Investigations of soil organic matter using solid-state nuclear magnetic resonance

• Main Author: Mao, Jingdong
• Language: ENG
• Published: ScholarWorks@UMass Amherst 2000
• Subjects: Soil sciences
• Online Access: https://scholarworks.umass.edu/dissertations/AAI9988818

Soil organic matter (SOM) plays an essential role in agriculture and environments. Humic substance (HS) is a major component of SOM. This research was to investigate HS using solid state nuclear magnetic resonance (NMR). Several experiments were concluded, and their results and conclusions are reported below. 13C direct-polarization magic-angle-spinning (DP/MAS) corrected by cross-polarization spin-lattice relaxation time (CP/T1) experiments with total sideband suppression (TOSS) were found to be quantitative. None of eight models proposed in the literature totally match compositions determined with this technique. Qualitative and quantitative analyses of HAs with five different 13C solid-state NMR techniques were assessed using different HAs. The techniques compared are: (1) DP/MAS at 13 kHz, (2) conventional CP/MAS at 5 kHz, (3) ramp-CP/MAS at 8 kHz, (4) CP/TOSS at 4.5 kHz, and (5) DP/MAS corrected by CP/T1-TOSS. DP/MAS or DP/MAS corrected by CP/T1-TOSS was used as a quantitative reference. Ramp-CP/MAS and CP/TOSS gave acceptable qualitative results. Conventional CP/MAS spectra at 13C frequency of 75 MHz was neither qualitatively nor quantitatively. Crystalline poly(methylene) exists in an untreated peat, soil humin and HAs but little or none is present in fulvic acids (FAs). The similar characteristics of crystallites in tree leaves and SOM indicate that crystallites in SOM might be from the biopolymers of the protective parts of higher terrestrial plants. New information on the chemical and physical structures of a peat HA was obtained using a series of two-dimensional 1H-13 C heteronuclear correlation solid-state NMR (HETCOR) experiments. COO carbons appeared to be prominently in OCHn-COO environments, but some were also bonded to aromatic rings and aliphatic chains. Compositional heterogeneity was also studied by introducing 1H spin diffusion into the HETCOR experiment. The chain mobility of different HAs was investigated with 1D proton and 2D WISE (wideline separation) solid-state NMR techniques. 1D proton spectra only gave an overall estimate of the mobility of HAs. 2D WISE showed that HS mobility difference may arise from bound water.