AGING EFFECT ON BIOAVAILABILITY OF MN, CO, ZN AND TC IN JAPANESE AGRICULTURAL SOILS UNDER WATERLOGGED CONDITIONS

Radionuclides or stable isotopes added to soils can change their forms physico-chemically and their fixation by soils can occur as a functio n of time. Thus, the amounts of these radionuclides or elements enteri ng the food chain decrease gradually with time, because of the aging e ffects. In this study, Ca-exchangeable and strongly adsorbed fractions of manganese (Mn-54), cobalt (Co-60), zinc (Zn-65) and technetium (Tc -95m) in two typical agricultural soils in Japan were observed after b eing freshly introduced into the soil samples under aerobic and anaero bic conditions. The Ca-exchangeable fractions of Mn-54, Co-60 and Zn-6 5 that varied slightly with soil type decreased rapidly to almost zero within a few days. The strongly adsorbed fractions of these radionucl ides decreased over time, although the rates of decrease were differen t. No obvious difference was recognized in either of the two fractions under aerobic or anaerobic conditions. However, it was clear that the amounts of Tc-95m in both fractions under the waterlogged condition w ere much lower than those under the upland field condition. The bioava ilable Te was presumably influenced by the soil redox conditions. Sinc e the anaerobic condition was established by waterlogging, Tc would be reduced to insoluble forms and sorbed onto the soil. (C) 1998 Elsevie r Science B.V. All rights reserved.