Temporal trends of 137Cs and 40K activity flux from soil to plant in grassland ecosystems

A data set of 137Cs and 40K activity values in plant and soil samples, obtained under identical conditions during a four-year period, was analysed in order to determine the temporal trend of the soil-to-plant transfer process. The aggregated transfer factor (Tag) was determined in each sampling site, showing the typical seasonal variations and the well-known uncertainties. The activity flux density (FD) and the activity flux density per soil activity superficial concentration (J) were used as more suitable variables to quantify the radionuclide soil-to-plant transfer process. A negative correlation between log J(137Cs) and t was obtained (R=−0.99). In contrast, log J(40K) remains approximately constant with time (R=−0.67). The influence of the 40K content in soil on 137Cs soil-to-plant transfer is discussed. Temporal mean value of FD was 50 times larger for 40K than for 137Cs. However, comparing temporal mean values, J(40K) was 26 times J(137Cs). The soil concentration level of 137Cs and 40K explains only 50% of the higher uptake of K than Cs in plants. The J(40K):J(137Cs) relationship was five for the first year and 32 for the last year, a behaviour that could be attributed to the radiocaesium fixation to soil components.