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

To understand the behavior of 137Cs in undisturbed soils after nuclear fallout deposition between the 1940s and 1980s, we investigated the speciation of 137Cs in soils in forest and its adjacent grassland from a volcano and subalpine area in Taiwan. We performed sequential extraction of 137Cs (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, 137Cs was mainly present in the persistently bound (31–41%) and residual (22–62%) fractions. The proportions of 137Cs 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 137Cs was mostly transferred to the persistently bound and resistant fractions after long-term deposition of fallout. The readily exchangeable 137Cs fraction was higher in soils with higher organic matter content or minor amounts of 2:1 silicate clay minerals.