Comparative dynamics of potassium and radiocesium in soybean with different potassium application levels
Introduction
The accident at Tokyo Electric Company's Fukushima Dai-ichi Nuclear Power Plant (FDNPP), caused by the Great East Japan Earthquake and subsequent Tsunami in March 2011, resulted in widespread contamination of agricultural land in eastern Japan with radionuclides, particularly in parts of Fukushima Prefecture. Immediately after the accident, the major radionuclides found in plants and environmental resources, such as soil and water, were 131I and radiocesium (134Cs and 137Cs). Although 131I (half-life: 8 days) decays within a few months, radiocesium is retained in the soil for much longer because of the length of the 137Cs half-life (30.2 years), and its transfer to crops is therefore a long-term problem. With some exceptions, the Japanese government commissioned physical decontamination works, which included topsoil (depth of 15 cm) removal and soil inversion for soils exceeding 5 kBq kg dry weight of radiocesium (Yamaguchi et al., 2016). Ten years have passed since the accident and all decontamination work has been completed, except in some difficult-to-return zones near the FDNPP, where the annual cumulative radiation doses are greater than 50 mSv, and entry and lodging are prohibited.
Studies on the absorption and accumulation of radiocesium in crops since the accident have shown that potassium fertilization is the most effective and practical countermeasure for reducing radiocesium transfer from the soil to the edible parts of crops. Because cesium has similar chemical and physiological characteristics to potassium, both potassium and cesium are expected to have similar behaviors in soil-plant systems, and cesium absorption from the soil is competitively decreased by the application of potassium fertilizer (Shaw and Bell 1991; Smolders et al., 1997). In paddy rice (Oryza sativa L.), radiocesium transfer to the grains decreased with increasing exchangeable potassium (1 mol L−1 ammonium acetate extractable potassium) concentrations in the soil (Tsukada et al., 2002a; Kato et al., 2015), with similar results obtained in buckwheat (Fagopyrum esculentum Moench) and vegetables (Kobayashi 2014; Kobayashi et al., 2014; MAFF 2014; Kubo et al., 2015, 2017). Thus, the additional application of potassium fertilizer has been widely implemented in low-contamination areas (<5 kBq kg −1 dry weight of radiocesium) and decontaminated areas. Information about the various case studies and the countermeasures published on websites and in technical reports in the early stages after the accident has been summarized by Yamaguchi et al. (2016).
Soybean (Glycine max (Merr.) L.) is one of the most important crops in Japan, with almost all being used in food processing, notably tofu production, which is made by concentrating soy protein. Other important soybean products, such as miso and soy sauce, which are essential seasonings in traditional Japanese cooking, are produced via fermentation. Immediately after the accident, we conducted a survey of exchangeable potassium concentrations in the soil and radiocesium concentrations in soybean grains to determine the level of soil-exchangeable potassium required to minimize the radiocesium concentrations. Accordingly, a negative correlation was observed between the soil-exchangeable potassium concentration and the soil-to-grain transfer factor (TF) of radiocesium, which is expressed as a ratio of radioactivity in the grains to that in the soil (Bq kg−1 (Bq kg−1)−1) (MAFF 2015). Based on these results, an exchangeable potassium concentration of ≥250 mg kg−1 of K2O in the soil prior to the use of basal fertilizer at a conventional rate is now widely recommended in soybean cultivation, except during initial cultivation after the accident or in fields with a high TF, where the recommended application level was increased to 500 mg kg−1 of K2O (MAFF 2015). The rate of additional potassium fertilizer application is calculated from the exchangeable potassium concentration of the soil before cultivation, assuming a soil bulk density of 1.0 Mg m−3 and soil depth of 15 cm. As a result, the percentage of soybean grains exceeding the present standard limits (100 Bq kg−1 fresh weight for radiocesium in food, established by the Japanese Ministry of Health, Labor and Welfare, 2011 in April 2012) has decreased annually, and the radiocesium concentration in soybean grains has not exceeded this limit since 2015 (Japanese Ministry of Agriculture, Forestry and Fisheries). According to MAFF (2015), the exchangeable potassium concentration of the soil is relatively low in fields where the radiocesium concentration of the soybean grains exceeds the above limit before 2015. Although additional potassium fertilization is a primary factor in reducing the percentage of soybean grains exceeding this limit, the decrease in soil-exchangeable radiocesium concentrations due to physical decay and fixation to clay minerals in the soil (Tsumura et al., 1984; Ehlken and Kirchner 2002) has also contributed to the reduction.
