Changes in the radioactive cesium concentrations of grasslands during the first year after the Fukushima Daiichi Nuclear Power Plant accident in east Japan

Radioactive cesium (Cs) concentration of vegetation and soil was monitored in grasslands in seven farms located at a distance ranging from 90 to 180 km from the Fukushima nuclear power plant during seven months following the reactor meltdown in March 2011. The monitored sites included six sown meadows used to produce hay or silage, three sown pastures and one native pasture used for cattle grazing. The radioactive Cs concentrations of the soil ranged from 264–1593 Bq kg⁻¹ dry matter (DM). The radioactive Cs concentrations in vegetation (aboveground parts of dominant grasses) were high with values ranging from 639–19 823 Bq kg⁻¹ DM for the meadows, 949–7161 Bq kg⁻¹ DM for the sown pastures and 5088–358 549 Bq kg⁻¹ DM for the native pasture. Although the radioactive Cs concentrations tended to decrease over time in most grasslands, there was no clear decreasing trend for grassland soils low in exchangeable potassium concentration and clay content. The transfer of radioactive Cs from soil to herbage tended to be lower in soils with higher exchangeable potassium concentration and clay content. Detailed measurements in one meadow showed highest radioactive Cs concentration in surface litter, followed by standing dead and live plant material. Approximately, 71, 21 and 7% of radioactive Cs in the meadows were present in the soil, litter and standing dead material, respectively. Further regular monitoring of radioactive Cs concentration in grasslands in the affected areas surrounding the nuclear power plant is required to amend the existing guidelines regarding livestock feeding.