ARS | Publication request: Solid-state NMR analysis of soil organic matter fractions from integrated physical-chemical extraction

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 25, 2011
Publication Date: June 23, 2011
Citation: Cao, X., Olk, D.C., Chappell, M., Cambardella, C.A., Miller, L.F., Mao, J. 2011. Solid-state NMR analysis of soil organic matter fractions from integrated physical-chemical extraction. Soil Science Society of America Journal. 75(4):1374-1384.

Interpretive Summary: Soil organic matter is a complex mixture of materials ranging in age from weeks or months to thousands of years that generally accounts for only five percent of the soil volume but is a major factor affecting productivity and many environmental interactions. To understand how soil organic matter affects soil processes, it is often separated into fractions using either physical or chemical techniques to determine the amount or chemical properties associated with the material. Several procedures exist for separating the various fractions, but all account for only a portion of the total soil organic matter. We found that by combining two different procedures, we could characterize the diversity found within total soil organic matter better than with either procedure alone. Our new procedure was also able to separate young and organic matter sources. These results will directly benefit other researchers who study the role of soil organic matter and indirectly affect producers and those managing soil resources using the research results.

Technical Abstract: Fractions of soil organic matter (SOM) are usually extracted from soil by either physical (size, density) or chemical (e.g., base, acid) procedures. In this study we used 13C nuclear magnetic resonance (NMR) spectroscopy to chemically characterize the fractions that were obtained by an integrated procedure that combined both of these approaches. For the 0-5 cm depth of a corn (Zea mays L.)-soybean (Glycine max. L.) soil in Iowa, we extracted in sequence the light fraction, two particulate organic matter (POM) fractions, and two NaOH-extractable humic acid fractions that differed in their binding to soil Ca+2. Whole SOM was obtained by dissolving the soil mineral component through HF washes. All samples were analyzed by advanced 13C NMR techniques, including quantitative direct polarization, spectral-editing techniques (dipolar dephasing, chemical-shift-anisotropy filter, CH and CH2 selection), and two-dimensional 1H-13C heteronuclear correlation. The NMR spectra were similar for the light fraction and the two POM fractions and were dominated by carbohydrates and significant lignins and also had proteins or peptides. Spectra of the two humic fractions were by contrast dominated by unsubstituted aromatic C and COO/N-C=O groups with smaller proportions of carbohydrates and NCH/OCH3 groups, indicative of more humified material. This trend was more pronounced in the calcium-bound humic fraction. The spectrum for whole SOM had signals intermediate between these two groups of SOM fractions, suggesting contributions from both. Our results for this soil suggest that either chemical or physical fractions alone will partially represent whole SOM, and their integrated use might provide greater insight into SOM structure and function than either approach alone.