Application of Diffuse Reflectance Spectroscopy (DRS) to study European Volcanic Soils: a preliminary examination

This study shows that the use of spectrophotometer measurements gives precise, reproducible results, and solves the problem of the visual and subjective observation of soil color, influenced by factors such as the eye’s adaptability, background and illumination. The visual estimates of Munsell color value and the instrumental measurements were moderately correlated. Discrepancies between the two measurements originate from different lighting, sample preparation, soil moisture and particles size. The measurements of the soil color of volcanic European soil using a spectrophotometer method range from 3.8Y (yellow) to 6.7 YR hue (reddish yellow), 3.7 to 7.3 value, 1.5 to 5.4 chroma. Minor changes in color were observed on samples with Munsell hues between 7.5YR and 10YR for considerable variations in the organic matter content (from 0.1% to 35%) while for samples with Munsell hues between 0.1Y and 4Y the organic matter caused significant changes in soil color. The high organic carbon content was responsible for the dark color of the volcanic soil studied with the reduction in Munsell value. A significant positive correlation between soil Fed (extracted by dithionite solution) and Munsell chroma indicated that both poorly ordered and crystalline Fe oxide minerals are especially important for color development in volcanic soils. Munsell chromas <2 could be considered as an indication of low Fe oxides content, although soils with high chroma (>3) generally exhibited higher amounts of Fe and showed more intense weathering. Differential chemical dissolution data clearly indicate that the color of volcanic soils is largely determined by Fed and Fed-Feo (organo-iron complexes, ferrihydrite and crystallline oxihydroxides such as goethite, hematite). Poorly ordered Fe oxides estimated with Feo and Feo-Fep (organo-iron complexes and ferrihydrite) are closely linked to organic carbon. Although these results are only preliminary, they show solid evidence that diffuse reflectance spectroscopy (DRS) can be a useful method for determining color in volcanic soils. In particular, we demonstrated that the yellow index was significantly correlated with dithionite citrate-extracted iron, confirming that not only soil organic matter but also some yellow Fe oxide minerals like goethite, strongly influence the color of volcanic soils. Analysis of the Fe minerals played a special role in clarifying the type of weathering in volcanic systems. More specifically, the presence of hematite and goethite could provide essential insights to understand the rate of alteration processes. Knowledge of the spatial distribution of color can be used to obtain important information on soil properties in order to improve soil cultivation and conservation of volcanic soils. These results also suggest that soil color calculated by reflectance spectra could be used to study soil weathering processes.