Chemical Differences in Soil Organic Matter Fractions Determined by Diffuse‐Reflectance Mid‐Infrared Spectroscopy

Abstract

We performed mid‐infrared (MidIR) spectral interpretation of fractionated fresh and incubated soils to determine changes in soil organic matter (SOM) chemistry during incubation. Soils from four sites and three depths were processed to obtain the light fraction (LF), particulate organic matter (POM), silt‐sized (silt), and clay‐sized (clay) fractions. Our results show that the LF and clay fractions have distinct spectral features regardless of site. The LF is characterized by absorbance at 3400 cm −1 , as well as between 1750 and 1350 cm −1 The clay fraction is distinguished by absorption near 1230 cm −1 , and absorption at 780 to 620 cm −1 The POM, like the LF, absorbs at the broad peak at 1360 cm −1 High SOM soils are characterized by absorbance at 1230 cm −1 , a band for aromatics, possibly associated with resistant C. Soils from different sampling depths have specific spectral properties. A band at 1330 cm −1 is characteristic of shallow depths. Because of their low organic matter (OM) content, the deeper samples are characterized by mineral bands such as quartz, clays, and carbonate. Spectroscopic data indicates that the clay fraction and the LF suffered measurable chemical transformations during the 800‐d incubation, but the POM and silt fraction did not. As the LF decomposes, it loses absorbance at 3400, 1223, and 2920 to 2860 cm −1 The band at 1630 cm −1 increased during incubation, suggesting a resistant form of organic C. The clay fraction suffered changes that were opposite to those of the LF, indicating that LF decomposition and clay decomposition follow different chemistries.