25th International Meeting on Organic Geochemistry IMOG 2011

P-512
The effect of fire intensity on alkane composition of plant
organic matter and soils affected by fire
Guido Wiesenberg 1 , Andreas Nestler 1,2 , Stefanie Birkner 1 , Ulrich Hambach 2
1 Department for Agroecosystem Research, University of Bayreuth, Bayreuth, Germany, 2 Geomorphology
Department, University of Bayreuth, Bayreuth, Germany (corresponding author:guido.wiesenberg@unibayreuth.de)
Plant-derived biomass entering soil commonly leads
to a typical chemical composition of soil organic
matter (SOM), whereas alteration of biomass during
microbial and thermal degradation results in chemical
changes of plant-derived SOM. On a molecular level
plant-derived SOM is characterized by a
predominance of long-chain alkanes, fatty acids and
alcohols within the lipid fraction with a strong relative
predominance of odd or even carbon-numbered
homologues depending on the lipid fraction. E.g.
plant-derived alkanes as typical epiculticular wax
components and degradation products of
functionalized lipids are dominated by odd long-chain
alkanes. Contrastingly, a progressive increase in
short-chain even-numbered alkanes was found in
charred biomass with increasing temperature
associated by a decrease in chain-length and a
decrease in the predominance of odd n-alkanes [1].
Thermal degradation of plant biomass during a fire
results in a modification of lipid distribution patterns
that differs from microbial degradation. Not only the
composition, but also the total amount of lipidic
components changes. The aim of the current study
was to assess effects of temperature oxygen
availability and source biomass on amount and
composition of thermal degradation products.
Additionally, we compared results from laboratory
experiments with soil and archaeological samples of
well known burning history.
During charring at low temperatures (400°C) a decrease in abundance of the all lipidic
fractions occurs including aromatic hydrocarbons with
a stronger depletion in aliphatic than in aromatic
components. At higher temperatures (>350°C) even
carbon numbered monoalkenes increase in
abundance presumably due to degradation of ester
bound lipids containing side chains with 14-18
carbons. In general this is in agreement with NMR
spectra [2], indicating a higher aromaticity at higher
charring temperatures.
The effect of oxygen availability improved degradation
of alkanes, whereas under nitrogen atmosphere
degradation was limited and resulted in larger
amounts of alkanes, alkenes and PAHs.
All these observations indicate that incomplete
burning results in large amounts of complex organic
remains in soil, where with increasing temperature
burning gets more complete leaving less burning
residues. Recent investigations indicate a structural
re-arrangement of lipidic components in burned plant
biomass not only as a function of temperature,
duration of thermal degradation, and oxygen
availability, but also as a function of the initial plant
biomass composition.
Additionally, we compared burning residues from the
laboratory experiment, where thermal degradation
was carried out from 200-700°C, the Roman Furnace
Project at Velzeke (Belgium) with furnaces managed
at 550°C and 1070°C, respectively, recent burnt
agricultural residues derived from agricultural plots in
Russia, material from ancient anthropogenic pits as
well as fire places from different ancient settlements.
The alkane distribution patterns clearly correlated with
proposed temperatures. Therefore, alkanes and
alkenes as molecular marker might be useful to trace
not only fire in recent and ancient soils, but also the
burning conditions and potentially the initial biomass.
References
[1] Wiesenberg GLB, Lehndorff E, Schwark L 2009.
Org Geochem 40, 167.
[2] Baldock JA, Smernik RJ 2002. Org Geochem 33,
1093.
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