Fjölrit RALA - 15.06.2004, Side 48

NaOH and Na4P207 extractable organic matter in two allophanic volcanic
ash soils of the Azores Islands - a pyrolysis-GC/MS study
P. Buurman1, K.G.J. Nierop2, P.F. van Bergen3 and B. van Lagen1
1 Department of Environmental Sciences, Laboratory ofSoil Sciences and Geology, Wageningen University, The
Netherlands.
2IBED-Physical Geography, University of Amsterdam, The Netherlands.
3Shell Global Solutions International - Amsterdam, The Netherlands.
Andosols are well known for their capacity to store organic matter. The 14C age of soil
organic matter (SOM) in Andosols ranges from modem to 7000 years BP in the Ap and Ah
horizons, similar to several other soil types, but increases with depth. Ages range from 5000
to 30,000 BP and even 150,000 years BP, which is a much larger mean residence time of
SOM than in any other soil type (Wada and Aomine, 1973, Tom et al. 1997). Despite this
notoriety of volcanic ash soils, the organic matter composition has hardly been studied at the
molecular level.
Two extensively analysed soil profiles from the Azores Islands were selected for organic
matter charaterization. These profiles were collected for the COST Action 622, European
Volcanic Soils, which was funded by the European Commission.
Profile N5 (Ah - C - 2Ahb - 2Bw - 2BwC - 3C) consists of two superposed profile,
developed in three ash layers. It is located in extensively grazed grassland on the Island of
Faial, Azores. The altitude is 510 m asl, and the parent material is pyroclastic material.
Temperature regime is mesic, and moisture regime is udic. The soil is tentatively classified as
a Hydric, acmdoxic Fulvudand (SSS, 1999).
Profile N6 (Ah - ABwl - ABw2 - 2Bwl - 2Bw2 - 2Bw3) is developed in two superposed
ash layers. It is located on the Island of Pico, Azores, in a plantation forest with Cryptomeria
japonica with undergrowth of Pittosporum undulatum, Erica azorica, Rubus ssp, and fems.
The altitude is 400 m, and the parent material is basaltic pyroclastics. The temperature regime
is mesic, and the moisture regime is udic. This profile is a Acmdoxic Hydmdand.
Samples were first extracted with 0.1 M NaOH and consecutively with 0.1 M Na4P207
(pH=10). The obtained extractable organic matter fractions were studied by pyrolysis-gas
chromatography/mass spectrometry (py-GC/MS). The pyrolysates of all samples were
dominated by polysaccharide-derived compounds. The Na4P207 extractable fractions were
slightly enriched by markers of lignin, chitin, proteins and Upids. Only in the topsoils (A
horizons) lignin was be present in significant amounts and, notwithstanding substantial side-
chain oxidation, the distribution of lignin-derived products could be related to the overlying
vegetation. A similar trend was observed for lipids, especially the high abundance of C26
alkanol in profile N5 clearly reflects the grass vegetation. Below the topsoils, lignin and lipids
were hardly detectable. With depth, markers of intact polysaccharides decreased relatively to
smaller compounds and also in relation to chitin. The occurrence of large amounts of
polysaccharides including chitin points to an important in situ production of SOM by fungi
and/or arthropods.
The topsoils of both soils are not dark enough to qualify for a melanic epipedon (SSS,
1999), and they lack the very high content of char-derived) aromatic moieties that are so
characteristic of such topsoils. The topsoils of the two soils were not bumt regularly.
The fact that the pyrophosphate extract is slightly enriched in lipids, lignin, chitin and
proteins may have two different explanations: (1) these components are relatively strongly
bound to amorphous silicates and therefore less extractable with NaOH, or (2) NaOH extracts
the majority of the polysaccharides and therefore the residue after NaOH extraction is
relatively enriched in the compounds other than polysaccharides. Because the two extractions
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