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- PublicationAccès libreBiologically induced accumulations of CaCO3 in orthox soils of Biga, Ivory CoastBiologically induced accumulations of calcium carbonate have been found inside orthox soils, under and around the native iroko tree Milicia excelsa (Moraceae) in Biga (Ivory Coast). The nature of these accumulations and their origin were studied in two soil profiles, directly under the tree and at a distance of 30 cm from the trunk. Microscale forms of CaCO3 include: (1) wood pseudomorphic structures such as parenchyma cells, cellulose fibers, and calcitic vessel infillings; (2) three types of rhombohedra; and (3) needle fiber calcite (NFC). In addition, large scale blocks exhibit three types of textures: (1) micritic calcite, which seems to be the original material; (2) light-colored sparite in moldic voids; and (3) asymmetrical radiaxial laminated fibrous cement. Some micritic aggregates and hemi-spherulites (vaterite) were found in the sap on the trunk as well as in soils on silica grains and the wood itself. The mineralogy of all these carbonate forms is mainly a stoichiometric calcite or a moderately enriched Mg calcite. However, some samples contain monohydrocalcite, as well as two polymorphs of calcium oxalate (weddellite and whewellite). Calcite precipitation is facilitated by the oxidation of oxalate by soil bacteria that contributes to the increase in pH in Biga soils. This is in contrast to conventional orthox soils. Therefore, three conditions are necessary for biologically induced precipitation of calcium carbonate in orthox soils associated with iroko trees: the presence of a large amount of oxalate (originating from the tree and fungi), the existence of an oxalotrophic flora for oxalate oxidation into carbonate, and a dry season.
- PublicationMétadonnées seulementCycle du carbone et biominéralisation carbonatée en milieu continental: la diagénèse des phases oxalate-carbonateIn terrestrial environments, the carbon sequestration pool in the global carbon cycle is almost entirely attributed to soil and plant organic carbon storage while soil mineral carbon is generally neglected. Nevertheless, significant biologically induced accumulations of carbonate have been observed in soils under the tropical tree "iroko" (Milicia excelsa). Without an input of calcium from carbonate sources in the system, these accumulations constitute a carbon sink by definition. Approximately 1.2. 10-4 to 2.3.10-3 PgC are not sequestered each year due to deforestation of irokos. This calculation is based on an average of 5.76 kg/yr of carbon sequestration by one tree and extrapolated for to the iroko population in Africa. This proposed quantification suggests that the iroko carbon sink is about one or two orders of magnitude less than some environments such as coral reefs or continental shelfes. Furthermore, this carbon sink is highly significant because the residence time of mineral carbon in carbonate-enriched soils associated with the irokos is 102 - 106 years, i.e. 100,000 times longer than soil organic matter residence time. These accumulations of carbonate are the result of the oxalate-carbonate transformation by oxalotrophic bacteria. The iroko tree provides a large amount of oxalate (a photosynthetic by-product), which once released in the soil, can be consumed by soil oxalotrophic bacteria. There are two consequences of this consumption: (i) carbonate ions are produced and available in the soil solution, (ii) and soil pH increases leading to favorable conditions for carbonate precipitation (in a primary acid soil with a pH around 5, the induced pH can reach 9). In the iroko ecosystem, biological agents are present and active at various scales. Termites and saprophytic fungi are involved in the release of oxalate crystals in the soil. Soil oxalotrophic bacteria are able to consume oxalate leading to CO32- production. Carbonate ions can be pumped by iroko roots, circulating into wood vessels in which they can form calcium carbonate crystals in the presence of calcium, when conditions are favorable. They can also precipitate as calcium carbonate in soil pores, depending on local conditions. Soil bacteria and fungi obviously influence precipitation of carbonate inside the soil. Moreover, regarding carbon isotopic signatures and crystallographic properties of needle fiber calcite (NFC) observed in surficial environments, including African soils, they result from a direct biogenic influence. The nomenclature of the various types of NFC has been reinterpreted regarding diagenesis and geochemical fingerprints. The possible biogenic origin of nanorods usually found in the presence of NFC is also discussed from new observations and experiments. In conclusion, this study has demonstrated the potential importance of the oxalate-carbonate pathway in the global carbon cycle, emphasizing the crucial interrelations between geological and biological processes.