Tuesday, November 14, 2006

Evolution of Lipids during the Podzolisation of Laterites in the Upper Amazon Basin.

Marion Bardy1, Sylvie Derenne1, and Emmanuel Fritsch2. (1) BIOEMCO, UMR CNRS 7618, ENSCP, 11 rue Pierre et Marie Curie, Paris Cedex 05, 75231, France, (2) Institut de Recherche pour le Développement, UR058, 213 rue Lafayette, Paris, 75480, France

In the upper Amazon Basin, podzolisation involves the remobilization of large amounts of potentially damaging chemical elements (Fe, Al, Si) previously accumulated in lateritic formations. Lipids generally accumulate in acidic soils such as podzols and can influence their functioning: for example, they can be toxic towards microorganisms, have chelating properties towards metals and their hydrophobicity can affect soil physical properties. In addition, the composition of specific classes of lipids can give information about the sources of organic matter, microbial activity, and the pathways of degradation and/or stabilisation of organic matter.

In order to better understand the mechanisms involved in the podzolisation process in the Amazon Basin, the evolutions of lipid abundance and composition were studied along a representative soil catena showing the transition between a latosol and a well-developed podzol. Total free lipid extracts were obtained from eight key samples of the catena which enable to follow both lateral and vertical evolutions. The extracts were derivatized and molecular analyses were performed using gas chromatography and mass spectrometry.

As expected, lipids accumulate with decreasing pH from the latosol to the podzol. Unsaturated acids such as C16:1 and C18:1 are observed, consistently with a low level of lipid degradation. Furthermore, in surface horizons, the occurence of short-chain branched alkanes, together with n-, iso- and anteiso- C15 and C17 fatty acids, indicate a microbial contribution, but the variations in the relative abundances of these isomers show that the microbial activity must be reduced in the developed podzol relative to the latosol and the transition. Finally, the contribution of alkanols relative to acids is lower in the transition, either due to a reduced contribution of acids or to their immobilization by complexation with metals released during acidocomplexolysis. Analysis after complex dissociation using pyrophosphate should allow to test this hypothesis.



Handout (.pdf format, 290.0 kb)