Earthworms and radionuclides, with experimental investigations on the uptake and exchangeability of radiocaesium

The potential influence of earthworm activity on the mobility of radionuclides in soils and their subsequent availability for uptake by plants and transfer to higher trophic levels is briefly reviewed. The accumulation of caesium by the earthworm Aporrectodea longa from soil and from plant litter was investigated in laboratory experiments, as was the effect of reworking (through burrowing and ingestion) soil and soil with added organic material, on the extractability of caesium (ammonium acetate extraction). Soil was spiked with (134)Cs, organic matter with (137)Cs. In soil-fed worms, most of the radioactivity measured was eliminated with the gut contents; 5-25% of the ingested radioactivity was retained or assimilated. Loss of caesium from soil-fed worms followed a two component curve, with an initial rapid loss due to gut clearance (half-life of loss (Tb1/2) of about 0.2-0.6 days) and a slower loss of assimilated caesium (Tb1/2 of 15-26 days). Loss rates of assimilated caesium from worms fed on fragmented apple leaves were found to have half-lives of 18-54 days. Assimilation of caesium from apple leaves was higher than from soil, ranging from 55-100% of the activity measured before gut clearance. Dry weight transfer factors (concentration in worm tissue/concentration in substrate) for worms cleared of their gut contents were similar for the two substrates 0.04 and 0.04 for two loss experiments with worms fed on radioactive soil, and 0.03 and 0.05 for worms fed on apple leaves. After three months of reworking soil and soil/organic mixtures, A. longa was found to have no measurable effect on the extractable fraction of caesium. If earthworms have any subtle effects they were masked by changes in availability that occurred when the spiked soil and organic substrates were mixed together. Only about half of the extractable fraction in soil was recovered when soil was mixed with organic material suggesting that some of the labile fraction in soil had become complexed with organic material. This exchange occurred in substrate mixtures with and without worms. The limitation of chemical extraction procedures is discussed and suggestions for further work are presented.