Soil samples spiked with five concentrations of pyrene and phenanthrene were aged for 0, 10, 40, and 120 d before toxicity was investigated using a standardized bioassay with the soil-dwelling collembolan Folsomia fimetaria L. Toxic effects were measured as reductions in survival and reproductive output after 3 weeks of exposure. Both pyrene and phenanthrene were degraded in the test system during storage, phenanthrene to a higher degree than pyrene. However, when toxic effects of the tested substances were calculated on the basis of measured concentrations of the parent compounds, toxicity was unaffected by storage for up to 120 d. Many studies have shown a negative correlation between aging and the biodegradation of polycyclic aromatic hydrocarbons (PAHs), indicating a reduction in the bioavailability of PAHs to micro-organisms with time. Our results indicate that a reduction in toxicity, as related to the measured concentrations of the parent compounds in the soil, may not always be expected for aged soils. Provided that metabolites of pyrene and phenanthrene did not significantly contribute to the toxicity in aged soil samples, a possible explanation for the absence of aging effects is the high test concentrations used in combination with the low content of organic carbon (1.6%) in the test soil.
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