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Countermeasures to reduce radiocesium uptake by crops have been implemented in farmlands affected by the accident at Tokyo Electric Power Company's Fukushima Daiichi Nuclear Power Plant in 2011. A widely practiced countermeasure is the application of K. Long-term maintenance of soil K is crucial due to the long physical half-life of 137Cs (30 years). Plant-available K budgets can be used as an indication of sustainable maintenance. Our previous study found that large water outflow, especially percolation, associated with excessive irrigation caused major K outflow and a negative K budget in a paddy field in Fukushima in 2018. The present study evaluated whether decreasing the water outflow by reducing the irrigation volume from 6300 to 1900 mm reduced K outflow and improved the K budget in the field during the cropping period in 2019. K concentrations in soil solution were comparable in both years, and the percolation water volume decreased to 37% that of 2018, resulting in a 53% reduction in the K output from 392 to 184 kg ha–1. The input via irrigation also decreased (−35 kg ha–1), but the output via surface runoff showed a greater reduction (−58 kg ha–1) owing to a 70% decrease in the water volume compared with 2018. Consequently, the K budget was substantially improved (from −289 to −57 kg ha–1), highlighting the importance of controlling water outflow, particularly percolation, for soil K maintenance in paddy fields with high permeability. [ABSTRACT FROM AUTHOR]

An in situ model paddy field experiment was conducted using non-contaminated soil to identify factors that locally increase the radiocesium activity concentrations in rice near the water inlet of paddy fields. The results demonstrated that irrigation water intake reduced exchangeable potassium concentration and increased radiocesium activity concentration in the soil near the water inlet; thus, the radiocesium activity concentration in rice was negatively correlated with the former and positively with the latter. Exchangeable potassium flushing and deposition of suspended solids in the soil owing to irrigation water intake facilitated the transfer of radiocesium to rice plants near the water inlet. [ABSTRACT FROM AUTHOR]

The reasons why rice 137Cs concentrations increase near the water inlet were investigated using rectangular experimental plots settled in paddy fields. The 137Cs concentrations in brown rice and rice straw were highest at 1 m from the inlet and decreased gradually with increasing distance, showing a negative correlation with soil exchangeable potassium content (Ex-K). The 137Cs concentration in brown rice was negatively and positively correlated with yield and sterility rate, respectively. These results suggest that soil Ex-K reduction and rice anomalies occurred near the water inlet and could contribute to the local elevation of 137Cs concentrations in rice. [ABSTRACT FROM AUTHOR]