Annual Reduction of Transfer Factors of Radiocesium from Soil to Rice Cultivated in a KCl Fertilized and Straw Plowed-in Paddy Field from 2015 to 2021

Abstract

AbstractWe performed consecutive field trials of rice cultivation for 10 years (2012–2021) to elucidate how to reduce radiocesium absorption by rice and to resume safe rice cultivation in a partially decontaminated paddy soil in the Iitate Village in Fukushima prefecture, Japan. Our previous report (Ii and Tanoi (2016) Consecutive field trials of rice cultivation in partially decontaminated paddy fields to reduce radiocesium absorption in the Iitate village in Fukushima prefecture. In: Nakanishi TM, Tanoi K (eds) Agricultural implications of the Fukushima nuclear accident. Springer, Tokyo, pp 55–76) of the results in 2012 and 2013 showed that K fertilization can reduce the radiocesium level of the brown rice less than the standard for food (100 Bq/kg). This report of the subsequent years (2014–2021) shows more than 80% reduction of 137Cs concentration in the brown rice and straw at KCl fertilized and straw plowed-in paddy soil. The transfer factor of 137Cs from soil to brown rice reduced from 0.0022 in 2015 to 0.0003 in 2019 and that to straw reduced from 0.0262 in 2015 to 0.0028 in 2019, respectively. Exchangeable positive ions of the soil were also analyzed. Multiple regression analyses of all data of transfer factor in 2015–2019 to year (ageing) and exchangeable K ion as variables show that the main causal factor is year (ageing) with some supportive effect of increase of exchangeable K ion. This implicates that radiocesium in soil was gradually transformed to a form more difficult to be absorbed by rice, that is, 137Cs immobilization or fixation on clay minerals by ageing, not only in early years after the accident (2011–2015), but also in later years (2015–2019). This implication was supported by comparative analysis of exchangeable 137Cs of dry soil of 2017, 2018, and 2019 (Ii et al., Radioisotopes 70:63–72, 2021). The results in 2020 and 2021 show those transfer factors have reached at bottom levels. Further this field work also shows the robustness of the rice cultivation that we could harvest rice far below the safety standard, even though the paddy field suffered floods by typhoons in September 2015 and in October 2019.