Characterization of organic matter in a northern hardwood forest soil by 13C NMR spectroscopy and chemical methods

Abstract

Soil organic matter plays an important role in soil properties and influences ecosystem cycles of C, N, Al, Fe, and other major and trace elements. We examined spatial variations in the structure and chemistry of soil organic matter at the Hubbard Brook Experimental Forest in New Hampshire, USA. Humic substances were extracted and isolated chromatographically into humic acid, fulvic acid, and polysaccharide fractions. Chemical methods and solid-state 13C NMR spectroscopy were used to determine structural chemistry. On average, extractable humic substances accounted for nearly 50% of soil organic matter, with alkyl and O-alkyl C (carbohydrate) being the largest C fractions in whole soils and isolated humic substances. Alkyl C ranged from 33% to 56% of C, while O-alkyl C comprised 20–45% of C. Alkyl C increased, while O-alkyl C decreased with soil depth in whole soils, humin, and humic acid. Aromatic C increased with soil depth in whole soils and humin, while carbonyl C increased with depth in whole soils and fulvic acids. Fulvic acids were more acidic than humic acids, and were less phenolic and aliphatic than humic acids. Carboxylic acidity accounted for about 80% and 50% of total acidity in fulvic acid and humic acid, respectively. Soil from higher-elevation sites exhibited greater alkyl C and lower O-alkyl and aromatic C in the Oa horizon, suggesting a greater degree of decomposition of the organic matter in the Oa horizon of these conifer-rich sites. Mineral soils in conifer-rich sites contained organic matter that was more aromatic than in hardwood sites. Variations in humification processes, source materials, and transport of organic matter could account for variations in the structure and chemistry of organic matter in these forest soils.