Vertical distribution of radiocesium affects soil-to-crop transfer coefficient in various tillage systems after the Fukushima Daiichi Nuclear Power Plant accident

After the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident in March 2011, radiocesium (Cs-134 and Cs-137) released into the atmosphere contaminated soil across a large part of northeastern Japan. In croplands, moldboard plow and rotary tillage systems redistribute the radiocesium to deeper layers, lowering the level of contamination. However, the long-term effects of tillage on radiocesium contamination of crops and the form and distribution of radiocesium in the soil after continuous tillage remain unclear. We investigated the distribution of radiocesium in soil and changes in plant uptake in organic soybean fields in Ibaraki (170 km from FDNPP) to clarify the tillage effect on radiocesium uptake while controlling contamination level. From 2011 to 2015, soybeans and winter cover crops (fallow weeds, rye, and hairy vetch) were cultivated using three tillage systems (moldboard plow/rotary harrow, MP; rotary cultivation, RC; and no tillage, NT). Radiocesium contamination in soil, and plant uptakes were considerably lower in 2012 compared with the first year of the accident in 2011. Four and 5 years after the accident, the NT treatment showed the highest absorption of radiocesium by soybeans and highest radiocesium deposit density in cover crops. Cover crop radiocesium contamination was significantly correlated with aboveground biomass. Soil surface (0-2.5 cm) radiocesium contamination in NT was significantly higher than at other depths and in other tillage treatments. A similar trend was observed in exchangeable radiocesium in the soil profile, although the values were only 23.9-48.0% (2014) and 32.6-71.9% (2015) of radiocesium in the soil. A positive correlation was also observed between exchangeable radiocesium at the soil surface and soybean radiocesium contamination, but this relationship was not observed for cover crop radiocesium contamination. The transfer factor (TF) of soybean grain and residue was highest in the NT treatment. The TF based on exchangeable cesium (eTF) was particularly high. Cover crops showed different TF and eTF values, but rye always had the lowest values. Soybean radiocesium contamination was higher in NT than in MP and RC. TF of soybean was significantly correlated with the depth of the vertical distribution of both radiocesium and exchangeable cesium. This research revealed that the TF value was significantly influenced by plant biomass accumulation and the soil radiocesium distribution.