Saturday, 15 July 2006

Availability and Immobilization of 137Cs in Subtropical High Mountain Forest and Grassland Soils.

Chih-Yu Chiu1, Chih-Jung Wang2, and Ching-Chung Huang2. (1) Research Center for Biodiversity, Academia Sinica, Nankang, Taipei, Taiwan, (2) Radiation Monitoring Center, Atomic Energy Council, Kaohsiung, 833, Taiwan

Fallout from nuclear weapon testing has been deposited throughout Taiwan over the last 40 years. In order to ascertain the influence of long-term deposition at radioactive 137Cs in soils of forest ecosystems, the existence, forms and fate of 137Cs in forest and adjacent grassland soils were studied. This study investigates the distribution and migration of radioactive 137Cs in soils of forest ecosystems. Six soil samples collected from three sampling sites, including Histosols and Inceptisols from Yuanyang Lake, forest and grassland soils from middle level mountain at Yangming and from subalpine region at Tatajia, were analyzed for radioactive 137Cs. Six forms of 137Cs in surface soils, i.e., (I) readily exchangeable, (II) bound to microbial biomass, (III) bound to Fe-Mn oxides, (IV) bound to organic matter, (V) persistently bound and (VI) residual fractions were separated from above samples by a sequential extraction of 137Cs method. Radioactivity of 137Cs was measured by r-ray spectrometry. Although the soils from Yuanyang Lake were enriched with high organic matter, 46 and 83 % in Inceptisol and Histosol, respectively, only minor amounts of 137Cs were bound to organic matter (4-10%) and trace amounts (less than 3%) bound to readily exchangeable, microbial biomass and Fe-Mn oxides. Limited fractions of 137Cs existed in the labile and moderately labile forms of soil, which was consistent with those to organic C. Most of 137Cs were bound to the persistent (52-62%) and residual fractions (28-41%). The results from Yangming Mountain and subalpine region of Tatajia are very similar to those from Yuanyang Lake. The largest amount of 137Cs was bound to persistent and residual fractions in the surface soils. Two obvious differences can be observed by this research. One is that the soil in Yangming Mountain is a type of an amorphous volcano soil which has less fixing ability for 137Cs in soil. This increased the opportunity of plants and microbial biomass for using 137Cs of soil, so larger amounts of 137Cs (19%) bounded to microbial biomass are found both in forest and grassland soil in Yangming Mountain. Another is that the soil in Tatajia forest has up to 71% organic matter, so larger amounts of 137Cs are bound to organic matter (23.4%) than that in residual fractions (15.4%). In contrast, the surface soil at the Tatajia grassland contains more soil mineral than the forest soil. Most of 137Cs are bound to persistent (35%) and residual fractions (62%). Only trace amounts of 137Cs existed in readily exchangeable forms in the surface soil. After a long-term deposition (40 years) of fallout in Taiwan, the results from previously described three sampling sites suggest that most of 137Cs are fixed by the soil mineral in the surface layer to prevent 137Cs into deeper horizons via vertical migration. Forest soils have two characters in the composition one is composed mostly by organic matters such as the soils in Tatajia and Yuanyang Lake, or composed with minor amounts of 2:1 silicate clays in the soil of Yangming Mountain, so they have less ability to fix 137Cs with clay minerals in the surface soil which means the ratio of readily exchangeable 137Cs in such soils are higher than those in others types of soil. The labile form of 137Cs is mostly transferred to the stable and resistant fractions after a long-term deposition of fallout. It also supports previous findings that 137Cs recycles through the soil and plant systems and will not be easily removed from perhumid forest ecosystems.

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