The influence of defoliation intensity, season and leaf age on radiocaesium concentrations in Agrostis capillaris.
Abstract
The effects of cutting frequency, time of year and leaf age on the appearance of 137Cs in shoots of Agrostis capillaris were studied in a pot experiment using topsoil from a peaty podzol, artificially contaminated with 137Cs. Grazing was simulated by frequent cutting to establish swards 3 and 6 cm tall. Aboveground biomass was cut on 10 dates from June to December and separated into five categories of leaf age. Significant differences in growth rate and 137Cs activity concentrations were measured between swards cut to different heights. These differences were not consistent over time, but did indicate that during the main growing season 137Cs in actively growing tissues was higher in the less frequently cut plants. The effects of season and leaf age were pronounced, with activity concentrations of 137Cs increasing rapidly in all living tissues from mid-July onwards and generally stabilizing during autumn; this occurred sooner in actively growing than in fully mature and dying tissues. In autumn, mature and dying tissues had higher contamination than younger tissues; the opposite was the case in summer. Dead leaves always had the lowest 137Cs activity concentrations, up to 5-fold lower than green leaves. Over the winter, a 29% decrease in contamination of plant biomass was measured. The 137Cs activity concentrations in plant material cut to maintain constant sward height were considerably higher than those in the remaining standing biomass. Since the cut material corresponds to the part of the sward most likely to be consumed by grazing animals, animals in the field would be ingesting the most contaminated herbage. Seasonal patterns of 137Cs contamination of vegetation are discussed in relation to climatic, soil and plant intrinsic factors.