Evaluation of 137Cs ageing by dynamics of 137Cs/133Cs ratio in Andosol paddy fields with/without potassium fertilizer application

Shokichi Wakabayashi; Shigeru Takahashi; Hisaya Matsunami; Shioka Hamamatsu; Mayumi Hachinohe; Nobuharu Kihou; Noriko Yamaguchi

doi:10.1016/j.jenvrad.2020.106252

Evaluation of ¹³⁷Cs ageing by dynamics of ¹³⁷Cs/¹³³Cs ratio in Andosol paddy fields with/without potassium fertilizer application

J Environ Radioact. 2020 Jul:218:106252. doi: 10.1016/j.jenvrad.2020.106252. Epub 2020 Mar 28.

Authors

Shokichi Wakabayashi¹, Shigeru Takahashi², Hisaya Matsunami³, Shioka Hamamatsu⁴, Mayumi Hachinohe⁵, Nobuharu Kihou⁶, Noriko Yamaguchi⁷

Affiliations

¹ Tohoku Agricultural Research Center, NARO (National Agriculture and Food Research Organization), 50 Harajukuminami, Arai, Fukushima, Fukushima, 960-2156, Japan. Electronic address: shokichi@affrc.go.jp.
² Central Region Agricultural Research Center, NARO, 2-1-18 Kannondai, Tsukuba, Ibaraki, 305-8666, Japan. Electronic address: shigeru@affrc.go.jp.
³ Tohoku Agricultural Research Center, NARO (National Agriculture and Food Research Organization), 50 Harajukuminami, Arai, Fukushima, Fukushima, 960-2156, Japan. Electronic address: hisaya@affrc.go.jp.
⁴ Headquarters, NARO, 3-1-1 Kannondai, Tsukuba, Ibaraki, 305-8517, Japan. Electronic address: shioka@affrc.go.jp.
⁵ Food Research Institute, NARO, 2-1-12 Kannondai, Tsukuba, Ibaraki, 305-8642, Japan. Electronic address: mhach@affrc.go.jp.
⁶ Institute for Agro-Environmental Sciences, NARO, 3-1-3 Kannondai, Tsukuba, Ibaraki, 305-8604, Japan. Electronic address: kihou@affrc.go.jp.
⁷ Institute for Agro-Environmental Sciences, NARO, 3-1-3 Kannondai, Tsukuba, Ibaraki, 305-8604, Japan. Electronic address: nyamag@affrc.go.jp.

PMID: 32421576
DOI: 10.1016/j.jenvrad.2020.106252

Abstract

The mobility of ¹³⁷Cs in soil decreases with time owing to fixation by micaceous minerals. Such ageing is a critical parameter for estimating and predicting annual change in ¹³⁷Cs contamination risk of agricultural products. The decrease in the exchangeable fraction of ¹³⁷Cs has traditionally been used as an index of the ¹³⁷Cs ageing. Under field conditions, however, exchangeable ¹³⁷Cs is influenced by several environmental factors. In this study, we propose a new index to evaluate the ¹³⁷Cs ageing with minimum influence of environmental factors. The ratio of the exchangeable ¹³⁷Cs fraction to exchangeable fraction of ¹³³Cs ((¹³⁷Cs/¹³³Cs)_exch) eliminates the influence of environmental factors on exchangeable ¹³⁷Cs. We assessed the applicability of the (¹³⁷Cs/¹³³Cs)_exch index, using a four-year field study of a rice paddy in allophanic Andosol, starting 200 days after the Fukushima Dai-ichi Nuclear Power Plant accident. The influence of K fertilization was also investigated. The ¹³⁷Cs and ¹³³Cs exchangeable fractions varied together, indicating that both were similarly controlled by environmental factors. The values of (¹³⁷Cs/¹³³Cs)_exch decreased with time, reflecting ¹³⁷Cs fixation by the ageing. The half-time of the (¹³⁷Cs/¹³³Cs)_exch decline was 6.6-17.7 years. Relative to K fertilization, the lack of K fertilization seemed to affect the ¹³⁷Cs ageing in two ways: the early ¹³⁷Cs fixation progressed more rapidly, probably because fewer competing K⁺ ions were present, and the long-term ageing process was occasionally hampered, probably by the release of reserve K from micaceous minerals. The (¹³⁷Cs/¹³³Cs)_exch values were similar to the ratio of the ¹³⁷Cs to ¹³³Cs transfer factor of the rice straw. Thus, we conclude that the (¹³⁷Cs/¹³³Cs)_exch index is reliable for evaluating the ¹³⁷Cs ageing, decrease in ¹³⁷Cs mobility caused by the diffusion into micaceous mineral interlayer, in the field.

Keywords: Ageing; Andosol; Cesium-137; Exchangeable fraction; Isotope ratio; Potassium fertilization.

MeSH terms

Cesium Radioisotopes / analysis*
Fertilizers
Fukushima Nuclear Accident*
Ions
Japan
Minerals
Potassium
Radiation Monitoring*
Soil Pollutants, Radioactive*

Substances

Cesium Radioisotopes
Fertilizers
Ions
Minerals
Soil Pollutants, Radioactive
Cesium-137
Potassium