Radiocesium Distribution Caused by Tillage Inversion Affects the Soil-to-Crop Transfer Factor and Migration in Agroecosystems

This study reports the migration of cesium-137( 137 Cs) into deep soil layers, and the 137 Cs transfer from soil to soybean in farmland under three tillage(no tillage, NT; rotary cultivation, RC; moldboard plow; MP) treatments and an undisturbed grassland(GL) at eight years after the Fukushima Dai-ichi Nuclear Power Plant(FDNPP) accident on 11 March 2011 in Japan. Tillage influences the 137 Cs distribution in the 0‒30 cm of soil; the distribution of 137 Cs in the soil was uniform under RC and MP treatments, while in the grassland, most 137 Cs was concentrated on the soil surface. The center of vertical 137 Cs activity concentration(the thickness of the soil from surface which containing half of the 137 Cs inventory) in GL was 5.5cm, which was shallower than that in farmland (9.5cm in NT, 13.6cm in RC and 15.2cm in MP). Hence, the total migration distance of 137 Cs 8 years after FDNPP accident showed the following trend: GL(2.4cm) < NT(7.0cm) < RC(10.0cm) < MP(12.3cm). Meanwhile, a significant positive correlation was observed between 137 Cs activity concentration and organic carbon and nitrogen content in the soil. On the other hand, the 137 Cs activity concentration in soybean grains was negatively correlated with the center of vertical 137 Cs activity concentration but positively correlated with the ratio of exchangeable 137 Cs(ExCs) and K content in the soil. The ExCs/K and 137 Cs distributions in the soil were combined into a statistical model to predict the 137 Cs activity concentration in soybean grain. The results revealed the magnitude of the impact of 137 Cs distribution on the 137 Cs transfer from soil to crop. The addition of the 137 Cs distribution dramatically improved the prediction model of 137 Cs activity concentration in soybean