25th International Meeting on Organic Geochemistry IMOG 2011

P-242
Changes of Rock-Eval HI, OI, and C-isotope ratios during decay
of soil organic matter in a temperate climate environment
(Villiers, Switzerland)
Olivier Hasinger 1 , Jorge E. Spangenberg 2 , Eric P. Verrecchia 1
1 Institute of Geology and Paleontology, University of Lausanne, Lausanne, Switzerland, 2 Institute of
Mineralogy and Geochemistry, University of Lausanne, Lausanne, Switzerland (corresponding
author:olivier.hasinger@unil.ch)
Four hypotheses have been formulated to explain the
13 C enrichment in soil organic matter (SOM) with
increasing depth: (i) a change in the isotopic
composition of atmospheric carbon, (ii) a higher
contribution of microbial biomass in deeper soils, (iii)
kinetic fractionation during plant respiration, and (iv)
preferential decay of labile organic compounds [1, 2].
In this study, we explore the combined use of C
isotope ratios and Rock Eval HI and OI values to get
further insight into the increase of � 13 CSOM along soil
profiles. Data have been gathered in a 1.4 m deep
reference soil in Villiers (Switzerland, 47°04‘N, 6°59‘E,
769 m asl). Bulk soil samples were collected along
the soil profile at different depth. Three fractions were
separated from each sample, including (i) lignin
(alkaline CuO oxidation with a microwave digestion
system); (ii) humic substances (alkaline extraction);
(iii) labile sugars (hot water extraction).
The C isotopic composition of TOC varies
between -26.9 and -25‰ VPDB with depth (Fig. 1A).
The sugar is the isotopically heaviest fraction of SOM
whereas the lignin is the most 13 C depleted. Trends
toward higher � 13 C values with depth suggest
(bacterial) decomposition of the SOM fraction with
release of isotopically light moieties (e.g., CO2, short-
C-chain lipids). The OI values increase with depth
from ~183 to ~377, and the HI values decrease from
~186 to ~84. The opposite trends between the
� 13 CTOC and the OI and HI values suggest that
oxidation of SOM through (bacterial) oxidative
decarboxylation along the soil profile is most probably
the main process triggering the 13 C-enrichment (Fig.
1B).
In conclusion, the Rock-Eval HI, OI, and � 13 C
values of the soil organic fractions bring new insights
into the processes responsible for the 13 C enrichment
of TOC during soil formation. These results support
the hypothesis of a preferential decay of given organic
components in soils. Nevertheless, these results
cannot be generalized to other soils, as different
humification pathways may occur in soils with
different SOM composition.
A
B
Fig.1. A. Vertical distribution of the � 13 C values of TOC and
various extracted fraction of SOM B. � 13 CTOC vs Rock-Eval
OI and HI values.
References
[1] Boström et al. (2007), Oecologia 153, 89-98.
[2] Wynn et al. (2005), Geochimica et Cosmochimica Acta
69, 1961-1973.
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