

Senbonmatsu, Nasushiobara, Tochigi

329-2793, Japan.

Email: [email protected]

Received 27 December 2012;

accepted 4 January 2014.

doi: 10.1111/grs.12044

Abstract

Radioactive cesium (Cs) concentration of vegetation and soil was monitored in

grasslands in seven farms located at a distance ranging from 90 to 180 km

from the Fukushima nuclear power plant during seven months following the

reactor meltdown in March 2011. The monitored sites included six sown

meadows used to produce hay or silage, three sown pastures and one native

pasture used for cattle grazing. The radioactive Cs concentrations of the soil

ranged from 264–1593 Bq kg�1 dry matter (DM). The radioactive Cs concen-

trations in vegetation (aboveground parts of dominant grasses) were high with

values ranging from 639–19 823 Bq kg�1 DM for the meadows, 949–7161 Bq kg�1 DM for the sown pastures and 5088–358 549 Bq kg�1 DM for

the native pasture. Although the radioactive Cs concentrations tended to

decrease over time in most grasslands, there was no clear decreasing trend for

grassland soils low in exchangeable potassium concentration and clay content.

The transfer of radioactive Cs from soil to herbage tended to be lower in soils

with higher exchangeable potassium concentration and clay content. Detailed

measurements in one meadow showed highest radioactive Cs concentration in

surface litter, followed by standing dead and live plant material. Approximately,

71, 21 and 7% of radioactive Cs in the meadows were present in the soil, litter

and standing dead material, respectively. Further regular monitoring of radio-

active Cs concentration in grasslands in the affected areas surrounding the

nuclear power plant is required to amend the existing guidelines regarding live-

stock feeding.

Introduction

The Fukushima Daiichi Nuclear Power Plant located on

the east coast of Japan was struck by an earthquake and

tsunami on 11 March 2011. Grasslands in the region sur-

rounding the power plant became heavily polluted with

radioactive materials, including cesium (Cs), due to a

meltdown in the reactors at the power plant. This pollu-

tion is predicted to remain in the land for many years. In

April 2011, the Japanese government outlined a tempo-

rary radioactive Cs guideline level of 300 Bq kg�1 fresh

weight (FW) for forage used as feed for dairy and beef

cattle (Ministry of Agriculture, Forestry and Fisheries

[MAFF] 2012). In March 2012, the limit was decreased to

100 Bq kg�1 FW for cattle feed (MAFF 2012).

Radioactive fallout following the Chernobyl accident in

1986 affected a large area including a number of Euro-

pean countries (De Cort et al. 1998). Many studies moni-

tored radioactive Cs in grasslands after the accident

(Prister et al. 1993; Ros�en 1996; Ros�en et al. 1998;

© 2014 Japanese Society of Grassland Science, Grassland Science, 60, 69–75 69

Japanese Society of Grassland Science ISSN1744-6961

Japanese Society of Grassland Science

Papastefanou et al. 2005). In Greece, the 137Cs concentra-

tion of grasses after the accident showed a decreasing

trend over time, with an estimated half-life of 40 months

(Papastefanou et al. 2005). Thus, there is a need for con-

tinued regular monitoring of radioactive Cs concentra-

tions in herbage growing in areas affected by the

Fukushima nuclear power plant accident to assess the

suitability of herbage as feed.

In the present study, we monitored changes in radioac-

tive Cs concentration in grasslands in farms located in

the Kanto and Tohoku regions of eastern Japan during

seven months following the reactor meltdown in March

2011. The aim of this study was to provide information

for improving the existing governmental guidelines

regarding the management of forage used as feed for ani-

mals.

Materials and methods

Study sites and sampling of grasslands

We monitored the radioactive Cs concentrations of grass-

lands in seven farms (farms A–G). All the farms are

located in the Kanto or Tohoku region of east Japan, at

an altitude of 19–1053 m and a distance of 90–180 km

from the Fukushima Daiichi Nuclear Power Plant. We

sampled six sown meadows (i.e. grasslands harvested by

machine to produce hay or silage) on farms A, B, C, F

and G (two meadows from farm G), and three sown and

one native pastures (i.e. grasslands enclosed by a fence for

grazing) on farms D, E and F and farm F, respectively.

