Influence of Soil Characteristics and Land Use Type on Existing Fractions of Radioactive 137Cs in Fukushima Soils

Radioactive cesium (137Cs) in distinct soil fractions provides key information to its bioavailability, and therefore determining the effect of soil characteristics and land use types on existing fractions of 137Cs in soils is important for predicting future 137Cs mobility in Fukushima. Thus, the objective of this study was to investigate the influence of soil characteristics and land use types on sequentially extracted fractions of 137Cs in Fukushima. In this study, five coniferous forest soils, eight arable soils, and eight paddy rice soils were sampled in 2012 and 2013. The 137Cs in the soils were separated into four fractions; water-soluble, exchangeable, organic matter-bound and residual fractions. More than 90% of the soil 137Cs fraction for arable and paddy rice soils was found in the residual fraction, implying significantly reduced bioavailable 137Cs with higher fixation. In contrast, forest soils measured higher exchangeable and organic matter-bound fractions of 5%–33% and 9%–44%, respectively, implying future 137Cs mobility in the forest ecosystem. Correlation analysis showed a significant negative correlation (p < 0.05) between the organic matter fraction and residual fraction in both arable and paddy rice soils. There was a significant positive correlation (p < 0.05) for both exchangeable and residual fractions with cation exchange capacity (CEC), total carbon (TC) and total nitrogen (TN) values in arable soils. Organic matter content influenced both exchangeable and residual fractions. It was not clear whether organic matter played a direct role in 137Cs fixation or mobility in the agricultural soils. In paddy rice soils, the organic matter fraction showed a significant negative correlation with TC and TN values. Soil pH was significantly negatively correlated (p < 0.05) with both water-soluble and residual fractions in forest soils but positively (p < 0.1) with the organically bound 137Cs fraction.