Changes in organic structural group composition of humic and fulvic acids with depth in sediments from similar geographical but different depositional environments

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A series of soil and swamp sediments from the Myall Lake National Park region, New South Wales, Australia have been studied by traditional wet chemical techniques and by solid state 13C NMR. Amino acid, carbohydrate, total acidity, phenolic, phenoxy and alkyl contents have been measured. Results obtained support previous literature reports that NMR carboxyl content measurements correlate better with total acidity than wet chemical COOH measurements. Large discrepancies (×20) between NMR and traditional chemical methods of measuring carbohydrate contents were also observed. The carbohydrate content of humic and fulvic acids measured by NMR or traditional methods drops rapidly with increasing soil or sediment depth probably because of metabolization by microorganisms. Fulvic acids contain more carboxylic acid and decreased alkyl carbon than their humic acid counterparts from the same source. The structure of humic acids however more closely reflect their depositional environment. For example, more phenolic and phenoxy carbon is present in forest humic acids because of the high lignin input.