Estimation of rooting depth of 137Cs uptake by plants

Understanding the soil-to-plant transfer process of 137Cs is essential for predicting the contamination levels of plants in contaminated areas. The rooting depth is considered one of the key factors explaining the difference in the activity concentration of 137Cs in different plant species. In this study, the distributions of 137Cs and 133Cs in soils and plants were investigated, and the plants' rooting depth of 137Cs uptake was estimated using the 137Cs/133Cs ratios in exchangeable fractions of soils and biological samples. The results showed that different plant species accumulate different levels of 137Cs and 133Cs. The 137Cs/133Cs ratios were fairly constant in plants of the same species. The average 137Cs/133Cs ratios in bamboo grasses and ferns were 0.015 ± 0.009 (n = 5) and 0.13 ± 0.04 Bq ng−1 (n = 10) in Yamakiya, respectively. The percentage of 137Cs in the exchangeable fraction of the uppermost soil layer was lower than that in the deeper soil layers. The activity concentrations of 137Cs in the soil profiles decreased sharply with depth, whereas the depth distributions of 133Cs were uniform. Therefore, the 137Cs/133Cs ratios were driven mainly by the 137Cs activity concentrations in soil. The plants’ rooting depths of 137Cs uptake were estimated on the basis of the relationships between the averaged 137Cs/133Cs ratio in the soil layer and the 137Cs/133Cs ratio in the plant. The results indicate that the deeper-rooted species such as bamboo grasses have a lower accumulation of 137Cs than the superficial-rooting species such as ferns. The soil-to-plant transfer factors would be determined using rooting depth by calculating the averaged activity concentration of 137Cs within the estimated rooting depth.