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Title: SPECTROSCOPIC CALIBRATIONS FOR THE DETERMINATION OF C IN SOILS

Author
item Reeves Iii, James
item McCarty, Gregory
item MIMMO, T - UNIVERSITY OF BOLOGNA, IT
item REEVES, V - FDA
item FOLLETT, R - SPNRU
item KIMBLE, J - NRCS
item GALLETTI, G - UNIVERSITY OF BOLOGNA, IT

Submitted to: World Congress of Soil Science
Publication Type: Proceedings
Publication Acceptance Date: 11/21/2001
Publication Date: 8/1/2002
Citation: REEVES III, J.B., MCCARTY, G.W., MIMMO, T.V., REEVES, V.B., FOLLETT, R.F., KIMBLE, J.M., GALLETTI, G.C. SPECTROSCOPIC CALIBRATIONS FOR THE DETERMINATION OF C IN SOILS. WORLD CONGRESS OF SOIL SCIENCE. 2002.

Interpretive Summary: Spectroscopy uses light to determine the composition of various materials. The objective of this work was to evaluate the ability of spectroscopy to quantify soil organic- and inorganic-carbon. Samples were scanned in the near- and mid-infrared, light regions beyond the range of human sight. In the near-infrared, samples were also scanned using a variety of sampling devices. In addition, samples were also scanned moist, dried but not ground, and dried and ground. In the mid-infrared, dried, ground samples were scanned using a custom made sample transport device designed to increase the sample area scanned. Four basic samples sets and several combined set were examined. Results demonstrated that: 1) Both inorganic- and organic-carbon can be accurately determined using either spectral range, but that mid-infrared results are almost always the more accurate. 2) mid-infrared results are generally more robust (less affected by sample differences and therefore apply to a wider range of samples) than are near-infrared results. 3) While accurate near-infrared results can be achieved using fiber optic probes, there is a decrease in calibration accuracy due to signal lost in some areas of the spectral region, and perhaps also to the optical alignment which may result in less sample being scanned. 4. The use of polyethylene bags as sample holders appears to degrade near-infrared results for dried, but not moist, soils. Finally, preliminary results indicate that accurate mid-infrared results covering a wide range of soil types are feasible. In conclusion, mid- infrared and near-infrared spectroscopy offer the possibility of rapid, inexpensive, and accurate methods for the determination of carbon in soils.

Technical Abstract: The objective was to evaluate the ability of spectroscopy to quantify soil organic- and inorganic-C. Samples were scanned in the NIR from 400 to 2498 nm on a NIRSystems 6500 scanning monochromator, and in the mid-IR from 2500 to 25,000 nm using a DigiLab FTS-60 FTIR. All samples were scanned using diffuse reflectance and non-KBr diluted samples. In the NIR, samples were scanned using a variety of sampling devices. In addition, samples were also scanned moist, dried but not ground, and dried and ground. In the mid-IR, dried, ground samples were scanned using a custom made sample transport device designed to increase the sample area scanned. Four basic samples sets and several combined set were examined. Total-C was determined on all samples by combustion, and for all but the western soils was equivalent to organic-C (acid soils). Results demonstrated that: 1) Both inorganic- and organic-C can be accurately determined using either spectral range, but mid-IR calibrations are almost always the more accurate. 2) mid-IR calibrations are generally more robust (less affected by sample diversity) than are NIR calibrations. 3) While accurate NIR calibrations can be developed using fiber optic probes, there is a decrease in calibration accuracy due to signal attenuation in the 1800 to 2500 nm region, and perhaps also to the optical alignment which may result in less sample being scanned. 4. The use of polyethylene bags as sample holders appears to degrade NIR calibrations for dried, but not moist, soils. Finally, preliminary results indicate that accurate mid-IR calibrations covering a wide range of soil types are feasible. In conclusion, mid-IR and NIR spectroscopy offer the possibility of rapid, inexpensive, and accurate methods for the determination of carbon in soils.