The aim of this work is to study main pedogenetic processes in soils developed from a basaltic parent material. Soils from a toposequence in Sardinia, localised in correspondence of the basaltic plateau of Sedilo, have been selected. Characterization of soils included: i)chemical and physico-chemical analysis, total iron content, ammonium oxalate and dithionite citrate bicarbonate extractable iron of fine earth (ö < 2 mm) fraction, ii) infra-red spectroscopy (FTIR) of fine clay fraction (ö < 0.1 ìm), iii) X-ray diffraction of fine sand-silt (200-2 ìm) and fine clay fractions and iv) micromorphological study of thin sections. In the lower part of the sequence (P3) results show the presence of a clay-rich (> 60% clay content) black Vertisol, 100 cm thick, with mainly sub-alkaline reaction and CEC due, up to about 70%, to accessible permanent charges. In the middle (P2) and on the top (P1) of the slope soils are reddish-brown Inceptisols, 80 and 40 cm thick, respectively, with 30-56 % of clay content, neutral and slightly acidic reaction, CEC due for 50-33% to pH dependent charges. XRD analysis show that clay minerals in the Inceptisol P1 are dominated by kaolinite and kaolinite-smectite mixed layers (K/S) with 90% of kaolinite, whereas the Vertisol contains mainly K/S with a smaller proportion of kaolinite (25-60% kaolinite). That proportion of kaolinite layers in the K/S increases from the deepest C (25-40% kaolinite) to the Ap horizon (60% kaolinite). DCB extractable and total Fe contents indicate that in the Vertisol, particularly in the C horizon, iron occurs mostly in silicates, whereas in the Inceptisol P1 iron is present predominantly as oxide and oxy-hydroxide forms. This is confirmed by the FTIR spectra showing Fe-beidellite and nontronite in the fine clays of the Vertisol C horizon. Different drainage conditions, induced by a gentle slope, at the two end-members of the sequence are thought to govern different pedogenetic processes, with a consequent contrasting mineralogical evolution, in soils developed from the same parent material. The slope favors downward movement of water, which removes silica and soluble cations, promoting the formation of high kaolinite contents, oxides and oxi-hydroxides of Fe and Al. Concentration of soil solutions induced by evaporation in the lower part of the sequence promotes formation of smectites. This relation between topography and soil clay-minerals distribution is similar to that reported for kaolinitic-smectitic red-black soil association in subtropical areas. Nevertheless, micromorphological studies and chemical analysis of iron content in its different forms put in evidence that the degree of complexity of this pedoenvironment is higher than that indicated by mineralogical analysis.

Evoluzione pedogenetica di una toposequenza di suoli sul plateau basaltico di Sedilo (Sardegna centrale)

VACCA, ANDREA;
2004

Abstract

The aim of this work is to study main pedogenetic processes in soils developed from a basaltic parent material. Soils from a toposequence in Sardinia, localised in correspondence of the basaltic plateau of Sedilo, have been selected. Characterization of soils included: i)chemical and physico-chemical analysis, total iron content, ammonium oxalate and dithionite citrate bicarbonate extractable iron of fine earth (ö < 2 mm) fraction, ii) infra-red spectroscopy (FTIR) of fine clay fraction (ö < 0.1 ìm), iii) X-ray diffraction of fine sand-silt (200-2 ìm) and fine clay fractions and iv) micromorphological study of thin sections. In the lower part of the sequence (P3) results show the presence of a clay-rich (> 60% clay content) black Vertisol, 100 cm thick, with mainly sub-alkaline reaction and CEC due, up to about 70%, to accessible permanent charges. In the middle (P2) and on the top (P1) of the slope soils are reddish-brown Inceptisols, 80 and 40 cm thick, respectively, with 30-56 % of clay content, neutral and slightly acidic reaction, CEC due for 50-33% to pH dependent charges. XRD analysis show that clay minerals in the Inceptisol P1 are dominated by kaolinite and kaolinite-smectite mixed layers (K/S) with 90% of kaolinite, whereas the Vertisol contains mainly K/S with a smaller proportion of kaolinite (25-60% kaolinite). That proportion of kaolinite layers in the K/S increases from the deepest C (25-40% kaolinite) to the Ap horizon (60% kaolinite). DCB extractable and total Fe contents indicate that in the Vertisol, particularly in the C horizon, iron occurs mostly in silicates, whereas in the Inceptisol P1 iron is present predominantly as oxide and oxy-hydroxide forms. This is confirmed by the FTIR spectra showing Fe-beidellite and nontronite in the fine clays of the Vertisol C horizon. Different drainage conditions, induced by a gentle slope, at the two end-members of the sequence are thought to govern different pedogenetic processes, with a consequent contrasting mineralogical evolution, in soils developed from the same parent material. The slope favors downward movement of water, which removes silica and soluble cations, promoting the formation of high kaolinite contents, oxides and oxi-hydroxides of Fe and Al. Concentration of soil solutions induced by evaporation in the lower part of the sequence promotes formation of smectites. This relation between topography and soil clay-minerals distribution is similar to that reported for kaolinitic-smectitic red-black soil association in subtropical areas. Nevertheless, micromorphological studies and chemical analysis of iron content in its different forms put in evidence that the degree of complexity of this pedoenvironment is higher than that indicated by mineralogical analysis.
basalti; toposequenza; suoli red-black
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11584/20941
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