Humic substances in soils: are they really chemically distinct?

Humic substances (HS) are an operationally defined fraction of soil organic matter, and they represent the largest pool of recalcitrant organic carbon in the terrestrial environment. It has traditionally been thought that extractable HS consist of novel categories of cross-linked macromolecular structures. In this study, advanced nuclear magnetic resonance approaches were used to study the major components (proteins, carbohydrates, aliphatic biopolymers, and lignin) that are known to be present in HS, and to identify their fingerprints in humic mixtures. Theoretically, once all known components have been identified, the remaining signals should be from materials with novel structures, themselves forming a distinct chemical category of humic materials. Surprisingly, nearly all of the NMR signals in traditional HS fractions could be assigned to intact and degrading biopolymers. We therefore suggest that the vast majority of operationally defined humic material in soils is a very complex mixture of microbial and plant biopolymers and their degradation products but not a distinct chemical category. It is important to note this work in no way rules out the existence of a distinct category of humic macromolecules, either at low abundance in the soluble fraction from young soils, in diagenetically evolved samples (for example lignites, etc.), or in the nonextractable humin fraction.