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

In this study, we evaluated the mobility of 90Sr 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 90Sr in soil and total 90Sr in wheat grain were determined. The concentration of exchangeable 90Sr in ploughed soil decreased exponentially with time. The environmental factor responsible for the decrease of exchangeable 90Sr 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 lambda(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 90Sr from ploughed upland soil and 90Sr uptake by wheat grain was the cation-exchange capacity (CEC) of soil. Our experimental results indicate that the entrapment of 90Sr on a cation-exchange site retards the downward migration and wheat uptake of 90Sr 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 90Sr in ploughed soil and soil-borne 90Sr transfer to wheat grain in the humid Japanese climate.