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Observations de concentrations argileuses dans les structures racinaires au cours des transformations pedologiques.
... Plants can locally induce the precipitation of minerals, either in the rootrhizosphere system or in their aerial parts, that is, stems, leaves, and flower parts. The precipitation of clay (kaolinite) in root cells accompanying decomposition of cell tissues was demonstrated byCallot et al (1984Callot et al ( , 1992). The precipitation occurs when the secondary cell wall is still intact, which rules out the hypothesis of a physical displacement of clay particles from the surrounding soil. ...
Key Words plant cycling, organometallic complexes, biological weathering, soil genesis, stability of soil minerals s Abstract The recycling of elements by plants and plant-induced biological ac-tivity cause the rates and products of weathering to be markedly different from what would result in abiotic processes. Plants directly control water dynamics, weathering, and the chemistry of weathering solutions, which is clearly exhibited in equatorial areas where old weathering mantles are greatly influenced by biological activity. Depending on the dynamics of plant-induced organometallic compounds, this weathering results in either clayey soils, which are in a dynamic equilibrium sustained by the forest's cycling of elements, or sandy soils. In most places (tropical as well temperate areas), the weathering mantle can be regarded as being in a dynamic equilibrium sustained by plants.
The ability of the roots of rape (Brassica napus) to promote the transformation and dissolution of a phlogopite mica was studied in the rhizosphere. Rape was cropped for 1 to 32 d on 2–105 μm phlogopite as sole source of both K and Mg. The chemical balance of K and Mg revealed that rape was able to induce a significant release of interlayer-K after 4 d, and even a significant release of octahedral-Mg after 8 d of cropping. After 32 d, the root-induced release amounted to 80 and 21 g kg−1 of total-K and -Mg, respectively. The weathering products sampled in the close proximity of the roots were analysed by X-ray diffractometry (XRD), and by energy dispersive X-ray (EDX) microanalysis of resin-embedded samples prepared for transmission electron microscopy. The XRD analysis of K-saturated, oriented plates showed that part of the vermiculite formed by root activity behaved as a typical hydroxy-aluminous interlayered vermiculite (HIV). The EDX microanalysis revealed that the release of interlayer-K which occurred during the vermiculitization was compensated mostly by A1 and Mg originating from the octahedral sheet of the phlogopite. Such crystallographic and crystallochemical changes necessarily involved an acid dissolution of the mica structure, which may be related to the root-induced pH decrease encountered simultaneously in the rhizosphere. Proton excretion by rape roots was thus the probable mechanism involved in the root-induced irreversible transformation of the phlogopite.
Many, if not most, physical and chemical properties of soils required for plant growth are affected strongly by biotic processes. Feedback processes involving primary producers and decomposers may be involved in the development of properties that favour net primary productivity in terrestrial ecosystems. However, both constructive and destructive effects of biota on soils can be observed. Apparently, effects favouring net primary production have accumulated and presently prevail in the various terrestrial ecosystems of the world. In some ecosystems, however, the dominant vegetation gains competitive advantage by making soils un “favourable” for most other plants. Ombrotrophic peat bogs and heathlands are cases in point.On the global scale, biotic processes can be seen as responsible for the persistence of water on the earth, through control of the earth's surface temperature under the influence of greenhouse gases. As a result, therefore, the large-scale geochemical and hydrological cycles, which are essential for chemical rejuvenation of the earth's surface, also depend on life processes. This is an aspect of Lovelock's Gaia hypothesis, which states that the earth has evolved over geologic time by feedback processes keeping the earth in a state comfortable for life by the action of living organisms.While the development of soils with “favourable” properties may be explained in evolutionary terms, such an explanation apparently does not suffice for the development of Gaia as a whole.
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