The sown meadows and pastures were dominated by or-

chardgrass (Dactylis glomerata L.), Italian ryegrass (Lolium

multiflorum Lam.) or timothy (Phleum pratense L.) or

combinations of the two. The native pasture was domi-

nated by Japanese lawn grass (Zoysia japonica Steud.). All

sown grasslands had been established for more than

three years before the nuclear accident. Fertilizer and graz-

ing management of the farms are shown in Tables 1 and

2. Most of the farms stopped or minimized fertilizer appli-

cation to grasslands after the accident, because herbage in

the grasslands could not be used for animal feed due to

radioactive Cs contamination levels exceeding the provi-

sional guidelines. The grazing periods on farms D and E

were later and shorter compared to those of a normal

year. Grazing on the native pasture was suspended during

5–7 days preceding individual samplings (see next section)

to ensure accumulation of herbage to be sampled.

We carried out soil sampling once (May or July) for

individual grasslands and herbage sampling two or three

times (before harvests in May–October) for individual

meadows and four to nine times (from May or July to

October) for individual pastures. On each sampling occa-

sion, soil samples (including surface litter) were collected

to a depth of 15 cm using a 5-cm-diameter core sampler

(HS-25; Fujiwara Ltd., Tokyo, Japan) at 39 locations in

three areas (13 locations in each area) in individual grass-

lands. Soil samples were also taken from a bare ground

area (i.e. no vegetation) on farm F. Herbage samples were

taken by manually cutting three areas (1.5–3 m2 each) to

a 10-cm height above the ground level in the meadows

and sown pastures, and by cutting three strips

(10 9 0.3 m each) to a 2-cm (April and May) or 3-cm

Table 1 Fertilizer management of seven farms in 2011

Farm Grassland

Fertilization

time (month

or date) Fertilizer type and rate

A Meadow May Compound (40 kg N, 40 kg P2O5,

27 kg K2O ha�1)

B Meadow March Urea (92 kg N ha�1)

C Meadow 15 May Compound (19 kg N, 24 kg P2O5,

19 kg K2O ha�1)

9 June Compound (19 kg N, 24 kg P2O5,

19 kg K2O ha�1)

D Pasture May Slaked lime (1000 kg ha�1)

E Pasture March Compound (28 kg N, 28 kg P2O5,

28 kg K2O ha�1)

F Meadow 3 June Compound (50 kg N, 50 kg P2O5,

50 kg K2O ha�1)

15 July Compound (50 kg N, 50 kg P2O5,

50 kg K2O ha�1)

8 September Compound (50 kg N, 50 kg P2O5,

50 kg K2O ha�1)

Pasture None None

G Meadow None None

Table 2 Grazing management of three farms in 2011

Farm

(pasture)

Area

(ha) Animal

Number of

animals

(head day�1) Grazing period

D 3.7 Dairy heifer 90 26–30 May

68 12–19 September

E 8.7 Breeding

beef cow

15 1–31 October

F (Dactylis

glomerata

pasture)

0.3 Breeding

beef cow

21 10–11 May,

12 June, 11 and

28 July, 12 and

14 September

F (Zoysia

japonica

pasture)

6.5 Breeding

beef cow

13 25 April–18

October†

†Grazing on the Zoysia pasture was suspended during 5–7 days pre-

ceding individual samplings to ensure accumulation of herbage to be

sampled.

© 2014 Japanese Society of Grassland Science, Grassland Science, 60, 69–7570

Radioactive cesium in grasslands Y. Yamamoto et al.

(June–October) height in the native pasture using a lawn

mower (Baroness LMB300; Kyoeisha, Aichi, Japan). At

the July sampling (at the second harvest) of the farm F

meadow, we further sampled herbage in the 0–10 cm

layer above the ground and litter on the ground surface,

and sorted the herbage samples into live plant material

and standing dead material. The litter samples were not

washed with water.

