There is a growing interest in the use of proximal Visible and Near-Infrared (VNIR) diffuse reflectance spectroscopy to rapidly, inexpensively and non-destructively quantify soil organic and inorganic C (McCarty et al., 2002; Shepherd and Walsh, 2002).  However, there are relatively few published studies on the potential of this technique for in situ soil characterization (Sudduth and Hummel, 1993).

Using conventional laboratory methods (total combustion and pressure-calcimeter) and VNIR spectroscopy we analyzed 315 surface and subsoil samples from calcareous soils at six sites in north central Montana.  VNIR scans were obtained from (i) undisturbed soil cores, field-moist and air-dry; (ii) crushed and sieved samples (< 2 mm); and (iii) fine milled samples.

All prediction errors were estimated using whole-site cross-validation.  To enhance predictions, we also augmented model calibration using the following datasets: (i) 283 samples from a different set of six sites in north central Montana (Brown et al., 2005a); and (ii) a global soil-spectral library with over 3000 samples (Brown et al., 2005b).  For all calibrations and sample preparations the standard error of prediction (SEP) was much larger than estimated error for the reference laboratory measurement (SEL).  Based upon these results, we suggest that VNIR diffuse reflectance spectroscopy cannot achieve analytical precision for soil C determination either in situ or in vitro—at least for low SOC, carbonate-rich soils like those found in north central Montana.  However, proximal VNIR could be used to map the spatial variability of organic and inorganic C, potentially in combination with other soil sensing techniques.

References

Brown, D.J., Bricklemyer, R.S. and Miller, P.R., 2005a. Validation requirements for diffuse reflectance soil characterization models with a case study of VNIR soil C prediction in Montana. Geoderma, 129(3-4): 251-267.

Brown, D.J., Shepherd, K.D., Walsh, M.G., Mays, M.D. and Reinsch, T.G., 2005b. Global soil characterization with VNIR diffuse reflectance spectroscopy. Geoderma, (in press).

McCarty, G.W., Reeves, J.B., Reeves, V.B., Follett, R.F. and Kimble, J.M., 2002. Mid-infrared and near-infrared diffuse reflectance spectroscopy for soil carbon measurement. Soil Sci. Soc. Am. J., 66(2): 640-646.

Shepherd, K.D. and Walsh, M.G., 2002. Development of reflectance spectral libraries for characterization of soil properties. Soil Sci. Soc. Am. J., 66(3): 988-998.

Sudduth, K.A. and Hummel, J.W., 1993. Soil Organic-Matter, CEC, and Moisture Sensing with a Portable NIR Spectrophotometer. Trans. ASAE, 36(6): 1571-1582.