The Fukushima accident in 2011 released radiocaesium (RCs) isotopes to the environment and renewed our interest in the radiation risks via the consumption of contaminated food. Radiocaesium (chiefly 137Cs) is of prime interest because 137Cs has its physical half-life of 30 years and because RCs can be potentially transferred from soil to the food chain via plant uptake. The soil mineralogy and agricultural management in the Fukushima affected area are different from those in Europe and, hence, the models to predict RCs transfer developed for temperate European soils in the post-Chernobyl studies required a critical evaluation. The objectives of the present research were to investigate the key factors affecting the RCs sorption in soils in the affected area and to better understand the mechanisms of RCs transfer from soils to flooded rice.
Representative soils were collected from the Fukushima accident affected area. The quantification of the RCs sorption in these soils in terms of their radiocaesium interception potential (RIP) suggested that the selective RCs sorption per unit clay mineral in the Fukushima soils was, on average, smaller than that for temperate European soils. A statistical model was developed to predict RIP from soil properties available in the soil maps in Japan, i.e. cation exchange capacity and soil organic matter content. A soil RIP map was created which indicates the spatial variability of potentially vulnerable soils for RCs transfer. The existing models developed for European soils was evaluated to predict RCs transfer for Fukushima soils. This prediction confirmed that parameters in these models need to be adjusted for typical soils in the Fukushima affected area. Pot trials showed that the mechanisms of RCs uptake by rice roots from flooded soils are identical to those in unsaturated soils, i.e. the plant RCs concentration increases with decreasing RCs sorption (increasing RCs mobility) and with decreasing potassium (K) concentration in soil solution. Potassium is a nutrient analogue that competes for Cs at root uptake. A new finding is that a submerged stem base of flooded rice contributes to RCs uptake and, hence, that the RCs concentration in irrigation water can affect the RCs concentration in rice. Reducing K supply to roots not only increased root uptake of RCs but also increased RCs uptake via the stem base of rice. This result suggested that the foliar RCs uptake via the stem base is controlled by the internal K status of rice plants.
This study supported the earlier developed concept that root uptake of RCs is controlled by the RCs:K concentration ratio in soil solution. It was shown that the existing models require recalibration to account for the differences in soil mineralogy between Fukushima and European soils. The role of irrigation water on RCs transfer to rice via a stem base of rice needs to be tested in field studies.
|Qualification||Doctor of Science|
|Date of Award||20 Mar 2017|
|Place of Publication||Leuven, Belgium|
|State||Published - 20 Mar 2017|