Yearly changes in radioactive cesium concentrations and quantities in tea plants and tea garden soil and factors affecting yearly changes in Chiba Prefecture

Our task was to measure the radioactive cesium concentrations and quantities in tea plants and tea garden soil in fiscal years 2011 - 2014. We studied three tea gardens that had been medium-pruned as a cesium reduction measures, as well as an abandoned tea garden. We investigated the effects of medium pruning, plucking and skiffing on the reduction of radioactive cesium in tea plants. 1. In both the three gardens subjected to cultivation and the abandoned garden tea plants, the radioactive cesium concentration of skiffed parts, the leaf layer, other aboveground parts, and underground parts fell year by year. Setting the sup(137)Cs concentration in 2011 at 100, its concentration in 2014 had fallen to 2 - 18 in the aboveground parts at both sites, and to 27 - 59 in underground parts. Similarly, at both sites, sup(137)Cs quantities in 2014 had fallen to 2 - 41 for aboveground parts and to 38 - 73 in underground parts. Setting the radioactive cesium concentration of crude tea of the first crop in 2011 at 100, the concentration in crude tea had fallen to 0.2 - 1.2 after four years. 2. The difference between sup(137)Cs concentrations in tea plants in both sites decreased year by year, although sup(137)Cs quantities in other aboveground parts and underground parts that have a large volume and weight contained larger quantities. 3. The sup(137)Cs concentration in the organic layer under the tea canopy was much higher, at 1.4 - 23.8-fold that of the sup(137)Cs concentration seen in the soil at depths of 0 - 5 cm in the years 2012 - 2014. In that three-year period, whereas sup(137)Cs concentration showed a generally downward trend in the organic layer, it showed an upward trend in the soil at depths of 0 - 5 cm depth under the tea canopy. There was little change at greater depths of 5 - 25 cm. Radioactive cesium present in the organic layer tended to move gradually but directly downwards into the soil. 4. Our results suggest that medeium pruning causes the greatest reduction of radioactive cesium in tea plants subjected to cultivation, followed by skiffing and plucking in the fallout year. In addition, these suggest that natural loss, such as defoliation and death of whole branches, causes primarily the reduction of radioactive cesium in abandoned tea plants.