Temporal dynamics of ¹³⁷Cs distribution in soil and soil-to-crop transfer factor under different tillage systems after the Fukushima Daiichi Nuclear Power Plant accident in Japan

The accident at the Fukushima Daiichi Nuclear Power Plant (FDNPP) in Japan in 2011 released a large amount of radionuclides, primarily radiocesium-137 (¹³⁷Cs; half-life: 30 years), resulting in long-term contamination of soil and consequently crops. Tillage is a common agricultural management practice that alters the vertical distribution of nutrients in the soil. However, the effect of tillage on ¹³⁷Cs contamination in soil and crops over time remains unclear. In this study, we investigated the temporal changes in the vertical distribution of ¹³⁷Cs in the soil, concentration of ¹³⁷Cs in soybean and cover crops, and the transfer factor (TF) of ¹³⁷Cs from the soil to crops under three tillage systems (rotary cultivation [RC], moldboard plow [MP], and no tillage [NT]; main factors) using three cover crops (hairy vetch, winter rye, and fallow weeds; side factors). The amount of ¹³⁷Cs in the soil decreased exponentially with soil depth under the NT and RC treatments. By contrast, ¹³⁷Cs showed uniform distribution at each soil depth tested under the MP treatment since 2012. The exchangeable ¹³⁷Cs demonstrated a similar tendency as ¹³⁷Cs. The ¹³⁷Cs concentration in soybean (including grain and residue) and cover crops decreased exponentially with time. Consistently higher ¹³⁷Cs concentration was observed in soybean grains under the NT treatment, suggesting that tillage continuously reduced the concentration of ¹³⁷Cs in soybean over 7 years since the FDNPP accident. The TF of ¹³⁷Cs from soil to soybean and cover crops decreased continuously over time; however, ¹³⁷Cs concentration of soybean grain showed a positive linear correlation with its annual variation rate. Additionally, TF showed a positive logarithmic correlation with ¹³⁷Cs relaxation depth in the soil. These results enhance our understanding of the long-term behavior and radioecology of ¹³⁷Cs in agroecosystems in Japan since the radionuclide accident.