Submitted to: Soil Science Society of America Journal
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/1/2010
Publication Date: 1/18/2011
Citation: Calderon, F.J., Reeves III, J.B., Collins, H.P., Eldor, P.A. 2011. Chemical differences in soil organic matter fractions determined by diffuse-reflectance mid-infrared spectroscopy. Soil Science Society of America Journal. 75(2)568-579. Interpretive Summary: We characterized important soil organic matter fractions using infrared spectroscopy. This experiment allowed us to gain an understanding of organic matter quality in different Corn Belt soils, especially chemical changes that occur in soils as they decompose. One of the important findings of this work is that the soil light fraction, made up of recent plant material, decomposes in a different and opposite way from the clay sized fraction, which is made up of partially decomposed and processed organic matter. Documenting these chemical changes in soil carbon is very important because these chemical processes can have very important implications for the C cycling at the regional and global scale.
Technical Abstract: We carried out mid-infrared (MidIR) spectral interpretation of fractionated fresh and incubated agricultural soils to determine changes in soil organic matter (SOM) chemistry during long-term incubation. Soils cores from four long-term sites under continuous corn in the US Corn Belt were obtained from 0-20, 25-50, and 50-80 cm depth. The samples were then subject to 440 d and 800 d of laboratory incubation. At each sampling time, the soils were processed to obtain the light fraction (LF), particulate organic matter (POM), silt-sized (silt), and clay-sized (clay) fractions. Time zero samples were also acidified to obtain decalcified soils. The dried and ground samples were scanned as neat samples in diffuse reflectance from 400 to 4000 cm-1. The objectives were to determine the differences in MidIR spectral properties between sites, depths, incubated samples, and fractions. Our results show that the LF and clay fractions have distinct spectral features regardless of site, suggesting that there is a common chemistry for each of these C pools. The LF is characterized by OH/NH band at 3400 cm-1, as well as carboxylic, protein, amides, aliphatic and aromatic C bond absorbance between 1350-1750 cm-1. The clay fraction is distinguished by aromatic ring CH absorption near 1230 cm-1, and absorption at 780-620 cm-1. Lamberton and Hoytville soils, which are high in organic C, are also characterized by absorbance at 1230 cm-1, suggesting that this band marks the presence of high SOM. In turn, the POM, silt and whole soil have different spectral properties from the LF or clay. The POM, like the LF, absorbs at the broad peak at 1360 cm-1 that encompasses CH2, CH3, COO, COOH in aliphatics and phenolics, as well as amines. Soils from different sampling depths also have specific spectral properties. The CH bending region between 1370-1450 cm-1 is characteristic of shallow depths. Because of their low OM content, the deeper samples are characterized by mineral features such as quartz, clays and calcite/dolomite. 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 OH/N-H bonds (3400 cm-1), and aromatic and aliphatic CH bonds (1223 cm-1, 2920-2860 cm-1). The clay fraction suffered changes that were opposite to those of the LF, indicating that LF decomposition and clay decomposition follow different chemistries.