Radiocaesium bioavailability to flooded paddy rice is related to soil solution radiocaesium and potassium concentrations

Aims This study was established to better understand the mechanisms of the variable radiocaesium bioavailability observed in flooded rice. Methods Paddy topsoils ( n = 9) with contrasting soil properties were collected from the Fukushima-accident affected area, spiked with 134 Cs and cropped in the greenhouse with rice under flooded conditions. Soil solution was collected. As a reference to earlier work, we cropped the same paddy soils and additional grassland soils ( n = 22 in total) with ryegrass under unsaturated conditions. Results Rice shoot 134 Cs concentrations varied 110-fold among soils and were unrelated to soil solution 134 Cs concentrations. However, the rice shoot 134 Cs concentrations clearly increased with the soil solution 134 Cs:K concentration ratio ( R 2 = 0.79; P < 0.001). The same trend was true for ryegrass shoot 134 Cs concentrations, which were, on average, 4.1-fold larger than those for flooded rice at an equal soil solution 134 Cs:K ratio. Conclusions Soil solution radiocaesium and K concentrations explain the radiocaesium bioavailability to flooded rice, confirming established knowledge for ryegrass. Transport modelling suggests that the overall smaller 134 Cs bioavailability to flooded rice than to ryegrass in unsaturated soils is related to the smaller K uptake rate of rice, alleviating K depletion in its rhizosphere with locally higher K + , blocking the root uptake of radiocaesium.