Long-term mobility of fallout Sr-90 in ploughed soil, and Sr-90 uptake by wheat grain

In this study, we evaluated the mobility of Sr-90 in ploughed upland soil, which affects the residual amount in the soil and plant uptake on the basis of long-term monitoring data. Paired samples of soil and wheat grain were taken annually from 1961 to 1995 from 8 agricultural fields in Japan, and the concentrations of exchangeable Sr-90 in soil and total Sr-90 in wheat grain were determined. The concentration of exchangeable Sr-90 in Ploughed soil decreased exponentially with time. The environmental factor responsible for the decrease of exchangeable Sr-90 in the ploughed layer lambda(e), was determined from the monitoring data of exchangeable 90Sr in the ploughed soil and the amount of fallout-derived deposition. The e was larger from 1970 to 1980 than it was from 1980 to 1995, suggesting that an easily removable fraction of 90Sr in soil was preferentially lost from ploughed soil. Among various soil properties that we investigated, the main factor controlling the long-term mobility of Sr-90 from ploughed upland soil and Sr-90 uptake by wheat grain was the cation-exchange capacity (CEC) of soil. Our experimental results indicate that the entrapment of Sr-90 on a cation-exchange site retards the downward migration and wheat uptake of Sr-90 from ploughed soil. The empirical parameters that we obtained based on the long-term observation of a wheat-cultivated upland field in Japan could be used as reference data in order to roughly estimate the mobility of Sr-90 in ploughed soil and soil-borne Sr-90 transfer to wheat grain in the humid Japanese climate. (c) 2006 Elsevier B.V. All rights reserved.