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

摘要

Radioactive Cs-137 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 2years after the Fukushima Daiichi Nuclear Power Station accident in 2011. The experiment field is located 110km southwest of the Fukushima Daiichi Nuclear Power Station, and the soil contains Cs-137 of 920Bq kg(-1) on average. For crop cultivation, nitrogen fertilizer was applied in addition to FYM. The Cs-137 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 Cs-137 concentrations, the FYM application rate should be more than and equal to 30Mgha(-1) when FYM is used as the major nutrient source. Exchangeable potassium oxide (K2O) greater than around 0.3gkg(-1) was mostly maintained with the FYM application rates. Corn Cs-137 concentration appeared to increase at exchangeable K2O levels below 0.15gkg(-1). These results suggest that continuous FYM application can maintain soil nutrients including K2O and thereby control radioactive Cs transfer from the soil. FYM application rate of 30Mgha(-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 Cs-137 concentrations in Italian ryegrass in this study, although low exchangeable K2O seemed to increase concentrations of stable Cs-133. Further experiments should be conducted to understand the observed differences between corn and Italian ryegrass.