Measurements of radioactive Cs

The soil samples were dried indoors and passed through

a 2-mm mesh, after being bulked to make batches of

three areas in individual grasslands. The herbage and lit-

ter samples were dried at 70°C for 3 days, and cut to 1–2 cm in length. All samples were then subjected to a ger-

manium semiconductor detector (GC3020, GC3520 and

GR3522–7500SL; Canberra, CT, USA) for analysis of134Cs and 137Cs. After measurement of radioactive Cs, all

samples were dried at 105°C for 24 h for determination

of dry matter (DM). Concentrations of Cs were corrected

to predict those at the sampling time based on the decay

constant.

The transfer factor (TF) of radioactive Cs from soil to

herbage was calculated as TF = (concentration of radioac-

tive Cs in herbage [Bq kg�1 DM]) /(concentration of

radioactive Cs in soil [Bq kg�1 DM]). We used Cs con-

centration data in herbage samples from the second and

third cuttings, i.e. excluding the data at the first cutting,

to avoid the influence of the contamination from the

nuclear fallout. Cs concentrations of soil (including sur-

face litter) were adjusted to those at the time of herbage

sampling based on the decay constant.

Measurements of soil physical and chemicalproperties

After determining the radioactive Cs concentrations, we

measured soil pH and electrical conductivity (EC) of the

soil samples using a pH meter (F–52; Horiba, Kyoto,

Japan) and an EC meter (CM–60G; Toa DKK, Tokyo,

Japan), respectively. Exchangeable cations were extracted

from the soil using an extracting solution (1 N NH4OAc,

pH 7.0). The cation exchange capacity was measured

using an atomic absorption spectrophotometer (Z–2300;Hitachi High-Tech, Tokyo, Japan). The size distribution

of the soil was determined by the pipette method (DIK–2021; Daiki, Saitama, Japan).

Data analysis

We calculated an average and a standard deviation (SD)

of Cs concentration using data from three areas in indi-

vidual grasslands (n = 3). The amount of radioactive Cs

in the soil at the July sampling (at the second harvest)

of the farm F meadow was calculated as the difference

between the amount of radioactive Cs in the soil sam-

ple with surface litter and that in the litter sample. A

linear regression analysis was used to examine the rela-

tionship of TF with soil physical and chemical proper-

ties.

Results

Radioactive Cs concentration of soil

Radioactive Cs concentration of soil varied considerably

both across grasslands (264–1593 Bq kg�1 DM) and

within individual grasslands (Table 3). On farm F, radio-

active Cs concentration of soil was 1.5–2.5 times higher

without vegetation (2365 Bq kg�1 DM) than with vegeta-

tion (950–1593 Bq kg�1 DM).

Radioactive Cs concentration of herbage

Radioactive Cs concentration of herbage in the meadows

ranged from 639–19 823 Bq kg�1 DM with high SD val-

ues within individual meadows (Table 4). The concen-

tration tended to decrease with cutting time except farm

G where the concentration remained almost constant

over the three harvests. Radioactive Cs concentration of

herbage in the sown pastures and the native pasture

ranged from 949–7161 and 5088–358 549 Bq kg�1 DM,

respectively (Table 5). In farm F, the Cs concentration

in the sown pasture remained relatively high from May

to July (4857–7161 Bq kg�1 DM) and decreased thereaf-

ter. The Cs concentration in the native pasture was

very high in May 2011 (358 549 Bq kg�1 DM), then

decreased rapidly from May to August, followed by a

gradual decrease until October. In contrast, the radioac-

tive Cs concentration of herbage in the pastures on

farms D and E showed no clear decreasing trend over

time.

Distribution of radioactive Cs in grassland

Radioactive Cs concentration in the vegetation–soilcomponents in the farm F meadow was highest in surface

litter (511 665 Bq kg�1 DM), followed by standing dead

material in the 0–10 cm layer (173 897 Bq kg�1 DM),

live plant material in the 0–10 cm layer (7565

Bq kg�1 DM), herbage above 10 cm (2146 Bq kg�1 DM)

and soil (1048 Bq kg�1 DM) (Figure 1). Approximately

71, 21 and 7% of radioactive Cs in the meadow were

present in the soil, litter and standing dead material,

respectively (Figure 2). The distribution of radioactive Cs


Y. Yamamoto et al. Radioactive cesium in grasslands

in the herbage (aboveground parts of plant) was very

small.

