Availability and immobilization of 137Cs in subtropical high mountain forest and grassland soils

To understand the behavior of (137)Cs in undisturbed soils after nuclear fallout deposition between the 1940s and 1980s, we investigated the speciation of (137)Cs in soils in forest and its adjacent grassland from a volcano and subalpine area in Taiwan. We performed sequential extraction of (137)Cs (i.e., fractions readily exchangeable, bound to microbial biomass, bound to Fe-Mn oxides, bound to organic matter, persistently bound and residual). For both the forest and grassland soils, (137)Cs was mainly present in the persistently bound (31-41%) and residual (22-62%) fractions. The proportions of (137)Cs labile fractions--bound to exchangeable sites, microbial biomass, Mn-Fe oxides, and organic matter--were lower than those of the recalcitrant fractions. The labile fractions in the forest soils were also higher than those in the grassland soils, especially in the volcanic soil. The results suggest that the labile form of (137)Cs was mostly transferred to the persistently bound and resistant fractions after long-term deposition of fallout. The readily exchangeable (137)Cs fraction was higher in soils with higher organic matter content or minor amounts of 2:1 silicate clay minerals.