Cadmium fixation in soils measured by isotopic dilution

There is conflicting evidence on the effect of time of contact between soil and Cd on Cd availability to plants. If Cd can be fixed in soil by aging, higher soil contamination may be tolerated. Fixation of Cd by soil can be studied by adding small quantities of Cd-109 to the indigenous soil Cd, The ratio of Cd-109 to indigenous Cd in soil extracts or in plants gives information on the lability of Cd in soil. This isotope exchange technique was used to measure the labile and fixed Cd fractions in 10 Belgian agricultural soils (Soils A-I) with both back-ground and elevated Cd content (0.33-6.5 mg Cd kg(-1)). The isotopically exchangeable Cd pool (E value) was measured after equilibrating Cd-109 spiked soil suspensions in CaCl2 0.01 M for 7 d. The %E values (the E value relative to aqua regia soluble Cd) ranged from 62 to 90% in the eight soils where %E values could be detected. The plant labile Cd pool, relative to aqua regia soluble Cd (%L value) was measured from the specific activities in wheat (Triticum aestivum L.) seedlings grown for 16 to 21 d on soils spilled with Cd-109. The Cd %L value varied from 55 to 109% (mean: 82%) with five soils having a significant (P < 0.05) fixed Cd fraction. Varying the soil incubation procedure after soil spiking and before plant growth marginally affected the specific activity of Cd in plants, The %L values always exceeded the respective %E value between 1.05- and 1.4-fold. It is concluded that Cd fixation, where found, is not very pronounced.