Seasonal variations in soil-to-grass transfer of fallout strontium and cesium and of potassium in North German soils

The uptake of 134Cs, 137Cs, 90Sr and 40K by grass plants growing on permanent pastures was studied between May 1990 and November 1992. Four experimental sites were chosen including both mineral and organic soils. At each site, some pasture plots were cropped repeatedly in order to simulate grazing of cattle, while other plots were cropped only once during the vegetation period. For all soils and radionuclides investigated, nuclide concentrations in grass showed marked seasonal fluctuations, though no regular pattern could be demonstrated in successive years or at different sites. No correlations with air temperature or rainfall were found, but cesium and strontium concentrations in plants tended to be negatively correlated with the soil moisture within the rooting zone which was calculated using the OPUS code. This result indicates that root uptake rates are controlled by concentrations of ions in solution which increase with decreasing moisture content of the soils. Differences in transfer factors between grass plants growing on different soils ranged up to a factor of about 100 for 137Cs, but were less than one order of magnitude for 90Sr. Transfer factors were higher if grass plants were cropped repeatedly. This effect might be attributed to the plant treatment: grass plants which are defoliated repeatedly develop a more shallow root system resulting in preferential uptake from the uppermost soil layers where concentrations of radionuclides are highest. Uptake of cesium by grass growing on the two organic soils was high enough to differentiate between Chernobyl and weapon fallout cesium. Transfer factors of Chernobyl cesium were consistently higher, and for one of the sites, showed a marked decrease with time. As the results of a simple model show, these observations can be explained by the impact of the differing depth distributions of the cesium fractions within the rooting zone of the grass plants. It is concluded that in interpreting soil-to-plant transfer data the distribution of the radionuclides in the plant rooting zone should receive much attention.