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SUMMARY Lipids generally accumulate in surface horizons of soils deficient in clay minerals. Samples from a podzol (A1 horizon–no clay) were incubated with a supply of hetero‐ or homo‐ionic (Mg)‐bentonite for 4 months at 28 °C. A significant decrease of unbound lipids was observed. A detailed study of the mainly hydrocarbon fraction revealed that several processes were involved. A transfer of lipids, particularly hydrocarbons, occurred from the free to the bound phase. Consequently, reduced toxicity of these lipids towards soil microorganisms can be expected. The addition of clay resulted in a stimulation of oxidation processes, the extent of which varied with the exchangeable cations; heteroionic bentonite led to increased mineralization, whereas Mg‐bentonite promoted mild oxidation reactions. The increased microbial activity also produced short‐chain hydrocarbons; with Mg‐bentonite, unusual C21‐C35 n‐alkanes were detected. All of these microbial hydrocarbons were found in the bound phase.

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