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| Titel |
Rhizoliths in loess - a new tool to estimate post-sedimentary incorporation of organic matter in terrestrial environments |
| VerfasserIn |
Martina Gocke, Guido L. B. Wiesenberg, Konstantin Pustovoytov, Yakov Kuzyakov |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250042365
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| Zusammenfassung |
| Pedogenic (secondary) carbonates are a typical feature of soils in arid and semiarid regions.
In calcareous parent material, they are formed by dissolution of lithogenic (primary)
carbonate and recrystallization with soil CO2. Usually, hundreds to thousands of years are
necessary for complete recrystallization of primary CaCO3. δ13C of pedogenic carbonates
reflects the photosynthetic pathway of the predominant local vegetation, because secondary
CaCO3 is formed in isotopic equilibrium with soil CO2 released by root and rhizomicrobial
respiration. Therefore, δ13C of pedogenic carbonates is used for paleoenvironmental
reconstructions.
Rhizoliths are a special form of pedogenic carbonate formed by encrustation of plant
roots by recrystallized carbonate. While the organic part of the root is mainly degraded during
decomposition, carbonatic, bone-like structures remain in the terrestrial sediments. They
occur locally abundant in calcareous sediments like loess. In contrast to very long formation
of other types of pedogenic carbonates, these calcified roots are built probably within
a few decades or even years. They are thought to be formed after sedimentation
and can be followed continuously in loess profiles over a depth of 8m (Nussloch,
Germany).
Radiocarbon ages indicate an age of 3-4 ka for rhizolith organic matter and carbonate,
while the surrounding loess was sedimented 13-20 ka before present. Due to their high
frequency in the Nussloch loess profile (app. 10-20 rhizoliths m-2) it can be assumed that
rhizodeposits led to a significant contribution of post-sedimentary organic matter in the loess.
Stable carbon isotopic (δ13C) signatures and other parameters did not show obvious
differences between loess and rhizolith organic matter due to potentially similar biogenic
sources. An estimation of the post-sedimentary organic matter has been performed using
molecular proxies, which allowed for a differentiation of the different biogenic sources.
These parameters included alkane and fatty acid compositions and molecular proxies based
on these organic matter fractions.
The molecular patterns clearly showed a significant contribution of root-derived organic
matter in loess adjacent to roots even in a distance of 5 cm. First estimations of
post-sedimentary root-derived organic matter based on the amounts of unsaturated fatty acids
yielded a high contribution of root-derived organic matter near the roots and decreasing
amounts with increasing distance to roots, which accounted to least 10-20% even in a
distance of 5 cm. Depending on alkane distribution patterns different biogenic sources of
rhizoliths could be determined that also varied in their contribution of root-derived
components in the vicinity of roots.
We present an approach, how post-sedimentary incorporation of higher plant biomass
can be assessed in terrestrial sediments and discuss the following corrections for
paleoenvironmental and paleoclimatic reconstructions that follow these findings. |
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