CAN NEAR- OR MID-INFRARED DIFFUSE REFLECTANCE SPECTROSCOPY BE USED TO DETERMINE SOIL CARBON POOLS?

Authors: Reeves Iii, James; Follett, Ronald; McCarty, Gregory; KIMBLE, JOHN - USDA-NRCS

Submitted to: Communications in Soil Science and Plant Analysis
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/5/2005
Publication Date: 7/1/2006
Citation: Reeves III, J.B., Follett, R.F., Mccarty, G.W., Kimble, J.M. 2006. Can near- or mid-infrared diffuse reflectance spectroscopy be used to determine soil carbon pools? Communications in Soil Science and Plant Analysis. Vol. 37:pp 2307-2325.

Interpretive Summary: Due to differences in plant metabolism between different types of plants, the type of carbon (isotopes) incorporated into plant tissues can vary. These variations in C isotopes can be used to differentiate the source of C in soils. Presently, the only method to determine the isotope values is very expensive, and an alternative would be useful. The objective of this work was to examine the use of optical spectroscopy (near- and mid-infrared) for determining the isotope differences (Soil*13C). Two hundred and thirty seven soil samples were collected from 14 sites in 10 western states (CO, IA, MN, MO, MT, ND, NE, NM, OK, TX). Two sub-sets of these were examined for a variety of C measures by conventional assays and near- and mid-infrared spectroscopy. Biomass-C and -N, soil inorganic C (SIC), soil organic C (SOC), total C, identifiable plant material (IPM) (20X magnifying glass), the ratio of SOC to the silt/clay content, and total-N were available for 185 samples, and mineral associated C, Soil*13C of the mineral associated C, Soil*13C of SOC, % C in the mineral associated C fraction, % organic matter and % organic matter C were available for 114 samples. For many analytical determinations methods (spectroscopy) based on light beyond the range of human sight (i.e., near- and mid-infrared) have come to be used. Results demonstrated that accurate calibrations (equations relating the spectra to composition) for a wide variety of soil-C measures, including measures of Soil*13C, are feasible using MIR spectra. Similar efforts using NIR spectra indicated that while NIR spectrometers may be capable of scanning larger amounts of samples, the results are generally not as good as those achieved using MIR spectra.

Technical Abstract: Two hundred and thirty seven soil samples were collected from 14 sites in 10 western states (CO, IA, MN, MO, MT, ND, NE, NM, OK, TX). Two sub-sets of these were examined for a variety of C measures by conventional assays and near- and mid-infrared spectroscopy. Biomass-C and -N, soil inorganic C (SIC), soil organic C (SOC), total C, identifiable plant material (IPM) (20X magnifying glass), the ratio of SOC to the silt/clay content, and total-N were available for 185 samples, and mineral associated C, Soil*13C of the mineral associated C, Soil*13C of SOC, % C in the mineral associated C fraction, % organic matter and % organic matter C were available for 114 samples. Near-infrared spectra (64 co-added scans) from 400 to 2498 nm (10 nm resolution with data collected every 2 nm) were obtained using a rotating sample cup and a NIRSystems model 6500 scanning monochromator. Mid-infrared diffuse reflectance spectra from 4000 to 400 cm-1 (2500 to 25,000 nm) were obtained on non-KBr diluted samples using a custom made sample transport and a Digilab FTS-60 Fourier transform spectrometer (4 cm-1 resolution with 64 co-added scans). Partial least squares regression was used with a one-out cross validation to develop calibrations for the various analytes using near- and mid-infrared spectra. Results demonstrated that accurate calibrations for a wide variety of soil-C measures, including measures of Soil*13C, are feasible using MIR spectra. Similar efforts using NIR spectra indicated that while NIR spectrometers may be capable of scanning larger amounts of samples, the results are generally not as good as achieved using MIR spectra.