Temporal changes in earthworm availability and extractability of polycyclic aromatic hydrocarbons in soil.

In this study, the earthworm, Aporrectodea longa, was used as a model soil organism to assess the impact of ageing upon the bioavailability of polycyclic aromatic hydrocarbons (PAHs) in soil. The objectives were to characterise the temporal changes in the extractability/availability of PAHs amended into soil and to examine how the decline in PAH availability to earthworms related to the decline in chemical availability as determined by solvent extraction. Two PAHs (pyrene and benz[a]anthracene) were spiked into sterilised soil and aged in microcosms for up to 240 days. The earthworms were incubated in the PAH spiked soils, at 0, 30, 60, and 240 days after spiking, for a period of 28 days. After exposure, the earthworm-PAH tissue concentrations were measured. Change in chemical extractability of the soil-PAHs was monitored throughout the incubation by employing a sequential extraction technique with two solvents of different polarities. The chemical extractability and bioavailability of both PAHs reduced with increased soil contact time. Pyrene and benz[a]anthracene both displayed biphasic profiles in chemical extractability and earthworm bioavailability, but the rates and extents differed. Thus, chemical extractability does not accurately predict the bioavailable fraction of PAHs in the soil and does not agree with work reported earlier using Eisenia fetida, hence it follows that the earthworm species may be important in determining the bioaccumulation of soil-associated PAHs. Further, the ecological niche occupied by the experimental species will influence feeding behaviour and thus, perhaps, the degree of accumulation. Therefore, the use of the manure earthworm, E. fetida, in screening of contaminated soils (as recommended by the USEPA) may underestimate toxicity or accumulatory potential