Physical and chemical properties of grasslandsoil

The pH and EC of soil ranged from 4.90–6.84 and

28.2–126.0 lS cm�1, respectively (Table 6). Exchangeable

potassium ranged from 3.2–21.6 mg (100 g DM)�1,

showing low values in farms B and G. Clay content

ranged from 1–32% with low values in farms D,

E and G.

The TF of radioactive Cs from soil to herbage was

lower at the third cutting than at the second (Figure 3).

The TF did not show a clear relationship with soil pH at

either cutting time, but tended to be lower in soils with

higher exchangeable potassium concentration and clay

content (P < 0.1) except for the relationship with clay

content at the second cutting.

Discussion

Monitoring of the air radiation dose rate by airplane

has shown that Cs contamination is spatially variable

Table 4 Radioactive cesium (Cs) concentration of herbage in six meadows on five farms in 2011

Farm

Dominant

species† Cutting Date

Herbage mass

(g DM m�2)

Concentration (Bq kg�1 DM)

134Cs 137Cs 134Cs+137Cs SD‡

A OG, IR First 9 May 570 1415 1495 2910 220

Second 23 June 76 671 716 1387 618

B OG Second 1 July 187 9464 10 359 19 823 3092

Third 26 September 113 4000 4675 8674 1754

IR Second 4 July 288 2995 3325 6320 1481

Third 2 August 111 1572 1721 3292 1429

C TI Second 6 July 530 4170 4616 8786 2653

Third 29 September 135 1651 1948 3598 836

F OG First 13 May 397 3042 3312 6355 1719

Second 6 July 283 1024 1122 2146 316

Third 31 August 196 301 338 639 104

G OG First 31 May 272 2413 2637 5050 840

Second 9 August 283 2788 3139 5927 1667

Third 7 October 173 2417 2839 5255 1576

†OG; Dactylis glomerata, IR; Lolium multiflorum, TI; Phleum pratense.

‡Standard deviation (n = 3).

Table 3 Radioactive cesium (Cs) concentration of soil in 10 grasslands on seven farms in 2011

Farm Soil type Grassland

Dominant

species† Date


134Cs 137Cs 134Cs+137Cs SD‡

A Andosol Meadow OG, IR 9 May 127 137 264 86

B Andosol Meadow OG 1 July 734 802 1536 374

IR 4 July 380 418 798 134

C Andosol Meadow TI 6 July 392 440 831 27

D Andosol Pasture OG, TI 11 July 201 245 445 113

E Andosol Pasture OG, TI 15 July 211 239 450 151

F Brown

lowland soil

Meadow OG 13 May 513 562 1075 310

Pasture OG 16 May 771 822 1593 434

ZJ 16 May 467 484 950 132

Bare land – 16 May 1148 1216 2365 546

G Andosol Meadow OG 31 May 354 377 731 153

†OG; Dactylis glomerata, IR; Lolium multiflorum, TI; Phleum patense, ZJ; Zoysia japonica.


Soil containing litter was sampled at a depth of 0–15 cm.



(Ministry of Education, Culture, Sports, Science and

Technology 2011). Tsuiki and Maeda (2012) demon-

strated the heterogeneous nature of radioactive fallout in

pastures by measuring air radiation dose rates. In the

present study, we also detected large variation in the

radioactive Cs contamination of soil both among and

within the grasslands (Table 3).

Following the reactor meltdown on 11 March, most

of the radioactive Cs fallout adhered to the surface of

the ground, vegetation and litter layer. In the farm F

meadow, high concentrations of radioactive Cs were

found for litter and standing dead plant material in July

(Figure 1). Approximately 71 and 21% of radioactive Cs

were present in the soil and litter, respectively (Fig-

ure 2). Radioactive Cs adhered to litter is considered to

be readily transferred to plants through absorption by

root mats formed between the ground surface and litter

layer (Sugiura et al. 1988). By contrast, radioactive Cs

Table 5 Radioactive cesium (Cs) concentration of herbage in four pastures on three farms in 2011

Farm

Dominant

species† Date

Herbage mass

(g DM m�2)


