Mineralogical characterization of the fine fraction (<2µm) of degraded volcanic soils and tepetates in Mexico

In Mexico, 70% of the land surface shows some degree of degradation. A substantial portion of these degraded soils are located in the central part of the country, where a high population density exerts unusual pressure on the land. The study of these degraded soils is important because of the ecological, social and economic consequence of this ecosystem component. Two types of degraded volcanic soils were studied in the present research: one coming from tepetates (a volcanic tuff, partially altered and ameliorated for production purposes) and the other, a highly eroded Acrisol developed from old volcanic materials. These soils have not been much studied and are here explored due to their potential to sequester carbon. In studies to focus the relationship between mineralogy and the carbon sequestration it will be necessary to clarify the characteristics of the fine fraction of the soil (<2µm). Clays have been reported to show different mechanisms of association with soil organic matter, in accordance with their nature. In this paper, a mineralogical characterization was made of the fine (2–1µm) and very fine (<1 µm) fractions of these soils, considered to be the most active in the sequestration process. The characterization was initially developed by using X-ray diffraction (XRD). However, the results obtained with this technique were not conclusive. In addition, due to the fact that XRD sometimes requires tedious chemical treatments and take time, it is proposed here to use spectroscopical techniques other that the traditional ones to more accurately define the mineralogical characteristics of the studied fractions. The diffuse reflectance infrared Fourier transform (DRIFT), 27Al magic angle spinning nuclear magnetic resonance (27Al MAS NMR), 29Si magic angle spinning nuclear magnetic resonance (29Si MAS NMR), transmission electron microscopic (TEM), high resolution transmission electron microscopy (HRTEM) and Mössbauer spectroscopy were employed. The fine fractions (<2µm) of the degraded soils are made up of low activity clays: tubular halloysites in the Te–Tl tepetates and kaolinites in the Acrisol. The coarse (2–1µm) ones in Te–Tl consist also of cristobalite and albite. In Ac–At, akaganeite, goethite and hematite are the principal Fe-mineral components. For this reason, the restoration techniques proposed for these degraded soils must be complemented with appropriate practices of fertilization providing basic elements (Ca, Mg, K and Na) to the soil that can be rapidly lost, and are associated with low activity of the fine fraction.