Keywords

1 Introduction

The Great East Japan Earthquake occurred on March 11, 2011, and it was immediately followed by the accident at the Fukushima Daiichi Nuclear Power Plant, Tokyo Electric Power Company. Radiocesium, the dominant nuclide released during the accident, reached agricultural lands in Fukushima and its neighboring prefectures and contaminated the soil and agricultural products [1, 2]. To guarantee the safe consumption and handling of agricultural, livestock, forestry, and marine products, monitoring inspections were established [3]. According to these inspections [4–6] the ratio of samples exceeding the new standard value of radiocesium [7] (100 Bq kg−1) were found to be 5.7 % for soybean, 2.6 % for rice, and 11 % for wheat in 2011; 2.6 % for soybean, 0.0007 % for rice, and 0 % for wheat in 2012; and 1.9 % for soybean, 0.0003 % for rice, and 0 % for wheat in 2013. The inspection results indicate that the ratio of soybean exceeding 100 Bq kg−1 was high compared with that for rice and wheat and that the tendency to decline was low compared with that for rice and wheat. To revitalize agriculture, Fukushima Prefecture has been promoting the decontamination of agricultural lands; consequently, it has implemented an increase in the exchangeable potassium content in soil used to grow rice to approximately 25 mg 100 g−1 (dry soil) or higher. This occurred because it recently became clear that potassium fertilization was effective for reducing the radiocesium concentration in agricultural crops [8, 9]. Nitrogen (N) has a large effect on crop growth, and some reports have suggested that it also promotes the radiocesium absorption [10, 11]. However, few studies have examined how nitrogen contributes to the radiocesium absorption in soybean apart from potassium.

For the recovery and revitalization of agricultural industries, the analysis of radiocesium absorption in soybeans is necessary. Focusing on this point, we studied the effect of nitrogen fertilizers on the radiocesium absorption in soybean seedlings.

2 Materials and Methods

We cultured soybean (Glycine max) in a greenhouse (experiment 1) and in a field (experiment 2). For experiment 1, nitrogen fertilizer in the form of ammonium sulfate was applied at two levels: 0.4 and 1.3 g per 1 L pot (hereafter low-N and high-N, respectively). The radiocesium activity of the soil which was taken in Fukushima in August 2012 was approximately 30 kBq kg−1, exchangeable potassium was 13.4 mg 100 g−1 soil, and pH was 6.0. The plants were grown until maturity, and the seed was collected. For experiment 2, soybean was grown in Iitate Village, Fukushima Prefecture. The radiocesium of the field was approximately 13 kBq kg−1 (15-cm depth), exchangeable potassium was 15.8 mg 100 g−1, and pH was 6.2. Nitrogen fertilizer in the form of ammonium nitrate was applied at three levels: 0, 50, and 100 kg ha−1 (hereafter non-N, low-N, and high-N, respectively). We sowed the seeds for experiment 2 on June 16, 2014, and collected the aboveground biomass on September 2, 2014. Next, we studied the effect of the different forms of nitrogen treatment on radiocesium absorption by soybean seedling (experiment 3). Nitrogen was applied as calcium nitrate, ammonium nitrate, and ammonium sulfate at three levels: 0, 0.01, and 0.05 g per treatment (hereafter non-N, low-N, and high-N, respectively). We cultured soybean in a vessel (6.5 × 6.5 × 6.5 cm) for 18 days in a biotron (28 °C, 16 h light). We collected the aboveground biomass. Moreover, we studied radiocesium activity in soil extracts following nitrogen application (experiment 4). Nitrogen was applied to the soil at 0.5 g kg−1 as ammonium sulfate. We collected the soil after 1, 5, and 15 days, extracted it with 1 M calcium chloride, and then measured the radiocesium activity. We used soil sourced from the same batch in Experiment 1, 3, and 4. The radiocesium activities of all samples were measured using a sodium iodide scintillation counter (Aloka AM-300). In experiment 3, potassium, calcium, and magnesium concentrations in the soybean seedling were measured after acid decomposition using inductively coupled plasma optical emission spectrometry (ICP-OES) (PerkinElmer, Optima 7300).

3 Results

Figure 15.1 shows the effect of nitrogen fertilizer on radiocesium absorption by soybean. The concentrations of radiocesium in the seed (experiment 1) or the aboveground biomass (experiment 2) were higher in the high-N treatments than in the non-N or low-N treatments. Table 15.1 shows the influence of the different forms of nitrogen treatment on radiocesium activity in the aboveground (experiment 3). The highest activity occurred with ammonium sulfate (approximately 3.7 times the non-N treatment), the next highest activity occurred with ammonium nitrate (approximately 2.4 times the non-N treatment), followed by calcium nitrate (approximately 2.2 times the non-N treatment). The concentrations of radiocesium in the aboveground biomass were higher in nitrogen-fertilized plants than in plants without added nitrogen. Figure 15.2 shows the radiocesium activity in soil with applied nitrogen (experiment 4). The amount of extracted radiocesium increased a day after fertilization and remained higher even after 15 days of nitrogen application.

Fig. 15.1
figure 1

(a) Experiment 1. The plants were cultured until maturity within 1 L pot, and the seed was analyzed. (b) Experiment 2. The plants were cultureed in Iitate village, Fukushima prefecture, and the aboveground biomass was analyzed

Full size image
Table 15.1 Radiocesium activities and concentrations of base cations in soybean aboveground biomass after nitrogen fertilization treatments (Experiment 3)
Full size table
Fig. 15.2
figure 2

Radiocesium extracted from the soil after nitrogen fertilization (Experiment 4)

Full size image

4 Discussion

The radiocesium concentration in seed and aboveground biomass increased as the amount of nitrogen fertilizer increased. The different forms of nitrogen treatment increased the radiocesium concentration of soybean in the order ammonium sulfate > ammonium nitrate > calcium nitrate. Hence, ammonium-nitorogen increased radiocesium absorption more than nitrate. Furthermore, the amount of radiocesium extracted from the soil, which is considered potentially available for plant absorption, was increased by ammonium-nitrogen fertilization.

Geometrically adapted cesium ions are fixed to the clay mineral, and the radiocesium adsorbed to the soil particles was probably not available for plant uptake. But the ionic radius of the ammonium ion is similar to that for the cesium ion [10], ammonium exchanged, and released radiocesium from the soil. We found that the amount of radiocesium extracted by ammonium fertilizer increased the day after fertilization; therefore, soybean could absorb radiocesium. In addition, the ammonium and cesium ions are both univalent cations, and ammonium has been found to restrict cesium absorption in the hydroponics [9–11]. This study was used by soil. It was considered that ammonium oxidizes to nitrate during cultivation, and ammonium fertilization did not restrict the radiocesium absorption of soybean. Moreover, we considered that potassium absorption might compete with ammonium absorption because both ions are univalent cations; hence, the soybeans may lack potassium. A lack of potassium has been found to increase cesium absorption [9, 12, 13]. However, the potassium concentrations in soybean did not decrease with ammonium fertilization. Therefore, we suggest that the increased activity of radiocesium in soybean due to ammonium fertilization was not because of a lack of potassium.

Soybean cultivation typically follows rice cultivation. To assist the recovery and revitalization of agricultural industries in Fukushima Prefecture, we suggest that special care is required to select the appropriate kind of fertilizer and to start cultivating soybean on fields that have higher available nitrogen than the regular cultivated fields. It is important to clarify the mechanism of cesium availability in response to nitrogen fertilizers to cope with cesium contamination in crops.