Seasonal variation in radiocaesium uptake by reseeded hill pasture grazed at different intensities by sheep.
On resown hill pasture in NE Scotland, two grass/clover swards were continuously grazed by sheep to maintain sward heights of 3 and 5 cm from May to Sep. in 1988 and 1989. Within small areas of pasture, 134Cs was injected into the peaty topsoil in June 1988 and the uptake by the vegetation was recorded during both years. 134Cs concentrations in the herbage increased in spring and decreased in autumn, but considerable fluctuations occurred during the growing season. The pattern of these fluctuations and the overall concentration of 134Cs in the herbage varied between years, whereas there was no change in 134Cs concentration in the top 5 cm of the soil. On both swards the seasonal patterns of Lolium perenne, Festuca rubra, Trifolium repens, and Cerastium fontanum were similar. In all species except C. fontanum, 134Cs concentrations were higher on the 5 cm sward than on the 3 cm sward. In summer, concentrations in C. fontanum were 4-6 times higher than those in the other species. Depending on season and sward height, 0.3-2% of the 134Cs injected into the soil was present in the sward. The total amount of 134Cs taken up by the sward during the growing season was equivalent to 1.5-8.5% of the amount injected.