Solid‐State NMR Analysis of Soil Organic Matter Fractions from Integrated Physical–Chemical Extraction

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Fractions of soil organic matter (SOM) were extracted by an integrated physical–chemical procedure and their chemical natures were characterized through 13C nuclear magnetic resonance (NMR) spectroscopy. For the 0‐ to 5‐cm depth of a corn (Zea mays L.)–soybean [Glycine max. (L.) Merr.] soil in Iowa, we extracted in sequence the light fraction, two size fractions of particulate organic matter (POM), and two NaOH‐extractable humic acid fractions based on their binding to soil Ca2+: the unbound mobile humic acid fraction and the calcium humate fraction. 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/magic angle spinning, spectral‐editing techniques, and two‐dimensional 1H–13C heteronuclear correlation NMR. The NMR spectra were comparable for the light fraction and two POM fractions and were dominated by carbohydrates and to a lesser extent lignins or their residues, with appreciable proteins or peptides. By contrast, spectra of the two humic fractions were dominated by 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 yet more pronounced in the calcium humate fraction. The spectrum for whole SOM had signals intermediate between these two groups of SOM fractions, suggesting contributions from both groups. Our results for this soil suggest that either chemical or physical fractions alone will partially represent whole SOM, and their integrated use is likely to provide greater insight into SOM structure and possibly function, depending on the research issue.