The monoacid part of soil lipids was studied in a hydromorphic sandy podzol under pine trees (Pinus maritima sp.). The undecomposed forest-litter layer (L), the fragmented mycelium-invaded litter layer (F) and the A1 soil horizon were sampled, and analysed for total lipid and total monoacid contents. Total monoacids were separated into straight-chain components and terpenic components.Straight-chain monoacids were determined as free acids and esters of fatty alcohols and of glycerol. Among these components, branched alkanoic acids occurred in the A1 soil horizon. Alkenoic acids were mainly determined as glycerides. Free n-alkanoic acids were mainly produced in soil from terminal oxidation of plant n-alkanes and plant n-alkanols. Free alkanoic acids with the longest carbon chains and alkanoic acids esterified as glycerides were concentrated in the A1 soil horizon.Terpenic monoacids were mainly diterpenic components from pine resin. Their concentration decreased markedly during the decomposition of plant debris.

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The proportion of alcohols in soil lipids was studied in an hydromorphic forest-podzol. The L and F litter layers and the soil A l horizon were sampled and analysed for total lipid and total alcohol contents. Total alcohols were separated into four fractions: primary alkanols, secondary alkanols, sterols and triterpenic alcohols.From the L layer to the soil A I horizon, all the fractions decreased sharply, except frcc primary alkanols and sterols. The decomposition of secondary alkanols was virtually complctc. Ncw odd-chain primary alkanols were generated and new hopanoid alcohols (triterpenic alcohols), considered as markers of bacterial origin, were detected in the A 1 horizon. In the sterol fraction, the same components were observed in all the samples with changes concerning only their rclative proportions.

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.