Long-term transfer of 137Cs and 90Sr to grass on contrasting types of Swedish pastures

In 1961 two contrasting types of Swedish pastures were artificially contaminated with 90Sr and 137Cs. One site, Loevsta, was a natural pasture on a sandy soil. The other, Risslinge, was a grazing ley on a clay soil. On both sites field experiments had been running for 10 years. Unique possibilities, to study the long term transfer to grass under equilibrated field conditions, were thus at hand. Results of the two experiments for the period 1961-1981 are treated in this article. In years of fallout direct deposition was the main transfer to grass. It was larger on the permanent natural pasture than on the grazing fey. For some years after a radioactive inventory, or during a lag period, the nuclides are retained in the surface mat zone of pastures and more available for uptake. The type of pasture has large impact on the behaviour of the radionuclides. The lag period is longer for 137Cs than for 90Sr TF8 decreases from year to year due to downward migration, which is always effective at surplus precipitation between growth periods. Although the migration of 90Sr is larger than of 137Cs the transfer to grass is much higher for 90Sr than of 137Cs of all types of pasture on mineral soils. Liming and fertilization has a large impact on nuclide transfer to grass on permanent pastures established on coarse soils. Use of acidifying N fertilizers, AS, may give a 90Sr/Ca ratio twice that of CaN. Fertilization with AS increases the leaching of 90Sr and thereby decreases the 90Sr/Ca ratios in the long run. On coarse soils the 137Cs transfer to grass is lower on P and K fertilized pasture than on unfertilized pasture. (author)