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Alteration, formation, and occurrence of minerals in soils

Pedogenesis of Chernozems in Central Europe — A review

Wet-chemical extraction of soil is a standard procedure to characterise pedogenic aluminium (Al) and iron (Fe) species, especially oxides, allophanic minerals and metal–organic associations. This article critically reviews the suitability of commonly used extractants (e.g. dithionite, oxalate and pyrophosphate) and the potentials and restrictions in their use for species identification and in soil classification. None of the commonly used extractants is completely selective and quantitative. The degree of completeness differs between the extractants and depends on soil composition. Dithionite-based methods provide a ‘pseudo-total’ content of pedogenic Fe oxides, as they are not always completely dissolved. Oxalate may attack further non-target species, releasing additional Al and Fe. Therefore, the extraction of Al and Fe exclusively from poorly crystalline species is not always guaranteed. As a consequence of dispersion of aggregates, pyrophosphate solubilises both mineral particles and metals from organic associations. Thus, quantification of species based on these extractions and their implementation in pedogenic thresholds may be questionable. Alternative extractants such as citrate–ascorbate and dithionite–citrate–oxalate could be used in addition, as applicable and reliable wet-chemical extractions will be still demanded for research and practical applications. The examination of the effectiveness and selectivity of wet-chemical extraction methods by spectroscopic techniques is recommended.

Wet-chemical extractions to characterise pedogenic Al and Fe species – a critical review

Preservation of fire-derived carbon compounds and sorptive stabilisation promote the accumulation of organic matter in black soils of the Southern Alps

Weathering of biotite in the presence of arbuscular mycorrhizae in selected agricultural crops

Many soils in southern Switzerland have a black color, contain a large amount of soil organic matter (SOM) and seem to have some andic properties although they did not develop on volcanic parent material. We investigated three typical 'black' soils to determine the mechanisms of (clay) mineral formation and transformation. We measured total element pools as well as the dithionite-, pyrophosphate- and oxalate-extractable fractions (Fe, Al, Si). The clay fraction (<2 μm) was analyzed using X-ray diffraction and FTIR spectroscopy. Iron speciation in the solid phase was determined by Mossbauer spectroscopy. With increasing weathering conditions, the plagioclase (albite) content decreases, trioctahedral species in the clay fraction such as biotite, chlorite or trioctahedral vermiculite either decompose or transform into a dioctahedral mineral such as dioctahedral vermiculite or hydroxy interlayered smectite (HIS). Typical weathering products were hydroxy interlayered vermiculite (HIV), HIS, interstratified minerals and kaolinite. The oxidation of Fe(II) into Fe(III) was evident and contributes to the transformation of trioctahedral mineral species into dioctahedral ones. In one soil, a large part of the Fe (up to 41%) was found in the form of Fe oxides. In the surface horizon, the poorly crystalline mineral ferrihydrite was dominant, while in the subsoil goethite prevailed. Maghemite (or maghemite/hematite mixture) was, furthermore, found in distinct concentrations down to a depth of ∼50 cm. The formation of this mineral requires high temperatures which means that a forest fire can influence soil mineralogy down to a considerable depth. The specific climatic conditions with periods of strong humidity alternating with periods of winter droughts, sporadic fire events and the relatively large content of poorly crystalline fractions of Fe and Al contributed to the stabilization of SOM.

M. Nötzli

Papers

'Black' soils in the southern Alps: clay mineral formation and transformation, X-ray amorphous Al phases and Fe forms