Diffuse-Reflectance Fourier-Transform Mid-Infrared Spectroscopy as a Method of Characterizing Changes in Soil Organic Matter.

Diffuse-Reflectance Fourier-Transform Mid-Infrared Spectroscopy (FTIR) can be used quickly and non destructively to identify and quantify the presence of important organic functional groups in environmental samples.  However, soils contain myriad organic and inorganic components that absorb in the Mid-infrared range.  Because of this, FTIR spectral analysis needs to be validated in order to correctly interpret changes in soil organic matter quality.  The objective of this project was to amend soils with known quantities of authentic standards and measure changes in FTIR spectra. Several authentic standards were added to the soil increasing the total C in the sample by 50%. We added adenine, cellulose, ergosterol, glucosamine, glycine, guanine, indole, methionine, palmitic acid, egg protein, chlorophyllin, tannic acid, xylose, urease, and vanillin. Two different soils were used in the mixing scheme: A Hoytville, Ohio soil (2.5% C, 0.3 %N, 19 % sand, and 36 % clay), and an Akron, Colorado soil (1.5% C, 0.2 %N, 35 % sand, and 14 % clay). Multivariate analysis of the data shows that the main spectral differences between the samples are explained by the soil type. Within each soil, the standard additions resulted in spectral changes according to what compound was added. Cellulose addition resulted in increase absorbance at the 1150 cm^-1 band for C-O stretching in polysaccharides. Egg protein additions increased absorbance at the 1510 cm^-1 N–H deformation and C=N stretching of amides (amide II band). Methionine-amended soils have increased absorbance at 1505 cm^-1 N–H deformation band and the 1420 cm^-1 band near the CH bending and/or C-O absorbance. Our results show that FTIR is sensitive to changes in a variety of soil organic matter components like carbohydrates and proteins, which are important for estimating soil properties related to biology and soil structure.