Influence of animal manure application on the chemical structures of soil organic matter as investigated by advanced solid-state NMR and FT-IR spectroscopy

Annual application of cattle manure to a farmer's field in eastern Nebraska for 5 yr caused improved soil N and P supply and increased corn (Zea mays L.) yield in less productive portions of the field compared to another field treatment receiving the same amount of N as inorganic fertilizer. As a first step toward identifying the soil processes that led to these changes, the effects of manure addition on the chemical nature of soil organic matter were investigated by extracting two humic fractions---mobile humic acid (MHA) and calcium humate (CaHA)---from the field soil for subsequent analysis by solid-state 13C nuclear magnetic resonance (NMR) spectroscopy and Fourier Transform-infrared (FT-IR) spectroscopy. Functional groups were selectively identified and quantified through NMR techniques that included quantitative direct polarization, cross polarization/total suppression of sidebands, 1H-13C heteronuclear correlation NMR, 13C chemical shift anisotropy filtering, CH-signal selection, and other spectral-editing techniques. The CaHAs exhibited high aromaticity and line shapes typical of Mollisols rich in oxidized charcoal, while the MHAs had lower aromaticity and greater contributions from lignin aromatic ethers. Results show that the chemical nature of each humic fraction did not differ significantly between an unfertilized control and the treatment receiving N as inorganic fertilizer. The FT-IR results were consistent with all these findings. The NMR results further showed that the CaHA fraction from the manure treatment was clearly depleted in aromatic rings and enriched in nonpolar alkyl compounds, most likely fatty acids, compared to the other two fertilizer treatments. Selective spectra further show strong indications of enhanced peptide contributions in the manure CaHA, consistent with the improved soil N supply associated with the manure treatment. The peptide enrichment was partially masked in unselective spectra by a simultaneous decrease in overlapping signals for OCH3 and COO groups that are associated with aromatic components, demonstrating the improved accuracy of the selective spectral-editing NMR techniques.