Biomarker distribution along a pedosequence recording a transition from pasture to conifer forest (Massif Central, France)
Résumé
Anticipating the consequences of future global changes on natural systems requires the understanding of the responses of past ecosystems under climatic and anthropic constraints. As natural archives, soils are particularly relevant since they potentially preserve indications of their previous uses. Such uses are known to have irreversible impacts on soils, in terms of bulk chemical properties, soil nutrients availability, soil structure and local vegetation communities, at least at the historical scale (e.g. [1]). However, little is known about the persistence of lipid imprints through time in soils. The aim of this study is to compare the lipid imprint of three soils that suffered distinct land use histories in order to evaluate the possibilities offered by lipids to preserve information on past land uses. Three soils were sampled in the catchment of Lake Aydat (Central France) and then subsampled into 2 cm thick slices. Two of these soils remained under the same overlying vegetation (pasture: S-p, and forest: S-f) for the last 60 years. They constitute "reference" soils. These soils are compared to a soil formerly covered by a pasture/grassland 60 years ago and now by a conifer forest (S-pf). Bulk organic matter (OM) characterisation was performed by Rock-Eval pyrolysis. Lipids were extracted by ASE using DCM:MeOH and then separated into neutral, acidic and polar compounds on aminopropyl bonded silica. The neutral fraction was submitted to further separation by flash chromatography using solvents of increasing polarity. Lipid fractions were then identified by gas chromatography-mass spectrometry. Rock-Eval results suggest that, in the three soil profiles, the time of turnover and OM stabilisation increases with depth, with a global tendency toward increasing oxidation with depth. Combined with the absence of aromatic compounds, this indicates that lipids were not strongly affected by alteration. Degradation can thus not be invoked to explain variations in molecular content between the three soil profiles. Several classes of compounds were tested as potential tracers of former land uses: n-alkanes, tricyclic diterpenes, wax esters, FAMEs, steroids and triterpenes (e.g. [2], [3]). Although the most abundant n-alkane homologue is clearly defined in reference soils (n-C29 in S-f and n-C31 S-p), there is no shift from n-C31 to n-C29 in S-pf. Tricyclic diterpenoids, that are conifer markers, are present all along the S-f profile and are totally absent in S-p. These compounds are only present in the upper part of S-pf. Methoxyserratenes follow tricyclic diterpenes, although they also appear in low amount at the base of S-pf. Reversely, triterpenyl acetates are detected in S-p but are totally absent from S-f. Most of detected triterpenyl acetates are present all along the S-pf profile. This study highlights the variable potential of soil molecular biomarkers to record former land-uses, at least on decadal scales. The most specific and resistant families, such as pentacyclic triterpenes and tricyclic diterpenes, provide the most useful information but did not allow observing an obvious vertical zoning of molecular imprints in S-pf. This could result from mixing processes such as leaching, bioturbation and root penetration that affected this soil. Finally, criteria based on the presence/absence of given compounds appear more powerful than compound relative abundances. References [1] Hurtt, G.C., Frolking, S., Fearon, M.G., Moore, B., Shevliakova, E., Malyshev, S., Pacala, S.W., Houghton, R.A., 2006. The underpinnings of land-use history: three centuries of global gridded land-use transitions, wood-harvest activity, and resulting secondary lands. Global Change Biology 12, 7, 1208-1229. [2] Lavrieux, M., Jacob, J., Le Milbeau, C., Disnar, J.R., Zocatelli, R., Bréheret, J.G., Masuda, K., 2011. First detection of triterpenyl acetates in soils: sources and potential as new palaeoenvironmental biomarkers. 25th International Meeting on Organic Geochemistry, Interlaken, Switzerland. [3] Le Milbeau, C., Lavrieux, M., Jacob, J., Zocatelli, R., Disnar, J.R., 2011. Methoxyserratenes as discriminant biomarkers for soils developed under conifer forests. 25th International Meeting on Organic Geochemistry, Interlaken, Switzerland.