Evolution of lipid abundance and molecular composition during the podzolisation of laterites in the upper Amazon basin
Résumé
In the upper Amazon basin, podzolisation involves the remobilization of large amounts of organic matter and chemical elements (Fe, Al, Si) previously accumulated in lateritic formations. In order to better understand the fate of organic matter in podzolic environments in this area, the evolution of lipid abundance and molecular composition were studied along a representative soil sequence showing the transition between a latosol and a well-developed podzol. Total solvent extracts were obtained from eight key soil samples from three profiles and their overlying litters, and enable us to follow both lateral and vertical evolutions. Lipid composition was determined by gas chromatography-mass spectrometry (GC-MS). Major compound classes include alkanes, alkanones, alkanols, alkanoic acids, ω-hydroxyacids, as well as aromatics, steroids and triterpenoids. Free lipids do not accumulate in the early stages of podzolisation but are abundant in well-developed podzolic horizons, possibly due to (i) combined acidity and waterlogging, (ii) limited amounts of complexing elements, (iii) a decreased microbial activity. The evolution of lipid composition is consistent with podzolisation mechanisms previously highlighted in the sequence. This paper provides further evidence for the occurrence of anoxic conditions in deep waterlogged podzolic horizons. It also shows that aromatic, phytotoxic compounds are stabilized in the well-developed podzol. This may play a role in the vegetation changes associated with podzolisation in the area.