Weathering half-life of radioactive cesium for winter rye (Secale cereale L.) and Italian ryegrass (Lolium multiflorum Lam.) directly contaminated by the 2011 Fukushima Daiichi Nuclear Power Station accident

Figures & data

Table 1 Soil chemical properties in the experimental fields.

	Field A		Field B
	2011 period	2012 period	2011 period	2012 period
Crop	Rye	Rye	Italian ryegrass	Rye
pH (H2O)	6.11 ± 0.12	6.52 ± 0.12	6.39 ± 0.16	6.56 ± 0.05
Total C (g kg^−1 DW)	48.8 ± 3.4	37.4 ± 2.1	72.5 ± 3.5	73.0 ± 0.6
Total N (g kg^−1 DW)	3.21 ± 0.15	2.56 ± 0.14	4.73 ± 0.16	4.75 ± 0.09
Inorganic N (mg kg^−1 DW)	23.7 ± 1.7	30.1 ± 1.1	30.1 ± 2.5	42.9 ± 8.3
Available P2O5 (mg kg^−1 DW)	98.6 ± 44.4	82.6 ± 2.4	147.3 ± 19.2	121.8 ± 13.4
Exchangeable cations (g kg^−1 DW)
K2O	0.495 ± 0.020	0.192 ± 0.010	0.938 ± 0.080	0.754 ± 0.010
CaO	2.38 ± 0.03	2.14 ± 0.22	4.44 ± 0.34	4.50 ± 0.04
MgO	0.636 ± 0.100	0.577 ± 0.080	1.110 ± 0.110	1.041 ± 0.010

All data are means ± standard deviation. All properties were determined for soil samples collected before each sowing. The 2011 and 2012 periods represent the harvesting year for 2010–2011 and 2011–2012 cultivation seasons, respectively. Rye, Secale cereale L.; Italian ryegrass, Lolium multiflorum Lam.; H₂O, water; C, carbon; N, nitrogen; P₂O₅, phosphorus pentoxide; K₂O, potassium oxide; CaO, calcium oxide; MgO, magnesium oxide; DW, dry weight.

Figure 1 Daily precipitation from the day of the earthquake on March 11, 2011, to the last harvesting date on May 6, 2011. Rye (Secale cereale L.) and Italian ryegrass (Lolium multiflorum Lam.) were harvested during their vegetative stage on March 31, 2011 (first harvest) and at the heading stage (second harvest) on April 27 and May 6, 2011, respectively.

Figure 2 Daily mean wind direction (circles) and wind speed (squares) from the day of the earthquake on March 11 to the last harvesting date on May 6, 2011. Rye (Secale cereale L.) and Italian ryegrass (Lolium multiflorum Lam.) were harvested during their vegetative stage on March 31, 2011 (first harvest) and at the heading stage (second harvest) on April 27 and May 6, 2011, respectively. NE, northeast; E, east; SE, southeast; S, south; SW, southwest; W, west; NW, northwest; N, north.

Figure 3 Changes in (a) plant lengths and (b) dry matter yields of rye and Italian ryegrass harvested in the spring of 2011 (first harvest, vegetative stage, March 31 for both species; second harvest, heading stage, April 27 for rye and May 6 for Italian ryegrass). Results are means ± standard deviation (n = 3). Values labeled with different uppercase and lowercase letters differ significantly between harvests for rye (Secale cereale L.) and Italian ryegrass (Lolium multiflorum Lam.), respectively (Welch’s t-test, P < 0.05).

Figure 4 Changes in (a) radiocesium concentrations per unit mass (dry weight basis) and (b) radiocesium amounts per unit area in rye and Italian ryegrass harvested in the spring of 2011 (first harvest, vegetative stage, March 31 for both species; second harvest, heading stage, April 27 for rye and May 6 for Italian ryegrass). Results are means ± standard deviation (n = 3). Values labeled with different uppercase and lowercase letters differ significantly between harvests for rye (Secale cereale L.) and Italian ryegrass (Lolium multiflorum Lam.), respectively (Welch’s t-test, P < 0.05).

Table 2 Comparison of weathering half-lives (T_w) for radiocesium in the present study with previously reported values.

