Cesium-137 concentration of forage corn and Italian ryegrass in a double cropping system under different rates of cattle farmyard manure application after the Fukushima Daiichi Nuclear Power Station accident in 2011

Radioactive ¹³⁷Cs concentrations of forage corn (Zea mays L.) and Italian ryegrass (Lolium multiflorum Lam.) in a double cropping system under continuous cattle farmyard manure (FYM) application were observed for more than 2 years after the Fukushima Daiichi Nuclear Power Station accident in 2011. The experiment field is located 110 km southwest of the Fukushima Daiichi Nuclear Power Station, and the soil contains ¹³⁷Cs of 920 Bq kg ⁻¹ on average. For crop cultivation, nitrogen fertilizer was applied in addition to FYM. The ¹³⁷Cs concentrations in corn decreased significantly between 2011 and 2012, but only differed significantly between 2012 and 2013 for the plot with no FYM application. For Italian ryegrass, no significant differences were observed between the harvest in 2012 and 2013 despite the FYM application rate. To minimize corn ¹³⁷Cs concentrations, the FYM application rate should be more than and equal to 30 Mg ha ⁻¹ when FYM is used as the major nutrient source. Exchangeable potassium oxide (K ₂O) greater than around 0.3 g kg ⁻¹ was mostly maintained with the FYM application rates. Corn ¹³⁷Cs concentration appeared to increase at exchangeable K ₂O levels below 0.15 g kg ⁻¹. These results suggest that continuous FYM application can maintain soil nutrients including K ₂O and thereby control radioactive Cs transfer from the soil. FYM application rate of 30 Mg ha ⁻¹ is within the levels recommended by the prefectural governments around Fukushima Prefecture for crop production before the accident. These levels are sufficient to decrease the radioactive Cs concentrations for corn. However, unlike corn, differences in soil chemical properties by FYM application did not affect ¹³⁷Cs concentrations in Italian ryegrass in this study, although low exchangeable K ₂O seemed to increase concentrations of stable ¹³³Cs. Further experiments should be conducted to understand the observed differences between corn and Italian ryegrass.