Plant uptake of radiocaesium on heather moorland grazed by sheep.
Abstract
A hill pasture on an iron podzol in NE Scotland comprising areas of heather-dominated vegetation and Deschampsia flexuosa grassland was grazed by sheep from May to early Oct. in 1989 and 1990. In each vegetation type small quadrats were artificially contaminated with 134Cs by soil injection and seasonal changes in 134Cs concentrations within these quadrats were recorded for the main botanical components. Calluna vulgaris accumulated more 134Cs than any other species. Current and previous years' shoots of C. vulgaris had consistently higher 134Cs concentrations than wood and dead shoots. Within the herbs 134Cs concentrations increased significantly in the order: fine-leaved grasses < broad-leaved grasses < Carex pilulifera < Galium saxatile. Lowest concentrations were measured in Vaccinium myrtillus, Erica cinerea, Nardus stricta, Juncus squarrosus and dead plant material. In the 1st year 134Cs concentrations in broad-leaved grasses, fine-leaved grasses, G. saxatile and C. pilulifera showed very similar seasonal trends with overall high, but fluctuating 134Cs concentrations in summer and decreasing concentrations in autumn. Pronounced seasonal changes in 134Cs concentrations occurred in flowering and non-flowering shoots of C. vulgaris, but not in woody tissues. In the 2nd year, 134Cs concentrations in all species varied little over the grazing season and, compared with the 1st year, the overall concentrations were 50-60% lower in the heather area and 70-85% lower in the grass area. Of the injected 134Cs, over 90% remained within the top 40 cm of soil. It was estimated that in the heather area 9% of the injected 134Cs was contained within the biomass of C. vulgaris during the summer, whereas in the grass area only about 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 134Cs taken up by the plant. Seasonal patterns of 134Cs concentrations were interpreted in relation to growth pattern and age structure of the sampled plant population. Possible effects of changing availability of 134Cs in the soil were discussed.