A compositional and solid-state nuclear magnetic resonance study of humic and fulvic acid fractions of soil organic matter

Four soil organic matter samples (two humic acid fractions and two fulvic acid fractions) were obtained from two proximate soils in the flood plain of the Altamaha River in south-eastern Georgia. Each samples was characterized by elemental and functional group analyses and by /sup 13/C nuclear magnetic resonance spectrometry using cross polarization magic angle spinning (CP-MAS) with spin counting. The elemental and functional group analyses provided input for a series of analytical constraints calculations that yield an absolute upper limit for the amount of aromatic carbon and most probable estimates of both aromatic and non-carboxyl aliphatic carbon in each sample. Spin counting experiments demonstrate that less than 50% of the carbon in three of the fractions is observed in the NMR experiment, and even after correction for differential relaxation, the amounts of aromatic and non-carboxyl aliphatic carbon determined by /sup 13/C CP-MAS NMR are dissimilar to those obtained by calculation. Unambiguous evidence is presented for the predominance of aliphatic carboxyl groups in one of the fulvic acid fractions.