Novel Advanced 13C and 15N Solid-State Nuclear Magnetic Resonance Techniques for Characterizing Soil Organic Matter.

Soil organic matter (SOM) can enhance soil fertility, improve soil structures and regulate the fate and transport of pollutants. A detailed understanding of the molecular structures of the SOM is critical in deciphering its roles.  Unfortunately, the complexity of SOM presents difficulties in determining its structure. Solid-state nuclear magnetic resonance (NMR) spectroscopy is regarded as the best tool to investigate the complex structures of SOM. However, the full power of this technique has not been exploited.  In this presentation, new, advanced solid-state ¹³C and ¹⁵N NMR techniques, developed especially for the structural elucidation of complex SOM, are demonstrated. With these robust techniques, more than 42 different functional groups can be distinguished in complex SOM whereas as reported in the literature, fewer than 10 functional groups have been distinguished by solid-state NMR. With spectral editing techniques coupled with reliable quantitative techniques, most of the functional groups can be quantified. In addition, advanced two-dimensional techniques can be used to detect the connections between different functional groups, and ¹H spin diffusion and long-range recoupled dipolar dephasing allow for the investigations of domains and heterogeneities in SOM. The applications of these new techniques are demonstrated in several case studies.