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| Titel |
Turning sunlight into stone: the oxalate-carbonate pathway in a tropical tree ecosystem |
| VerfasserIn |
G. Cailleau, O. Braissant, E. P. Verrecchia |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 8, no. 7 ; Nr. 8, no. 7 (2011-07-05), S.1755-1767 |
| Datensatznummer |
250006040
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| Publikation (Nr.) |
 copernicus.org/bg-8-1755-2011.pdf |
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| Zusammenfassung |
| An African oxalogenic tree, the iroko tree (Milicia excelsa), has the property to enhance
carbonate precipitation in tropical oxisols, where such accumulations are
not expected due to the acidic conditions in these types of soils. This
uncommon process is linked to the oxalate-carbonate pathway, which increases
soil pH through oxalate oxidation. In order to investigate the
oxalate-carbonate pathway in the iroko system, fluxes of matter have been
identified, described, and evaluated from field to microscopic scales. In
the first centimeters of the soil profile, decaying of the organic matter
allows the release of whewellite crystals, mainly due to the action of
termites and saprophytic fungi. In addition, a concomitant flux of carbonate
formed in wood tissues contributes to the carbonate flux and is identified
as a direct consequence of wood feeding by termites. Nevertheless, calcite
biomineralization of the tree is not a consequence of in situ oxalate consumption,
but rather related to the oxalate oxidation inside the upper part of the
soil. The consequence of this oxidation is the presence of carbonate ions in
the soil solution pumped through the roots, leading to preferential
mineralization of the roots and the trunk base. An ideal scenario for the
iroko biomineralization and soil carbonate accumulation starts with
oxalatization: as the iroko tree grows, the organic matter flux to the soil
constitutes the litter, and an oxalate pool is formed on the forest ground.
Then, wood rotting agents (mainly termites, saprophytic fungi, and bacteria)
release significant amounts of oxalate crystals from decaying plant tissues.
In addition, some of these agents are themselves producers of oxalate (e.g. fungi). Both processes contribute to a soil pool of "available" oxalate
crystals. Oxalate consumption by oxalotrophic bacteria can then start.
Carbonate and calcium ions present in the soil solution represent the end
products of the oxalate-carbonate pathway. The solution is pumped through
the roots, leading to carbonate precipitation. The main pools of carbon are
clearly identified as the organic matter (the tree and its organic
products), the oxalate crystals, and the various carbonate features. A
functional model based on field observations and diagenetic investigations
with δ13C signatures of the various compartments involved in
the local carbon cycle is proposed. It suggests that the iroko ecosystem can
act as a long-term carbon sink, as long as the calcium source is related to
non-carbonate rocks. Consequently, this carbon sink, driven by the oxalate
carbonate pathway around an iroko tree, constitutes a true carbon trapping
ecosystem as defined by ecological theory. |
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