Evidence for long-term changes in the soil composition of selected organic compounds, brought about by exchanges with the atmosphere, is briefly reviewed. In the case of some compounds - such as benzo(a)pyrene and octachlorodibenzo-p-dioxin, soils may be significant long-term environmental sinks for atmospherically-derived material. In other cases - such as phenanthrene and some of the lighter PCBs, de-gassing or volatilisation from soil back to the air can occur under certain conditions. Hence the soil may act as a "short-term" sink, and a potential source to atmosphere. Indeed, for some 'semi-volatile' compounds used in large quantities in the past - such as PCBs, soil outgassing may actually be an extremely important source to contemporary air. Furthermore, soil outgassing from areas of former high use may provide an important driving mechanism for continued "global cycling" of a range of semi-volatile organochlorine compounds.

The removal of phenanthrene, anthracene and benzo(a)pyrene added at three different concentrations was investigated with or without earthworms (Eisenia fetida) within 11 weeks. Average anthracene removal by the autochthonous micro-organisms was 23%, 77% for phenanthrene and 13% for benzo(a)pyrene, while it was 51% for anthracene, 47% for benzo(a)pyrene and 100% for phenanthrene in soil with earthworms. At 50 and 100 mg phenanthrene kg−1E. fetida survival was 91% and 83%, but at 150 mg kg−1 all died within 15 days. Survival of E. fetida in soil amended with anthracene ≤1000 mg kg−1 and benzo(a)pyrene ≤150 mg kg−1 was higher than 80% and without weight loss compared to the untreated soil. Only small amounts of PAHs were detected in the earthworms. It was concluded that E. fetida has the potential to remove large amounts of PAHs from soil, but more work is necessary to elucidate the mechanisms involved.

The simulation of ecosystems by mesocosms might be affected by artefacts due to the scale reduction implied in such experiments. The physico-chemical properties of water leaching out of mesocosms made of only 50 kg of agricultural soil were measured to demonstrate that they constitute reproducible systems. The soils were artificially contaminated with lead, atrazine, benzo(a)pyrene and Phenochlor. Physico-chemical parameters (redox potential, conductivity, pH, dissolved oxygen) and major ions were monitored in the leachates for six months. Even though these changed with time, in each sampling campaign they were similar for all of the mesocosms despite possible independent development of microbial and biological activity.