25th International Meeting on Organic Geochemistry IMOG 2011

P-228
Soil organic matter in drylands: insights into selective
degradation?
Arnoud Boom 1 , Andrew S Carr 1 , Zoë Roberts 1 , Alex Cumming 1 , Brian M Chase 2 , Michael
E Meadows 3 , Matthew Britton 3
1 University of Leicester, Leicester, United Kingdom, 2 CNRS, Institut des Sciences de l’Evolution de
Montpellier, UMR 5554 Université Montpellier 2, France, 3 University of Cape Town, Capetown, South Africa
(corresponding author:ab269@le.ac.uk)
Warm conditions and strongly oxidising conditions in
semi-arid environments typically promote the rapid
turnover of the SOM pool in such regions. The theory
of selective degradation implies that the most labile
OM fractions are rapidly mineralised, resulting in the
preferential preservation of more recalcitrant OM (de
Leeuw et al 2006). The concept of recalcitrant OM in
soils is currently debated (Marschner et al. 2008). A
source of recalcitrant soil OM is potentially the
resistant leaf cuticles of succulent plants (cutan;
Boom et al., 2005). Cutan, an aliphatic biopolymer,
which is resistant to acid hydrolysis and base
saponification should produce soils characterised by
highly aliphatic OM. Despite debate concerning the
presence of cutan in samples of geological antiquity
(Gupta et al. 2007), we have isolated cutan in a
variety of dryland succulent plants from southern
Africa. The confirmation of this potential cutan source
in this semi-arid setting allows further study of cutan
existence/persistence in the wider geosphere.
Here, we examine the OM composition of
contemporary soils and sediments in the Succulent
Karoo of southern Africa. A programme of field
sampling provided plants and soils across a number
of major climatic and ecological transitions in this
region. SOM and plant cuticles were then
characterised by pyrolysis-GC/MS and FTIR
analyses.
Typical Succulent Karoo SOM comprises a diverse
array of plant-derived aliphatic and aromatic pyrolysis
products. These soils were taken from plots
containing plant genera containing cutan. There is
however a consistent occurrence of an aliphatic signal
extending beyond n-C30. This is the dominant signal in
a number of lipid-extracted soil residues (R1). Further
isolation of the most resistant organic fraction by
means of acid hydrolysis and saponification (R3)
reveals the persistence and enhancement of this
aliphatic signal in a number of samples. The wider
implications of these findings for SOM
turnover/preservation in dryland environments will be
discussed.
A) Isolated cutan pyrolysate from the succulent plant
Aloë jacksonii; B) Typical R1 pyrolysate from Karoo
soils, displaying a prominent aliphatic signal.
References:
Boom A. et al (2005) Cutan, a common aliphatic
biopolymer in cuticles of drought-adapted plants.
Organic Geochemistry, 36(4), 595-601.
de Leeuw, J.W. et al (2006) Biomacromolecules of
algae and plants and their fossil analogues. Plant
Ecology 182, 209-233.
Gupta, N.S., et al (2007) Experimental evidence for
the formation of geomacromolecules from plant leaf
lipids. Organic Geochemistry, 38(1), 28-36.
Marschner, B. et al (2008) How relevant is
recalcitrance for the stabilization of organic matter in
soils? J Plant Nutrition Soil Sci 171, 91-110.
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