134Cs 137Cs 134Cs+137Cs SD‡

D OG, TI 11 July 607 484 574 1058 142

11 August 95 710 810 1520 689

13 September 193 1157 1310 2467 1967

13 October 213 426 523 949 415

E OG, TI 15 July 355 965 1054 2019 224

11 August 327 857 934 1791 310

13 September 238 915 1026 1941 382

13 October 422 467 533 1000 225

F OG 10 May 327 3307 3567 6874 3030

8 June 164 2763 2996 5758 1706

22 June 97 3410 3751 7161 3396

6 July 191 2373 2686 5060 1390

27 July 145 2309 2547 4857 2052

16 August 64 997 1182 2179 1041

6 September 71 897 1001 1898 1181

28 September 51 1158 1320 2478 1373

18 October 33 1057 1331 2388 1278

ZJ 10 May 41 175 462 183 086 358 549 72 681

8 June 46 67 117 71 893 139 011 82 775

8 July 27 7801 33 319 41 120 42 667

9 August 28 4118 4497 8615 2206

9 September 20 2522 2892 5414 2169

7 October 19 2340 2748 5088 1373

†OG; Dactylis glomerata, TI; Phleum pratense, ZJ; Zoysia japonica.


1048

511 665

173 897 7565

2146

0 200 000 400 000 600 000

Soil (0–15 cm)

Litter

Standing dead (0–10 cm)

Aboveground live parts(0–10 cm)

Aboveground parts (>10 cm)

Radioactive Cs concentration (Bq kg–1 DM)

Figure 1 Radioactive cesium (Cs) concentration in the meadow on

farm F at the second cutting in July 2011. The soil samples included

surface litter.

70.7%

21.4%

7.3%0.3%

0.3%Soil (0–15 cm)

Litter

Standing dead (0–10 cm)

Aboveground live parts (0–10 cm)

Aboveground parts (>10 cm)

Figure 2 Distribution of radioactive cesium in the meadow on farm F

at the second cutting in July 2011. The value for the soil was calcu-

lated to exclude surface litter (see Data analysis section).



adhered to the soil surface is considered to be only

partly transferable to plants after being dissolved in the

soil solution because of the strong affinity of Cs for soil

(Yamaguchi et al. 2012). However, despite the low rate

of relative transfer, the contribution from the soil can-

not be overlooked as the soil was the major stock of Cs

in the vegetation–soil system (Figure 2). Radioactive Cs

absorbed into plants is then transported from the base

to the upper part of vegetation, as reflected in the

higher Cs concentration in the former than in the latter

(Figure 1).

Radioactive Cs concentration of herbage tended to

decrease with cutting time in most of the meadows

(Table 4), which is in line with a previous report (Ros�en

et al. 1998). By contrast, radioactive Cs concentration of

herbage in the meadow on farm G remained almost con-

stant over the three harvests. In addition, radioactive Cs

concentration of herbage in the pastures on farms D and

E showed no clear decreasing tend with time although

that in the sown pasture on farm F decreased after July

(Table 5). These may be explained by the low contents of

exchangeable potassium and clay in the soil on farm G

and the low contents of clay in the soil on farms D and E

compared to those on the other farms (Table 6). Low

concentrations of potassium in the soil solution are

known to promote uptake of Cs by plants (Smolders

et al. 1997; Waegeneers et al. 2001, 2009). Low contents

of clay in soil are also considered to aid Cs uptake by

plants due to small amounts of Cs adsorbed to this soil

component (Absalom et al. 2001). The TF of radioactive

Cs from soil to herbage tended to be higher in soils with

lower exchangeable potassium concentration and clay

content (Figure 3). The results highlight the importance

of soil physical and chemical properties in Cs transfer to

plants. Moreover, TF of radioactive Cs was lower at the

third cutting compared to the second cutting, regardless

Table 6 Soil physical and chemical properties (depth 0–15 cm) of 10 grasslands on seven farms in 2011

Farm Grassland

Dominant

species†

pH

(H2O)

EC

(lS cm�1)

Exchangeable cation

(mg [100 g DM]�1) Cation exchange

capacity (meq

[100 g DM]�1)

Saturation

(%)

Size distribution

(%)

