Radiocesium Absorption by Rice in Paddy Field Ecosystems
Although most of the radiocesium fallout that deposited in paddy fields after the Fukushima nuclear disaster in March 2011 was expected to be bound to clay in the soil resulting in a very low soil-to-plant transfer function, a radiocesium contamination level of >500 Bq/kg was detected in brown rice grown in several hilly areas of Fukushima Prefecture in the autumn of the same year. The likely source of the radiocesium was fallout deposited on organic matter in the paddy fields and litter in mountain forests, from which runoff water flowed into irrigation channels that ultimately lead to the paddy fields. This problem appears to have been caused by conditions specific to lowland rice paddy fields, which are wetland ecosystems. Integrated studies of the soil, water, and plants from an ecological viewpoint are necessary to understand the mechanism of radiocesium absorption by rice before commercial rice production in the affected areas can be resumed.
KeywordsIrrigation water Paddy field ecosystems Radiocesium Rice (Oryza sativa L.)
The nuclear disaster in Fukushima in March 2011 released considerable amounts of various radionuclides and contaminated extensive areas of farmlands. Radiocesium comprised the majority of the radionuclides released and special attention needs to be paid to 137Cs because of its long half-life.
In general, plants absorb radioactive materials: (1) directly from the surface through their aerial parts or (2) through root uptake. After the radioactive material was released into the atmosphere, it adhered to the surfaces of the aerial parts of plants where direct absorption occurred, which was the major source of contamination in plants immediately after the release of the radioactive material. The radioactive material then entered the environment and moved into the soil, where it was absorbed through the roots resulting in long-term contamination of crops. The level of root absorption is greatly affected by the behavior of the radioactive material in the soil and the soil-to-plant transfer factor depends on the specific soil type because radioactive materials such as cesium are strongly bound to the soil granules (Yamaguchi et al. 2012). Because the typical farm soil in the affected area (mainly Fukushima) was a gray lowland soil, which binds cesium strongly, the radiocesium contamination of various crops through root absorption was much lower than the provisional regulation level (500 Bq/kg) in most cases during 2011.
A low level of radiocesium contamination was also expected in rice (Oryza sativa L.). Half way through the investigation of radioactive contamination (pilot survey) of rice in mid-September of 2011, the radiocesium concentration in brown rice was below the detection limit at most test sites and the highest contamination level was approximately 1/4th of the provisional regulation level. However, in the subsequent investigation, the radiocesium concentration was close to or >500 Bq/kg in many samples of brown rice from several localities in the hilly areas of Fukushima Prefecture (e.g., Obama district in Nihommatsu City, Onami district in Fukushima City, and Oguni district in Date City), which surprised the whole of Japanese society. The radiocesium absorption level in the rice grown in these paddy fields was extraordinarily—100 times or more than that in the flatland areas of Fukushima Prefecture such as Koriyama City. The mechanism underlying this unusual absorption was unclear, and a rapid investigation is necessary before the crop production can be resumed in the affected area. In this paper, we will summarize our most current knowledge about this problem.
3.2 “Seasonality” in the Radiocesium Absorption Level of Rice
The temporal pattern of radiocesium absorption by plants (i.e., the time course of absorption throughout the plant growth stages) can be estimated from the spatial distribution of the absorbed material in the plant body, provided the material has low mobility in the living plant (Tanoi et al. 2011). We used this approach to investigate the distribution of radiocesium in rice plants.
3.3 Radiocesium Absorption Reflects the Features of the Paddy Field Ecosystem
Given the remarkably high efficiency of radiocesium absorption from water by roots, particular attention should be given to radiocesium contamination of the water used for commercial hydroponic vegetable production, which is often recommended as a way of avoiding radiocesium transfer from the soil to vegetables.
The effect of the exchangeable potassium concentration in the soil is another key factor. It is widely known that a low potassium concentration in the soil enhances cesium absorption by plants. A joint research project in Onami district by the Ministry of Agriculture, Forestry, and Fisheries (MAFF) and Fukushima Prefecture reported a low concentration of exchangeable potassium in paddy fields where the radiocesium concentration was high in brown rice. This report suggested that the low exchangeable potassium concentration in the paddy soil may have reinforced the radiocesium absorption by rice roots in areas where brown rice contamination exceeded the provisional regulation level. Our pot experiment using contaminated paddy soil from Obama also showed that the radiocesium uptake by rice seedlings was reduced to approximately 1/10th after the application of potassium chloride, which indicated the effectiveness of potassium fertilizer application to contaminated paddy fields as a countermeasure for absorption of radiocesium by roots.
