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Mar. Environ. USEPA. 1995. Test methods for evaluating solid waste. Vol. IA: Res. 48:311–333. Laboratory manual physical/chemical methods. SW 846. 3rd ed. Van Kauwenbergh, S.J. 1997. Cadmium and other minor elements in U.S. Gov. Print. Office, Washington, DC. world resources of phosphate rock. Paper presented at the Fert. USEPA. 1996. Soil screening guidance: User’s guidance. EPA 540/ Soc. London. Int. Fert. Soc., York, UK. R-96/018. U.S. Gov. Print. Office, Washington, DC. Mid-Infrared and Near-Infrared Diffuse Reflectance Spectroscopy for Soil Carbon Measurement G. W. McCarty,* J. B. Reeves III, V. B. Reeves, R. F. Follett, and J. M. Kimble ABSTRACT Diffuse reflectance spectroscopy offers a nondestruc- tive means for measurement of C in soils based on the The ability to inventory soil C on landscapes is limited by the reflectance spectra of illuminated soil. Both the NIR ability to rapidly measure soil C. Diffuse reflectance spectroscopic (400–2500 nm) and MIR (2500–25 000 nm) region have analysis in the near-infrared (NIR, 400–2500 nm) and mid-infrared been investigated for utility in quantifying soil C (Dalal (MIR, 2500–25 000 nm) regions provides means for measurement of soil C. To assess the utility of spectroscopy for soil C analysis, we and Henry, 1986; Meyer, 1989; Janik et al., 1998; Reeves compared the ability to obtain information from these spectral regions et al., 1999; McCarty and Reeves, 2000; Reeves et al., to quantify total, organic, and inorganic C in samples representing 14 2001). The characteristics of spectra obtained in these soil series collected over a large region in the west central United regions varies markedly, with the MIR region domi- States. The soils temperature regimes ranged from thermic to frigid nated by intense vibration fundamentals, whereas the and the soil moisture regimes from udic to aridic. The soils ranged NIR region is dominated by much weaker and broader considerably in organic (0.23–98 g C kg -1 ) and inorganic C content signals from vibration overtones and combination bands. (0.0–65.4 g CO 3 -C kg -1 ). These soil samples were analyzed with and These divergent spectral characteristics may be ex- without an acid treatment for removal of CO 3 . Both spectral regions pected to have substantial influence on the ability to contained substantial information on organic and inorganic C in soils obtain quantitative information from spectral data. studied and MIR analysis substantially outperformed NIR. The supe- Over the last two decades, NIR spectroscopy (NIRS) rior performance of the MIR region likely reflects higher quality of has developed as a major tool for quantitative determi- information for soil C in this region. The spectral signature of inorganic nations of components within often complex organic C was very strong relative to soil organic C. The presence of CO 3 matrices whereas MIR spectroscopy (MIRS) has been reduced ability to quantify organic C using MIR as indicated by improved ability to measure organic C in acidified soil samples. The used mainly in research for qualitative analysis involving ability of MIR spectroscopy to quantify C in diverse soils collected spectral interpretation of chemical structures. The main over a large geographic region indicated that regional calibrations reason for the exclusion of MIRS in quantitative analysis are feasible. has been the belief that quantitative analysis using the MIR region required KBr dilution because of the strong absorptions present (Perkins, 1993; Olinger and Grif- fiths, 1993a, 1993b). The strength of these absorptions I ncreasing CO 2 content of the atmosphere from an- can lead to spectral distortions and nonlinearities (Cul- thropogenic sources has stimulated research to assess ler,1993), and could make quantitative analysis difficult the role of terrestrial ecosystems in the global C cycle. or impossible in undiluted samples. Recent work, how- The terrestrial biosphere is an important component of ever, with a number of sample matrices including food the global C budget, but estimates of C sequestration (Downey et al., 1997; Kemsley et al., 1996; Reeves and in terrestrial ecosystems are partly constrained by the Zapf, 1998), forage (Reeves, 1994), and soil (Janik and limited ability to assess dynamics in soil C storage. Ag- Skjemstand, 1995; Janik et al., 1998; Reeves et al., 2001) ricultural croplands have a great potential for sequester- has demonstrated that good quantitative measurements ing atmospheric C (Lal et al., 1998), but current technol- are possible in the MIR region. These reports have ogies for monitoring soil C sequestration in terrestrial demonstrated that quantitative MIRS analysis can be ecosystems are not cost effective, or they depend on performed on neat (as is) samples with good accuracy. intensive methods. Recent work has demonstrated good ability to estab- lish local (within-field) NIRS and MIRS calibrations for G.W. McCarty and J.B. Reeves, Environmental Quality Laboratory, soil C (Reeves et al., 1999; McCarty and Reeves, 2000; Building 007 Room 201, BARC-West, Beltsville, MD 20705; V.B. Reeves et al., 2001). The diversity of samples included Reeves III, FDA, Rockville, MD; R.F. Follett, USDA-ARS Fort Collins, CO; and J.M. Kimble, USDA-NRCS Lincoln, NE. Received Abbreviations: MIR, mid-infrared; MIRS, MIR spectroscopy; NIR, 4 Jan. 2001. *Corresponding author (mccartyg@ba.ars.usda.gov). near-infrared; NIRS, NIR spectroscopy; PLS, partial least squares; RMSD, root mean squared deviation; SD, standard deviation. Published in Soil Sci. Soc. Am. J. 66:640–646 (2002).