25th International Meeting on Organic Geochemistry IMOG 2011

P-239
Differentiation of organic matter in soil, loess and rhizoliths at
the Nussloch sedimentary sequence via n-alkane molecular
proxies
Martina Gocke, Guido Wiesenberg
Department for Agroecosystem Research, University of Bayreuth, Bayreuth, Germany (corresponding
author:martina.gocke@uni-bayreuth.de)
Loess-paleosol sequences are important terrestrial
archives to study Quaternary climate, based on
proxies like pollen, stable carbon isotope composition
(δ 13 C) of organic matter (OM) and many others.
Commonly, loess OM is thought to represent the
signal of the vegetation prevailing during deposition,
i.e. mainly grass vegetation without large abundances
of trees or shrubs.
However, the presence of large calcified roots
(rhizoliths) in the loess profile of Nussloch (SW
Germany) gives an example, how loess-paleosol
sequences have been penetrated postsedimentary by
deep-rooting plants. These plants could have a
different size and shape compared to synsedimentary
grass vegetation and likely modified the initial loess
OM via release of root exudates and fine root hairs.
This can entail uncertainties for paleoenvironmental
studies based on loess OM. Therefore, identification
and source apportionment of OM of different origin in
loess is required to reduce related uncertainties.
However, this remains difficult due of low organic
carbon (Corg) contents in loess (usually < 0.5%). Lipid
molecular proxies, e.g. from n-alkanes and fatty acids,
enable the required source apportionment of OM in
recent and ancient soils and sediments [1].
The aim of this study was to identify different sources
of OM in the loess sequence of Nussloch, SW
Germany, and to demonstrate the relevance of
postsedimentary incorporation of root-derived OM for
an overprint of the original Corg signal in terrestrial
sediments. Therefore, pairs of rhizoliths and reference
loess without visible root remains were sampled from
the profile in depths between 0.8 and 6.9 m. For
comparison, the modern soil and recent vegetation,
consisting of grasses, herbaceous plants and robinia,
were sampled. Further, loess in direct vicinity to the
rhizoliths up to a distance of 5 cm (rhizoloess), was
sampled. The samples were compared for their
alkane composition, lipid and Corg contents.
Alkane composition with C31 as most abundant long
chain homologue revealed that both, loess and soil
organic matter (OM), were derived from grass
vegetation. While arable crops mainly contributed to
the recent soil OM, these were natural grasses for
loess. The recent vegetation did not contribute to OM
in rhizoliths and rhizoloess. Rhizoliths were formed
under tree and/or shrub vegetation more than 3000
years BP and reflect the natural former vegetation,
which likely was dominated by beech, oak, alder and
hazelnut. Low values for the carbon preference index
(CPI) in rhizoliths and rhizoloess revealed the
presence of former root tissue and associated
rhizomicrobial remains [2]. The overprint of loess
organic matter even distant to former roots cannot be
excluded as revealed by molecular proxies like
average chain length (ACL) or CPI. Therefore,
paleoenvironmental investigations of terrestrial
sediments based only on bulk carbon analyses should
be regarded with caution.
Besides further investigations on recent root biomass
and rhizoliths, rhizoloess must be investigated in
more detail to assess the postsedimentary
modification of loess OM near to the former roots and
to determine the extension of the former rhizosphere.
Fig. 1: CPI and ACL of rhizoliths, loess, soil and
modern vegetation at the Nussloch site.
References
[1] Harwood J.L., Russel N.J. 1984. Lipids in plants
and microbes. Allen & Unwin, London.
[2] Cranwell P.A. 1981. Organic Geochemistry.
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