Mid-Infrared and Near-Infrared Diffuse Reflectance Spectroscopy for Soil Carbon Measurement

Diffuse reflectance spectroscopy offers a nondestructive 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 domiStates. 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 CO3-C kg 1 ). These soil samples were analyzed with and These divergent spectral characteristics may be exwithout an acid treatment for removal of CO3. 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 supeOver 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 determiinformation 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 CO3 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 Griffiths, 1993a, 1993b). The strength of these absorptions I CO2 content of the atmosphere from ancan lead to spectral distortions and nonlinearities (Culthropogenic 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, howThe 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. AgSkjemstand, 1995; Janik et al., 1998; Reeves et al., 2001) ricultural croplands have a great potential for sequesterhas demonstrated that good quantitative measurements ing atmospheric C (Lal et al., 1998), but current technolare 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 establish 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 ([email protected]). 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). MCCARTY ET AL.: INFRARED DIFFUSE REFLECTANCE SPECTROSCOPY 641 Fig. 1. Geographic location of the 14 sampling sites within the west central United States. of soil carbonates involved addition of 100 mL of 0.33 M in these evaluations was limited to a few agricultural H3PO4 to 5 to 6 g of soil and shaking for 1 h. The procedure fields, and a question remained concerning the ability was repeated until the pH of the soil solution remained within to establish broader calibrations across diverse soil types. 0.2 pH unit of that of the original acid solution (Follett et al., The purpose of this study was to compare the abilities 1997; Follett and Pruessner, 2000). These acidified soil samples of MIRS and NIRS to measure total, organic, and inorwere oven dried at 60 C, ground to pass a 180m screen ganic C in a highly diverse set of soils and to assess opening, and analyzed for C by dry combustion. Follett and feasibility of establishing regional diffuse reflectance Pruessner (2000) reported that acidification removed soil inorcalibrations for soil C. ganic C (carbonates), but little or no organic C. However, they did caution that for some soils, acidification may remove neutral sugars and possibly other soluble organic compounds MATERIALS AND METHODS and the significance of this influence needs further investigation. Soil Collection and Conventional Analyses The 273 samples used in this study were soil profile samples Infrared Spectroscopy collected as described by Follett et al. (2001) from 14 geographically diverse locations in the central United States (Fig. Samples were scanned in the MIR from 4000 to 400 cm 1 1). Soil temperature regimes ranged from thermic to frigid (2500–25 000 nm) at 4 cm 1 resolution with 64 coadded scans and soil moisture regimes from udic to aridic. From each per spectra, on a DigiLab FTS-60 Fourier transform spectromlocation, the soil samples were collected from adjacent parcels eter (Bio-Rad, Randolph, MA) equipped with standard DRIFT of land under crop production, native vegetation (never cultioptics under purge and with a custom fabricated sample transvated), and conservation reserve program (CRP) manageport which allowed a 50 by 2 mm sample to be scanned ment. The soils were sampled to a depth of 200 cm by genetic (Reeves, 1996). Samples of ground soil were placed in the horizons with the surface layer sampled at 0 to 5, 5 to 10, and sample cell without sample dilution and no precautions were 10 to 25 cm (bottom of the Ap for cultivated soils). Before used to avoid specular reflection. Log-transformed reflectance analyses, soil samples were air dried, mixed, sieved, and data was used in analysis. Near infrared spectra were obtained ground by a roller mill (180m mesh size). Soil C analyses using a NIRSystems model 6500 scanning monochromator were performed by dry combustion (1500 C) on a Carlo Erba (Foss-NIRSystems, Silver Spring, MD). Samples were scanned C/N analyzer (Haake Buchler Instruments Inc., Saddle Brook, from 1100 to 2498 nm (PbS detector) using a rotating cup. NJ ). Total soil C was determined on unamended soil samples Data were collected every 2 nm (700 data points per spectra) and organic soil C was determined on acidified soil samples. at a resolution of 10 nm. Inorganic soil C was determined by difference between total and organic soil C. The acidification procedure for removal Statistical Analysis Descriptive statistics on soil properties were performed us1 Trade and company names are included for the benefit of the ing SAS data analysis software (SAS, 1988),and analyses of reader and do not imply endorsement or preferential treatment of the product by the authors or the USDA. NIRS and MIRS spectral were performed by Partial least 642 SOIL SCI. SOC. AM. J., VOL. 66, MARCH–APRIL 2002 Table 1. Location, soil series, texture, and classification of soils studied. Location Map symbol† Soil series Texture Taxonomic classification Akron, CO COS Weld silt loam Fine-loamy, smectitic, mesic Aridic Argiustolls Indianola, IA IAS Macksburg silty clay loam Fine, smectitic, mesic, Aquic Argiudolls Dorothy, MN DOS Radium loamy sand Sandy, mixed, frigid, Oxyaquic Hapludolls Glencoe, MN GCS Nicollet clay loam Fine-loamy, mixed, superactive, mesic Aquic Hapludolls Roseau, MN ROS Percy loam Coarse-loamy, mixed, superactive, frigid Typic Calciaquolls Columbia, MO MOS Mexico silt loam Fine, smectitic, mesic Aeric Vertic Epiaqualfs Sidney, MT MTS Bryant loam Fine-silty, mixed, superactive, frigid Typic Haplustolls Lincoln, NE NES Crete silt loam Fine, smectitic, mesic Pachic Argiustolls Mandan, ND MDS Farnuf loam Fine-loamy, mixed superactive, frigid Typic Argiustolls Medina, ND MES Barnes loam Fine-loamy, mixed, superactive, frigid Calcic Hapludolls Boley, OK BOS Stephenville loamy fine sand Fine-loamy, siliceous, active, thermic Ultic Haplustalfs Vinson, OK VIS Madge loam Fine-loamy, mixed, superactive, thermic Typic Argiustolls Bushland, TX BLS Pullman clay loam Fine, mixed, superactive, thermic Torrertic Paleustolls Dalhart, TX DHS Dallam fine sandy loam Fine-loamy, mixed, mesic Aridic Paleustalfs