Thursday, 13 July 2006

Application of Visible-Near-Infrared Diffuse (DRS) and Bi-directional Reflectance Spectroscopy (BRS) to Characterize Volcanic Soil Properties.

Vincenzo Michele Sellitto1, Vidal Barrón2, Giuseppe Palumbo1, and Claudio Colombo1. (1) Dip. SAVA Molise Univ, Via De Sanctis, Campobasso, Italy, (2) Universidad de Córdoba, Edificio C4, Campus de Rabanales, Córdoba, Spain

Volcanic soils, recently recognized as pedological units, are characterized by physics, chemical and mineralogical properties very interesting which rarely we find in soils derived from others parental material. These distinctive properties are attributable mainly to the formation of amorphous materials. e.g. non-crystalline materials such as allophane, imogolite, ferrihydrite and Al-humus complexes. Diffuse (DRS) and bidirectional reflectance spectroscopy (BRS) has been successfully used for characterizing iron crystalline minerals. Although the spectra of soils with unknown mineralogy may be qualitatively evaluated for comparison with spectra of reference minerals, a quantitative approach requires the adoption of suitable procedures of spectra parameterization, such as the second derivative of the remission function or colour calculations. Iron oxide mineralogy is well characterized by spectral reflectance curves or by the Kubelka-Munk remission function as an alternative technique to more expensive laboratory analyses, such as differential X-ray diffraction or Mossbauer spectroscopy. This alternative technique analysis and in particular, second derivative spectroscopy, was used in this research to characterize the spectral properties of volcanic soils. We measured the reflectance (diffuse and bidirectional) in the visible-near infrared region (350–2500 nm) of 17 volcanic soil profiles from different European sites (Italy, Portugal, Iceland, Spain, France and Hungary). Volcanic soils were analysed by selective chemical dissolution, and in particular Fe, Al, and Mn were extracted by sodium dithionite-citrate (Fed, Ald, Mnd), ammonium oxalate (Feo, Alo, Mno) and pyrophosphate sodium (Fep, Alp and Mnp). Diffuse reflectance (DRS) was measured in the visible region (350–770 nm) using a Jasco V-560 UV/VIS spectrophotometer with the help of an integrating sphere and calibrated white standard. We also measured bidirectional reflectance (BRF) in the visible and infrared region (350 -2500 nm) using VIS-NIR spectroradiometers, both with a resolution of 1 nm. The laboratory measurements were conducted on air-dried soils, previously sieved at < 2 mm and vigorously ground in an agate mortar for at least 10 minutes in order to exclude the influence of micro-aggregation. Then the samples were gently pressed against unglazed white paper in order to avoid undesired particle orientation. Diffuse and bi-directional spectral data were transformed into their remission functions and then the second derivative curves of these functions were calculated. The amplitude of (Y1) and (Y2) absorption bands were then computed as follows: • (Y1) = difference between the second derivative minima at ca. 421 nm and maxima at ca. 450 nm, • (Y2) = difference between the second derivative minima at ca. 926 nm and maxima at ca. 946 nm. Using correlation analysis, significant relationships were observed between amplitude of (Y1) and Fed (r = 0.6) and between amplitude of band (Y2) and organic matter content (r = 0.7). In addition, the data showed that soil organic matter content, Ald and Fep were moderately correlated with visible and infrared region (350 -2500 nm) reflectance values centred at 546, 579 and 2048 nm. Selective chemical dissolution data clearly indicate that visible and infrared region (350 -2500 nm) reflectance can be useful in a volcanic soil mineralogy. Significant correlation between the amplitude of (Y1) and (Y2) and the position of the absorption bands could be used to estimate crystalline Fe oxides in order to understand the rate of alteration processes. Knowledge of the spatial distribution Fe mineralogy can be used to obtain important information on soil properties in order to improve soil management and conservation in volcanic systems.

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