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| Titel |
Rhizoliths in the loess-paleosol sequence of Nussloch (SW Germany): Differentiation between ancient and modern vegetation using n-alkanes |
| VerfasserIn |
Martina Gocke, Yakov Kuzyakov, Guido L. B. Wiesenberg |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250051569
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| Zusammenfassung |
| Loess-paleosol sequences are important terrestrial archives for studying Quaternary climate
changes. They often contain secondary carbonates which were formed under arid to
semihumid climatic conditions. Rhizoliths, a special form of secondary carbonate, are
formed by encrustation of plant roots with secondary carbonate, potentially leading to
conservation of former root tissue and rhizodeposits. Probably, rhizoliths were
formed after loess sedimentation, as suggested e.g. by their occurrence over several
dm within a profile. Rhizoliths at the loess-paleosol sequence of Nussloch (SW
Germany) were formed after cessation of loess deposition (younger than 12 kyears),
and thus by different vegetation than synsedimentary loess organic matter (LOM).
Further, the potential overprint of original LOM by postsedimentary deep-rooting
plants was demonstrated on a molecular level, using lipid molecular proxies derived
from n-alkanes and fatty acids. Especially in sediments with low organic carbon
content like loess, this contribution of OM from sources (rhizodeposits of shrubs or
trees) other than the synsedimentary grass vegetation might entail difficulties for
paleoenvironmental studies using LOM. In few studies that mentioned this potential error, the
overprint of LOM was attributed e.g. to percolating soil solution from the modern
soil.
The aim of this work was to distinguish between rhizoliths as relicts of an ancient
vegetation and roots of modern vegetation at the Nussloch site. Moreover, based on
the occurrence of rhizoliths from 0.5 to ~ 7 m, we hypothesized that rhizoliths
from different depth can be derived from different source vegetation. Therefore, we
compared n-alkane composition and molecular proxies of modern soil (Ap, Bw1 and
Bw2 horizons) and roots therein, as well as rhizoliths and reference loess without
visible root remains, which were taken at depth between 0.8 and 6.9 m below present
surface.
The alkane distribution pattern with dominance of long chain odd homologues showed
higher plant biomass as main source of OM in the whole sample set. The most abundant long
chain alkane (LCA), which was used in previous studies for source apportionment of OM,
differed between sample types: The alkanes in modern soil were dominated by
C31, confirming its former agricultural use (grass vegetation). In contrast, roots
in modern soil were dominated either by C27 or C29 LCA, deriving from recent
shrub vegetation. C31 as most abundant LCA in LOM was in agreement with the
general assumption of loess deposition taking place during glacial periods with scarce
grass cover. Rhizoliths, on the other hand, were dominated either by C29 or C31
LCA, making an attribution of their OM remains to one functional plant group
impossible.
Molecular proxies of carbon preference index (CPI) and average chain length (ACL)
allow for differentiation of fresh plant biomass (high values) and degraded plant biomass and
microbial remains (low values). CPI and ACL were highest in aboveground biomass
of the modern vegetation, while modern roots showed high ACL but lowest CPI
values of the sample set, because of abundant microbial biomass connected to living
roots. Slightly higher CPI in rhizoliths compared to modern roots indicated the
rather good preservation of former root tissue by calcium carbonate encrustation,
while even higher CPI in loess showed the aboveground biomass source of LOM.
These differing ACL and CPI values, together with different most abundant LCA,
suggest different biogenic sources for modern roots, rhizoliths, modern soil OM and
LOM.
Together with previous data, these results imply that roots of modern vegetation are not
the origin of postsedimentary incorporated OM in the loess-paleosol sequence of Nussloch.
This postsedimentary overprint can be attributed clearly to deep-rooting plants which
entered the sequence after loess deposition, but prior to modern soil development. |
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