Adsorption and fixation of radiocesium compared to properties, landuse and sampling location of contrasting soils from Taiwan

Adsorption and desorption of radiocesium has been measured on 57 soils from different climates, landuses and altitudes in Taiwan, and compared to soil properties. A large range in radiocesium adsorption (assessed by the distribution coefficient, Kd) was found. Largest values of Kd were observed for arable soils. These were sampled near sea-level, tended to have neutral to alkaline pH, and to be depleted in water-extractable potassium and organic matter. Kd values of soil from undisturbed and secondary forests fell in a smaller range, and were on average smaller than those of arable soils, and decreased with increasing altitude. Kd was not significantly related to clay content. A fraction of adsorbed radiocesium was not readily desorbed in a solution containing stable cesium. This fixed fraction, denoted f_fixed, tended to increase with increasing Kd, and was greater in arable soils. The f_fixed tended to decrease with increasing altitude for soils from undisturbed forests. Fixation increased with pH and was inversely related to carbon, extractable potassium and clay contents. We conclude that content of clay-sized fraction is a poor predictor of radiocesium dynamics, because small amounts of mineral clay are sufficient to adsorb trace amounts of radiocesium. Mineral weathering, controlled by landuse and climate, determines both affinity and fixation of radiocesium.