RADIONUCLIDE DYNAMICS DURING LITTER DECOMPOSITION IN A HOLM-OAK FOREST

摘要

Migration of ~(134)Cs and ~(110)Ag~m in a Mediterranean forest soil was studied by means of field incubation of artificially contaminated holm-oak leaves in plastic cylinders. The contaminated green leaves in the cylinders replaced the original litter layer, and the litter decomposition processes were studied. The following layers were sampled: L (remaining litter), F (fermented litter), H (humus layer) and mineral soil layers. The fraction of radionuclides that had migrated from the L-layer accumulated mainly in the F- and H-layers, whereas very little activity reached the mineral soil after an initial leaching period. During the second year of incubation, a significant transfer from the F-layer to the H-layer was observed for both radionuclides. At the first sampling, it was found that leaching of radionuclides from the L-layer was equivalent to the water extractable fraction of radionuclides obtained by sequential extraction. From this period onwards, radionuclide release was dependent on litter decomposition. A positive linear correlation was found between mass loss and radionuclide release after the first leaching period. The regression coefficient was 0.57 for ~(134)Cs and 0.56 for ~(110)Ag~m. In agreement with these observations, radionuclide release from the litter layer followed a double negative exponential function. Radionuclide dynamics in the F-layer followed three different periods: In the first period, rapid accumulation of radionuclides occurred in the F-layer as a consequence of initial leaching from the L-layer. In the second period, there was both transfer from the L-layer and transfer to the H-layer of radionuclides, which was mainly due to degradation of easily decomposable compounds. The F-layer lost about 70% of its weight, but the radionuclide content remained constant. It is suggested that in this period, retention of radionuclides by recalcitrant compounds occurred. Micro-organisms could also play an important role in consuming carbon, with subsequent immobilization of radionuclides. Finally, a third period, with both carbon and radionuclide mineralization, was observed at the end of the incubation period. It can be concluded that decomposition processes and organic matter dynamics in the L- and F-layers are key factors controlling cycling of radionuclides in forest systems.