Chemical–structural information from solid-state 13C NMR studies of a suite of humic materials from a lower montane forest soil, Colorado, USA

Chemically and physically fractionated samples extracted from the surface horizon of a soil developed under a mix of coniferous and deciduous vegetation in southwestern Colorado were studied. 13C NMR data on this soilʹs organic matter and its HF(aq)-washed residue, as well as the classic acid/base-separated humic fractions (humic acid, fulvic acid, humin), were examined for chemical–structural detail, e.g., the various structural functionalities present (especially lipids, carbohydrates, aromatics, polypeptides and carbonyl/carboxyls). Among the humic fractions, it was found that the lipid concentrations are in the order humic acid>fulvic acid= humin; for carbohydrates the order is fulvic acid>humin>humic acid; for aromatic carbons the order is humic acid>humin>fulvic acid; for polypeptides it is humic acid>fulvic acid>humin and for carbonyl/carboxyl species it is humin>humic acid>fulvic acid, but the differences are small. 13C spin–lattice relaxation times indicate that at least two types of “domains” exist in each, corresponding to “higher” and “lower” concentrations of paramagnetic centers, e.g., Fe3+.