Analysis of Factors Causing High Radiocesium Concentrations in Brown Rice Grown in Minamisoma City

Abstract

Despite a concentration of exchangeable K of >208 mg kg−1 dry weight in soil, the brown rice grown in Minamisoma City in 2013 had a higher concentration of radiocesium than the new Japanese standard (100 Bq kg−1) for food. To analyze the factors affecting the radiocesium concentration in brown rice, we carried out pot tests using paddy soil and irrigation water collected in Minamisoma City. Rice seedlings were planted in 5-L pots containing Minamisoma soil, in which the exchangeable K was 125 mg kg−1 dry weight, and were irrigated with tap water or irrigation water collected in Minamisoma City. There was no difference in the Cs-137 concentration in brown rice between the two types of irrigation. Then we grew rice in the Minamisoma soil and two soils collected in Nakadori, Fukushima Prefecture. Cs-137 uptake in the Minamisoma soil was intermediate between the uptake rates in the Nakadori soils, showing that the Minamisoma soil was not special in radiocesium uptake. Finally, we grew rice in soil without radiocesium near the Fukushima Daiichi Nuclear Power Plant in 2014. Although the maximum value of Cs-137 in brown rice was 18 Bq kg−1, below the standard, radiocesium was attached to the surface of the foliage.