13C CPMAS-NMR spectroscopy for characterization of soil organic carbon in terra rossa

Large amounts of Fe(III) oxides in terra rossa is a strong limiting factor for using solid state NMR spectroscopy in studying its soil organic carbon (SOC) composition and structure. The intense paramagnetic source of signal depression, which is originated by the high magnetic moment of the unpaired electron, can broaden NMR signals up to several kHz. In order to demonstrate the feasibility of SOC characterisation in samples with large amount of paramagnetic species, we characterised three terra rossa soils from different pedoenviroments. To remove selectively Fe(III) compounds (i.e. crystalline, amorphous, bound to SOC, etc.), we employed different soil purification procedures. This approach allowed us to determine the main functional groups, in most cases masked in the NMR spectra of bulk soils, and to characterize in detail the SOC composition in terra rossa. Furthermore, it was possible to assess indirectly Fe(III) organization within the heterogeneous organic domain. Finally, we found that in the case of SOC, the C-13 CPMAS-NMR spectrum of bulk terra rossa soil, i.e. without any purification, can be in some cases used to describe the rough chemical composition of SOC, most likely when Fe(III) is compartmentalized far away from SOC.