Improvement in potassium budget with water outflow reduction through reducing excessive irrigation in a paddy field

ABSTRACT 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.