25th International Meeting on Organic Geochemistry IMOG 2011

P-250
Biomarker distribution along a pedosequence recording a
transition from pasture to conifer forest (Massif Central, France)
Marlène Lavrieux 1 , Jean-Gabriel Bréheret 2 , Jean-Robert Disnar 1 , Jérémy Jacob 1 , Claude
Le Milbeau 1 , Renata Zocatelli 1
1 Institut des Sciences de la Terre d’Orléans (ISTO), Université d’Orléans, CNRS/INSU, Université de Tours,
UMR 6113, Orléans, France, 2 Institut des Sciences de la Terre d’Orléans (ISTO), Université de Tours,
CNRS/INSU, Université d’Orléans, UMR 6113, Tours, France (corresponding
author:marlene.lavrieux@etu.univ-tours.fr)
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, woodharvest
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. <strong>25th</strong>
<strong>International</strong> <strong>Meeting</strong> 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. <strong>25th</strong>
<strong>International</strong> <strong>Meeting</strong> on Organic Geochemistry, Interlaken,
Switzerland.
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