Relationships between the type and carbon chemistry of humic acids from some New Zealand and Japanese soils

Humic acids from a total of 14 soils, 10 from New Zealand and 4 from Japan, were selected to provide a range from Rp to A-types. Semi-quantitative assessment of their carbon chemistry by cross polarization magic angle spinning (CPMAS)13C-nuclear magnetic resonance (NMR) revealed variations in composition that were generally related to humic acid type. Aromatic-C was the predominant form of C in most A-type humic acids whereas in Rp, P, and B-type humic acids, the aliphatic-C and aromatic-C contents were comparable. Differences in humic acid types were not always reflected in their carbon chemistry, possibly because of their mode of formation. The effects of fire on vegetation or soil organic matter may at least partly explain the occurrence of A-type humic acids in two different New Zealand soils.