There is a growing interest in the use of diffuse reflectance spectroscopy to rapidly, inexpensively and non-destructively quantify soil organic and inorganic C (McCarty et al., 2002; Shepherd and Walsh, 2002). However, while a number of infrared chemometric or data mining models have been presented, there are relatively few published studies that attempt to map diffuse reflectance absorption features for soil organic matter (Baes and Bloom, 1989; Henderson et al., 1992). While correlations have been found between SOC and a range of wavelengths, it is not clear to what extent these are due to (i) SOM absorptions or (ii) SOM association with light absorbing materials (e.g. Fe-oxides, clays minerals, and water).
Using conventional laboratory methods (total combustion and pressure-calcimeter), Visible and Near-Infrared (VNIR) and Mid-Infrared (MIR) spectroscopy, we analyzed 315 surface and subsoil samples from calcareous soils at six sites in north central Montana. For the fine earth fraction (< 2 mm), we milled and scanned both whole soil and soil with SOM removed using Na-hypochlorite (Mikutta et al., 2005). For the MIR region we scanned both neat samples (with and without SOM) and samples diluted to 1% in KBr (a non-absorbing matrix). Wavelength-specific SOM light absorption was computed from the difference in whole soil and SOM-removed reflectance values, as a fraction of SOM-removed reflectance.
In the VNIR region, we found that SOM absorption was generally featureless, with the primary effect of SOM being a lower overall albedo. However, absorption was highest at approximately 600 nm and declined exponentially with increasing wavelength. Our results refute the common assumption that VNIR SOM reflectance features are due only or even primarily to overtones and combinations of fundamental MIR vibrational absorptions (that decline in intensity with smaller wavelengths). Correlations of SOC with SOM absorption were greatest between 500 to 1000 nm with a peak correlation at 630 nm. Empirical chemometric models, however, show significant information at greater wavelengths, suggesting that calibrations could be at least partially dependent upon the association of SOM with other soil materials.
In the MIR region, distinct SOM vibrational absorption features correlated with SOC concentrations can be readily identified in the KBr-diluted soil materials. However, with undiluted soil materials specular reflectance can distort these features, raising the issue of whether sample dilution in a non-absorbing matrix is required for consistent analysis (Reeves et al., 2005).
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