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Cesium137 in Japanese Soil and Rice
Abstract
Japanese soil and rice collected at 15 localities have been analyzed for cesium-137 since 1959 through 1963. The time trend of rice-levels indicates the minimum in 1960 and 1961 and the maximum in 1963, showing more than threefold value of the minimum. A higher dependency was observed of the level of cesium-137 in rice on the fallout rate than either of the cumulative ground deposit or the N-ammonium acetate extractable cesium-137 in soil. Variation with locality is great and the highest rice-level is observed on the northern Japan Sea side, the lowest in the central and southern parts, and intermediate on the northern Pacific side.
In Asian countries, paddy rice (Oryza sativa) has been cultivated as a main crop for many years, but cesium uptake by rice has only been studied recently. As a result, the effect of land use on cesium uptake by rice is not well understood, and there is little information about the interaction of cesium with alkali metals in the rice–soil system. The objective of this study was to elucidate the stable cesium (Cs), potassium (K), and rubidium (Rb) levels in rice cultivated under paddy and upland conditions and in the soil (a Hapludept). We cultivated 13 rice cultivars (temperate and tropical japonica) under both conditions, and determined the concentrations of Cs, K, and Rb in brown rice and soil (total, exchangeable, and water-soluble fractions) by means of inductively coupled plasma mass spectrophotometry and atomic absorption spectrophotometry. We found that the soil concentrations of water-soluble Cs, K, and Rb were higher under upland conditions than paddy conditions, and Cs and Rb concentrations in brown rice were also higher under upland conditions. The upland conditions may increase soil ion concentrations by water evaporation, and the higher water-soluble Cs and Rb concentrations would increase their concentrations in brown rice. However, more K accumulated in brown rice under paddy conditions. This suggests that the roots absorbed K rather than Cs and Rb from the soil mainly by means of ion diffusion, because paddy conditions would increase ion diffusion but not the soil ion concentration. We conclude that the land use (here, paddy and upland conditions) changed the Cs, K, and Rb levels in the brown rice and soil by different mechanisms because root uptake of these elements would be different.
In this Chapter we consider radionuclide uptake and translocation in tropical crops and ecosystems. There are many commonalities across all ecosystems because of the consistent, underlying mechanisms controlling the fate and behaviour of radioactivity in any environment. The basic radioecological concepts and models are described to cover these processes.However, the tropics and sub-tropics include much dissimilarity by way of soil types, agricultural methods, climate, plants and animals which give rise to different outcomes from those processes. Billions of people across the tropics and sub-tropics are supported by agricultural systems very different from those traditionally applied in more developed regions of the planet. Higher populations will do so in the foreseeable future. Given the push for nuclear developments in the region, the tropics will need greater attention now. Much of the science is under review but the available data, pertinent to tropical systems, has been summarised or the database identified for the reader.The conditions of tropical soil types and the factors influencing radionuclide biogeochemistry (which affects bioavailability and bioaccessibility) are discussed. Specific sections covering rice, tropical fruits and the limited data for tropical animals are included.
The levels of Sr and Cs in human placenta, fetus and diet were determined by radiochemical methods during the period from March 1966 to December 1968. The possible effects of these radionuclides on the pregnant woman and the fetus were discussed. The metabolic characteristics of both Sr and Cs were also discussed on the basis of estimates of discrimination factors and observed ratios of these radionuclides.As a result of the analyses, the dose rate to the human placenta resulting from deposition of Sr in the organ was estimated at about 0.003 mrem/month and that of Cs at about 0.013 mrem/month. The dose rate to the total body resulting from the body burden of these radionuclides in the mother was estimated at about 0.012 mrem/ month. These values are much smaller than the monthly dose from K normally existing in the body.It was rather difficult to draw definitive conclusions concerning the possible correlation between the Sr fallout and the annual changes of infant mortality rates in Japan.
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