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Title: Diffuse-reflectance fourier-transform mid-infrared spectroscopy as a method of characterizing changes in soil organic matter

Author
item Calderon, Francisco
item HADDIX, MICHELLE - Colorado State University
item CONANT, RICHARD - Colorado State University
item MAGRINI-BAIR, KIM - Department Of Energy
item PAUL, ELDOR - Colorado State University

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/16/2013
Publication Date: 9/20/2013
Publication URL: http://handle.nal.usda.gov/10113/61922
Citation: Calderon, F.J., Haddix, M., Conant, R., Magrini-Bair, K., Paul, E.A. 2013. Diffuse-reflectance fourier-transform mid-infrared spectroscopy as a method of characterizing changes in soil organic matter. Soil Science Society of America Journal. 77:1591-1600.

Interpretive Summary: In this experiment, we added different organic substances to soil in order to test the sensitivity of infrared spectroscopy to changes in soil carbon chemistry. This will be very useful knowledge because infrared spectroscopy can be an inexpensive and rapid way to analyze soil organic matter and test the effect of farming practices or environmental disturbances on soil quality.

Technical Abstract: Diffuse-Reflectance Fourier-Transform Mid-Infrared Spectroscopy (MidIR) can identify the presence of important organic functional groups in soil organic matter (SOM). Soils contain myriad organic and inorganic components that absorb in the MidIR so spectral interpretation needs to be validated in order to correctly assess changes in soil organic matter quality and quantity. We amended soils with known standards increasing the total C in the sample by 50%, and measured changes in MidIR spectra. Adenine, casein, cellulose, ergosterol, glucosamine, glycine, guanine, indole, methionine, palmitic acid, egg protein, chlorophyllin, tannic acid, xylose, urease, and vanillin standards were used. Two different soils were used in the mixing scheme: A Hoytville, Ohio soil (2.5% C, and 36 % clay), and an Akron, Colorado soil (1.5% C, and 14 % clay). Addition of standards with higher than 10% N content resulted in increased amide-like absorbance at 1670 cm-1, 1588 cm-1 and 1513 cm-1. Bands at 2970-2800 cm-1, 2200-2000 cm-1 and 1030-1160 cm-1 were sensitive to added polysaccharide. Protein addition increased absorption at 2970-2800 cm-1, but also increased the 1691 cm-1, 1547 cm-1 amide bands. Vanillin, an aromatic standard, resulted in higher absorbance at the 1592 cm-1, 1515 cm-1 and 1295 cm-1aromatic C=C bands. Our results show that MidIR is sensitive to relatively small changes in SOM, and that provided some assumptions about the soil mineralogy are met, specific spectral bands can be used to follow changes in SOM chemistry. Our results show that MidIR can provide a wealth of qualitative chemical information, but quantitative interpretation across the MidIR range requires further research.