K2O Na2O CaO MgO Lime Base Clay Silt Sand

A Meadow OG, IR 6.84 75.6 21.5 3.6 499.3 149.8 27.7 64 93 25 23 52

B Meadow OG 5.73 53.2 3.3 1.2 186.1 38.3 21.0 32 41 11 12 77

IR 6.03 41.6 3.2 1.4 175.4 47.8 15.8 40 56

C Meadow TI 4.96 126.0 8.3 2.1 90.5 13.3 24.7 13 17 8 14 78

D Pasture OG, TI 6.17 95.9 10.8 1.2 509.8 166.0 34.7 52 77 1 10 89

E Pasture OG, TI 4.90 108.5 20.2 1.4 82.8 24.7 24.9 12 19 2 14 84

F Meadow OG 5.86 43.3 21.6 1.7 169.9 57.5 19.5 31 48 16 15 69

Pasture OG 5.50 83.5 12.0 1.9 110.5 29.5 15.6 25 37 12 12 76

ZJ 5.42 28.2 7.3 2.2 45.1 9.1 21.0 8 11 32 21 47

G Meadow OG 5.68 84.3 3.5 2.1 146.3 45.2 22.8 23 33 3 31 66

†OG; Dactylis glomerata, IR; Lolium multiflorum, TI; Phleum patense, ZJ; Zoysia japonica.

pH (H2O)

K2O (mg [100 g DM]–1)

Clay (%)

Tran

sfer

fact

or o

f rad

ioac

tive

Cs fr

om s

oil t

o he

rbag

e

(a)

(b)

(c)

y = –3.99 x + 31.09 R² = 0.15

y = –0.97 x + 10.22 R² = 0.02

0

2

4

6

8

10

12

14

4.5 5 5.5 6 6.5

y = –0.46 x + 12.19 R² = 0.74

y = –0.28 x + 6.93 R² = 0.71

0

2

4

6

8

10

12

14

0 5 10 15 20 25

y = –0.53 x + 13.79 R² = 0.37

y = –0.50 x + 9.60 R² = 0.82

0

2

4

6

8

10

12

14

0 5 10 15 20

, P < 0.1

, P < 0.1

, P < 0.05

, P > 0.1

, P > 0.1

, P > 0.1

Figure 3 Relationship between the transfer factor of radioactive

cesium (Cs) from soil to herbage and the physical or chemical proper-

ties of soil in the meadows at the second (●) and third (○) cuttings.



of the soil physical and chemical properties (Figure 3).

Further research is warranted to collect data to validate

the relationship between the TF of radioactive Cs and soil

physical and chemical properties.

The native pasture on farm F showed an extremely high

level of Cs concentration of herbage on 10 May, which was

followed by a rapid decrease until August (Table 5). The

initial high Cs level may be explained by the contamination

of the herbage samples with Cs-rich surface litter and sur-

face soil due to the low sampling height (2 cm above the

ground). Then, the rapid decrease in the Cs level is attrib-

uted to the removal of Cs-rich herbage through grazing by

cattle in May–July. Japanese lawn grass, a low-growing,

creeping plant dominant in the native pasture, was closely

grazed by cattle near to the ground surface, whereas the

tall-growing, bunch-type grasses in the sown pastures were

rarely grazed down below 10 cm.

The IAEA (2006) reported that, after the Chernobyl

accident, major and persistent problems occurred in areas

of extensive agricultural systems that contained soils with

large amounts of organic matter and in unimproved pas-

tures that were not plowed or fertilized. The transfer of Cs

from the soil to plants is known to be depressed by potas-

sium in the soil (Smolders et al. 1997; Waegeneers et al.

2001, 2009). Most of the grasslands in the present study

received no or minimum fertilizer after the accident. If the

grasslands had been fertilized sufficiently with potassium,

the Cs concentration of herbage would have been much

lower. The long-lasting effects of radioactive Cs in the veg-

etation–soil system (Papastefanou et al. 2005) warrants

continued regular monitoring of radioactive Cs concentra-

tion of herbage in the areas affected by the nuclear fallout.

Acknowledgments

This study was supported in part by a Grant-in-Aid

(Research and development projects for application in

promoting new policy) from the Ministry of Agriculture,

Forestry and Fisheries. The authors express their gratitude

to Ms. Hiroko Suzuki for her generous help.

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changes in the radioactive cesium concentrations of grasslands during the first year after the...

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