	Tw (days)
Plants	Concentration per unit DW	Amount per unit area	Source of contamination	Reference
Rye (Secale cereale L.)	8.0	11.0	FDNPS	Present study
Italian ryegrass (Lolium multiflorum Lam.)	14.1	23.1	FDNPS	Present study
Grasses	10.5		Chernobyl	Mück et al. (1994)^†
	7.9–11.1		Chernobyl	Kirchner (1994), Ishida et al. (1988), Monte (1991); summarized by Leclerc and Choi (2009)
		10	Windscale	Booker (1958); summarized by Miller and Hoffman (1983)
Ryegrass		17	Sprayed	Bartlett et al. (1961); summarized by Miller and Hoffman (1983)
Bottlebrush squirreltail (Elymus elymoides (Raf.) Swezey)	9		Aerosol released	Fraley and Chavez (1993)
Sagebrush (Artemisia tridentata Nutt.)	17		Aerosol released	Fraley and Chavez (1993)
Cereal	35		Sprayed	Aarkrog (1969); summarized by Leclerc and Choi (2009)
Rice (Oryza sativa L.)		32.1–49.4	Sprayed	Choi et al. (2002)
Cabbage (Brassica oleracea L.)		9	Sprayed	Middleton and Squire (1961); summarized by Miller and Hoffman (1983)
Chinese cabbage (Brassica rapa L.)		11.5–34.5	Sprayed	Choi et al. (1998); summarized by Leclerc and Choi (2009)
Radish (Raphanus sativus L.)		16–36.7	Sprayed	Choi et al. (2002)

DW, dry weight. “FDNPS” (= Fukushima Daiichi Nuclear Power Station), “Chernobyl” and “Windscale”, contamination sources resulting from nuclear power station accidents; “Sprayed” and “Aerosol released” represent experimental application of radiocesium to the tested plants. ^†This T_w value was derived from the reported value of effective half-life (T_eff) by correcting for the process of radioactive decay of ¹³⁷Cs tested in the reference.

Table 3 Yield of rye grown in the 2012 period, and radiocesium levels in the rye and the soil.

	Field^†
	A	B	Mean^‡
Rye
Yield (kg DW m^−2)	0.847 ± 0.05	0.886 ± 0.03	0.866 ± 0.04
Concentration (Bq kg^−1 DW)
^134Cs + ^137Cs	13.4 ± 7.2	19.2 ± 8.3	16.3 ± 7.7
^134Cs	5.1 ± 1.7	7.6 ± 3.3	6.3 ± 2.7
^137Cs	8.3 ± 5.6	11.6 ± 5.0	10.0 ± 5.1
Amount (Bq m^−2)
^134Cs + ^137Cs	11.5 ± 6.8	16.9 ± 7.2	14.2 ± 6.9
Soil
Concentration (kBq kg^−1 DW)
^134Cs + ^137Cs	1.85 ± 0.05	1.35 ± 0.45	1.60 ± 0.39
^134Cs	0.75 ± 0.04	0.56 ± 0.19	0.66 ± 0.16
^137Cs	1.09 ± 0.02	0.79 ± 0.26	0.94 ± 0.23
Amount (kBq m^−2)
^134Cs + ^137Cs	283 ± 7	201 ± 67	242 ± 62

^†Values represent means ± standard deviation (SD) of three repetitions in each field.

^‡Values represent means ± SD of Fields A and B with three repetitions. Fields A and B were used for rye (Secale cereale L.) and Italian ryegrass (Lolium multiflorum Lam.) cultivation, respectively, in the 2011 period. No values differed significantly between the fields (Welch’s t-test, P < 0.05). Radiocesium concentrations were corrected to a value on May 1, 2012, based on the radioactive decay rates of cesium-134 (¹³⁴Cs) and cesium-137 (¹³⁷Cs). Amounts of radiocesium per unit area in the soil were calculated by multiplying the soil concentration by the soil weight to a depth of 20 cm based on a bulk density of 0.767 Mg m⁻³ for field A and 0.743 Mg m⁻³ for field B. DW, dry weight.

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