However, we would like to point out that the conventional rice cultivation methods used in the problem area are still quite reasonable and effective given the low input requirements of sustainable agriculture. Local farmers intensively utilized the rich mineral nutrients in runoff water to produce highly palatable rice while minimizing the use of chemical fertilizers. Thus, rice plants absorbed sufficient potassium from the paddy fields, although the potassium concentration of the soil was low. However, the nuclear disaster destroyed the well-intended efforts of farmers who wanted to produce sustainable, high quality rice in Fukushima.
3.4 Behavior of Radiocesium in Organic Matter
As discussed above, the problem appeared to be caused by the specific conditions in lowland rice paddy fields, which are wetland ecosystems. Many useful agricultural and plant studies were reported after Chernobyl (e.g., Ehlken and Kirchner 2002), but the experience and knowledge acquired from the areas affected by the Chernobyl disaster are not directly applicable to solving the rice production issues in Fukushima. Thus, original perspectives and approaches based on agricultural research in monsoonal Asia are required to prevent radiocesium contamination of rice, which are different from those required in upland farm areas. To understand the mechanism of radiocesium contamination of rice and solve the problem, we need to analyze the flow of radiocesium through forests, mountain streams, and paddy fields.
3.5 Can Breeding Resolve the Problem?
We have discussed the possible mechanism of radiocesium contamination in rice from an ecological perspective; besides, breeding new rice cultivars that absorb less radiocesium is another important approach that may solve the problem. Thus, we screened rice varieties to acquire basic information relevant to future breeding efforts. Over 100 rice varieties were grown using highly contaminated soil and their radiocesium uptake capacities were measured during the vegetative stage. The radiocesium uptake during the vegetative phase was generally higher in japonica varieties compared with indica varieties. The only exception was Pokkali rice, a famous salt-tolerant indica variety, which had a higher radiocesium uptake capacity than any of the japonica varieties we tested. However, the overall range of genetic variation in the radiocesium uptake capacity was only threefold.
We also need to study the radiocesium accumulation level during the reproductive stages before we conclude the study; however, the present results suggest that the genetic variation in radiocesium uptake by rice is not sufficiently high to breed a new variety that could resolve the problem. Therefore, the integration of breeding with other countermeasures such as cultivation methods and civil engineering with special attention to ecophysiological and environmental aspects will be required before rice culture in the affected areas can be resumed.
3.6 The Diagnosis of Radiocesium Absorption in Individual Paddy Fields
It appeared that the high levels of radiocesium contamination in brown rice were caused by a complex interaction between local factors and features of the individual paddy fields. The local factors included large amounts of radioactive fallout and contaminated water that tended to accumulate in areas with mountainous geography, whereas the paddy field-specific factors included the types of organic matter and runoff water that provided a source of radiocesium, the exchangeable potassium content, the type of clay in the soil, and water percolation. Furthermore, because the decomposition rate of organic matter can differ in the leaf litter produced by various trees, the end of the radiocesium contamination risk for rice will also depend on the organic matter type.
Solving the problem of radiocesium contamination in rice demands the clarification of general factors in affected areas and the detection of radiocesium absorption in individual paddy fields. Unfortunately, detailed data were not available for individual paddy fields during 2011 because the rice harvested from individual paddy fields was gathered when the brown rice contamination level exceeded 500 Bq/kg. Thus, it is necessary to conduct further field studies to diagnose individual paddy fields. We are conducting rice cultivation field experiments in some areas of Date City this year (2012), where radiocesium contamination levels exceeding 500 Bq/kg were detected in some brown rice samples in 2011. We intend to diagnose individual paddy fields to support the resumption of rice production by native farmers in these areas.
The studies reported in this chapter were performed as collaborative research by three groups from the Graduate School of Agricultural and Life Sciences of the University of Tokyo (Laboratory of Radioplant Physiology, Laboratory of Land Resource Science, Laboratory of Crop Ecology and Morphology) and Fukushima Agricultural Technology Centre.
- Shiozawa S (2012) Mechanisms of soil-to-plant transfer of radiocesium in rice grown in paddy fields. In: Abstract for 2nd progress report meeting of studies on the effects of radioactivity on agricultural, livestock and fishery products (in Japanese, Title translated by present authors) http://www.a.u-tokyo.ac.jp/rpjt/event/20120218-4.pdf. Retrieved on 20 Nov 2012
- Yamaguchi N, Takata Y, Hayashi K, Ishikawa S, Kuramata M, Eguchi S, Yoshikawa S, Sakaguchi A, Asada K, Wagai R, Makino T, Akahane I, Hiradate S (2012) Behavior of radiocaesium in soil-plant systems and its controlling factor: a review. Bull Natl Inst Agro-Environ Sci 31:75–129 (in Japanese with English summary)Google Scholar
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