THE INFLUENCE OF DEFOLIATION INTENSITY, SEASON AND LEAF AGE ON RADIOCESIUM CONCENTRATIONS IN AGROSTIS-CAPILLARIS

1. The effects of defoliation intensity, time of year and leaf age on the appearance of Cs-137 in shoots of Agrostis capillaris were studied in a pot experiment using topsoil from a peaty podzol, artificially c ontaminated with Cs-137. Grazing was simulated by frequent clipping to establish swards 3 and 6 cm tall. Above-ground biomass was harvested on 10 dates from June to December and separated into five categories o f leaf age. 2. Significant differences in growth rate and Cs-137 activ ity concentrations were measured between swards clipped to different h eights. These differences were not consistent over time, but did indic ate that during the main growing season Cs-137 in actively growing tis sues was higher in the less intensively defoliated plants. 3. The effe cts of season and leaf age were pronounced, with activity concentratio ns of Cs-137 increasing rapidly in all living tissues from mid-July on wards and generally stabilizing during autumn; this occurred sooner in actively growing than in fully mature and dying tissues. In autumn, m ature and dying tissues had higher contamination compared to younger t issues; the opposite was the case in summer. Dead leaves always had th e lowest Cs-137 activity concentrations, up to five times lower than g reen leaves. Over the winter, a 29% decrease in contamination of plant biomass was measured. 4. The Cs-137 activity concentrations in plant material clipped to maintain constant sward height were considerably h igher than those in the remaining standing biomass. Since the clipping s are the part of the award most likely to be consumed by grazing anim als, animals in the field would be ingesting the most contaminated par t of the sward. 5. Seasonal patterns of Cs-137 contamination of vegeta tion are discussed in relation to climatic, soil and plant intrinsic f actors.