Whole-crop silage corn and Italian ryegrass were cultivated during summer and winter in fields contaminated by radionuclide fallout caused by the Fukushima Daiichi Nuclear Power Plant accident, with three different tillage treatments: shallow tillage treatment (tilled with a rotary tiller to a depth of about 10 cm), conventional tillage treatment (plowed with a normal moldboard plow to a depth of about 20 cm and harrowed with a rotary tiller to a depth of about 15 cm), and deep tillage treatment (plowed with a moldboard plow to a depth of about 35 cm and harrowed with a rotary tiller to a depth of about 15 cm). Vertical distribution of radioactive cesium (Cs) in the soil layers of 0–10 cm, 10–20 cm, and 20–30 cm, and concentrations of radioactive Cs in forage samples were compared among the tillage treatments, as well as the soil chemical properties of those soil layers.
Radioactive Cs in the soil surface layer (0–10 cm) moved into the deeper layers at 10–20 cm and 20–30 cm due to plowing in the conventional and deep tillage treatments. However, significant differences were not observed for both species, either in the radioactive Cs concentration in forage samples or the radioactive Cs transfer factor from soil to plants among the tillage treatments. Moreover, the radioactive Cs concentrations in those plants and their TFs were relatively low for both species. These results suggest that radioactive Cs transfer was reduced by mixing the surface soil, even in the shallow tillage treatment. Furthermore, the exchangeable K2O content of soil was higher than 0.32 g/kg DW in all soil layers of the experimental fields, and such high content of exchangeable K2O in the soil was apparently another major reason why radioactive Cs uptake by both species was significantly restricted in all tillage treatments.