Radiocesium transfer from the soil to the edible parts of a plant has been widely studied, but most research has focused on radiocesium, with few reports on the comparative dynamics of radiocesium and potassium. Cesium and potassium are expected to have similar behaviors in soil-plant systems; however, the radiocesium/potassium ratio is not uniform in paddy rice, indicating different behaviors (Tsumura et al., 1984; Tsukada et al., 2002b; Kondo et al., 2015). Soybean and buckwheat show considerably high TF of 134Cs (Broadley and Willey 1997), and monitoring inspections after the accident also indicated that the percentage of soybean grains exceeding the limit was higher compared with other crops (Nihei and Hamamoto 2019). Potassium application is costly and labor-intensive; therefore, it is important to understand the behavior of potassium and radiocesium in soybean soil-plant systems to establish strategies for reducing radiocesium concentrations with minimal labor and cost. In the present study, we therefore examined the comparative dynamics of radiocesium (137Cs) and potassium (K) in soybean. To do so, we conducted a field experiment with five levels of soil-exchangeable K and examined the variability in the absorption and distribution of 137Cs compared with K.
Section snippets
Field management and experimental design
The field experiment was conducted in the city of Date in Fukushima Prefecture in 2015. The soil type in the field was gray lowland soil based on the classification of cultivated soils in Japan. The soil texture was sandy clay (27.0% clay, 7.5% silt, and 65.5% sand), and the 137Cs concentration from the surface to a depth of 15 cm was 2.74 kBq kg−1 in mid-May 2015. The physical and chemical properties of the soil are shown in Table 1.
Although the additional application of K fertilizer had been
Plant growth
Fig. 1 shows the dry weights and distribution patterns of the dry weights of the shoots. The dry weight of the shoots increased dramatically from the R2 to the R6 growth stage and the weight of the grains at the R8 growth stage was 1.7–2.2 times higher than that at the R6 growth stage (Fig. 1a, Table S1 in supplementary material), with 42% of the total weight of the shoots represented by the grains at the R8 stage (Fig. 1b). K application did not affect the dry weight or distribution patterns
Discussion
The dry weight of the shoots increased significantly from the R2 to the R6 growth stage (Fig. 1a). The inventories of K and 137Cs in the shoots increased remarkably with this increase in the dry weight (Fig. 2, Fig. 3 and ). Harper (1971) reported that the maximum rate of nutrient uptake occurred during the R2 and R5 growth stages. Thus, nutrient accumulation in the shoots coincides closely with dry matter accumulation. The rapid increase in K demand after the R2 growth stage (full bloom stage)
Conclusions
We conducted a field experiment in soybean with different levels of K application to elucidate the comparative dynamics of 137Cs and K. The inventory of K in the shoots increased greatly from the fifth trifoliate stage to the full seed stage, and as the absorption of K increased, so too did the absorption of 137Cs. The effect of K application was much greater in terms of the dynamics of 137Cs than those of K or biomass production. Although K application reduced not only the accumulation of 137
Funding
This work was conducted as a part of the “Development of Decontamination Technologies for Radioactive Substances in Agriculture Land” project funded by the Japanese Ministry of Agriculture, Forestry and Fisheries.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We thank the following staff of the Agricultural Radiation Research Center, NARO Tohoku Agricultural Research Center for their field and laboratory assistance: Ms. Aya Miura, Ms. Etsuko Shibayama, Ms. Yurie Yoshida, Mr. Yoshihiko Takahashi, Mr. Masakatsu Ito, Mr. Takao Sakurai, Mr. Masanori Yoshida, and Mr. Rikio Shishido.
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