Comparative dynamics of potassium and radiocesium in soybean with different potassium application levels

We conducted a field experiment in soybean with different levels of K application to elucidate the comparative dynamics of ¹³⁷Cs and K. The inventory of K in the shoots increased substantially from the fifth trifoliate stage to the full seed stage, and as the absorption of K increased, so too did the absorption of ¹³⁷Cs. Overall, the effect of K application was much greater in terms of ¹³⁷Cs dynamics than K dynamics or biomass production. K application reduced not only the accumulation of ¹³⁷Cs in the shoots, but also the distribution of ¹³⁷Cs to the grains. However, the decrease of ¹³⁷Cs distribution to the grain had a much smaller effect on ¹³⁷Cs accumulation in the grains than ¹³⁷Cs absorption. A positive correlation was also observed between the exchangeable ¹³⁷Cs/K ratio in the soil and the ¹³⁷Cs/K ratio in the shoots for each growth stage, and the ¹³⁷Cs/K ratios in the shoots at the full seed and full maturity stage were much higher than those at the fifth trifoliate and full bloom stage under the same exchangeable ¹³⁷Cs/K ratio in the soil. These findings suggest a decrease in the discrimination of ¹³⁷Cs from K during absorption after the full bloom stage. As a result of this and the increase in soil-exchangeable ¹³⁷Cs/K with growth, radiocesium was more transferable to the shoots after the full bloom stage. Overall, these results suggest that lowering the soil-exchangeable radiocesium/potassium ratio after the full bloom stage by increasing K availability could efficiently reduce the transfer of radiocesium to the grains.