PLANT UPTAKE OF RADIOCESIUM ON HEATHER MOORLAND GRAZED BY SHEEP

1. A hill pasture on an iron podzol in north-east Scotland comprising areas of heather-dominated vegetation and Deschampsia flexuosa grassla nd was grazed by sheep from May to early October in 1989 and 1990. In each vegetation type small quadrats were artificially contaminated wit h Cs-134 by soil injection, and seasonal changes in Cs-134 concentrati ons within these quadrats were recorded for the main botanical compone nts. 2. Calluna vulgaris accumulated more Cs-134 than any other specie s. Current and previous years' shoots of C. vulgaris showed consistent ly higher Cs-134 concentrations than wood and dead shoots. Within the herbs Cs-134 concentrations increased significantly in the order: fine -leaved grasses < broad-leaved grasses < Carex pilulifera < Galium sax atile. Lowest concentrations were measured in Vaccinium myrtillus, Eri ca cinerea, Nardus stricta, Juncus squarrosus and dead plant material. 3. In the first year Cs-134 concentrations in broad-leaved grasses, f ine-leaved grasses, Galium saxatile and Carex pilulifera showed very s imilar seasonal trends with overall high, but fluctuating Cs-134 conce ntrations in summer and decreasing concentrations in autumn. Pronounce d seasonal changes in Cs-134 concentrations occurred in flowering and non-flowering shoots of Calluna vulgaris, but not in woody tissues. In the second year, Cs-134 concentrations in all species varied little o ver the grazing season and, compared to the first year, the overall co ncentrations were 50-60% lower in the heather area and 70-85% lower in the grass area. 4. Of the injected Cs-134, over 90% remained within t he top 4 cm of soil. It was estimated that in the heather area 9% of t he injected Cs-134 was contained within the biomass of C. vulgaris dur ing the summer, whereas in the grass area only approximately 0.5% was present in the herbage. The shoots of C vulgaris accounted for 30-40% of its biomass, but contained 65-80% of the Cs-134 taken up by the pla nt. 5. Seasonal patterns of Cs-134 concentrations are interpreted in r elation to growth pattern and age structure of the sampled plant popul ation. Possible effects of changing availability of Cs-134 in the soil are discussed.