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.
Soil organic matter is a heterogeneous mixture of organic compounds of plant, animal, and microbial origin in various stages of decomposition. Humic substances are the amorphous, highly transformed, darkly colored component of organic matter. They consist of a wide range of structures and functional groups (Stevenson, 1994; Hayes and Swift, 1978). Humic substances are ubiquitous in the environment, occurring in all soils, waters, and sediments. In soils, humic substances comprise up to 80% of soil organic matter(Hayes, 1998). Humic substances form an important pool of C in the global C budget. According to Schlesinger (1997), the total mass of organic C stored in soils ranges from 1100 1015 to 3000 1015 g C, greater than atmospheric and living biomass C combined. In addition to their role in C cycle, humic substances also influence soil fertility, soil development, and various soil chemical properties, including cation exchange capacity (CEC), buffer capacity, pH, acid–base chemistry, and metal transport (Petersen, 1980; Buol et al., 1997), and serve as a source of energy for soil macro- and microorganisms(Paul and Clark, 1987).
While chemical investigations of humic substances go back more than 200 years, much remains unknown about their structure and properties. Research has shown that humic acids and fulvic acids extracted from soils formed under the same environ- mental conditions have broadly similar analytical characteristics and chemical struc- tures (Griffith and Schnitzer, 1975; Schnitzer, 1977; Chen et al., 1978; Burch et al., 1978). The chemical nature of the litter, composition of the microbial community, and environmental factors such as temperature and moisture play important roles in influencing the chemical and structural components found in humic substances. Forest floors that develop under broadleaf or mixed forests are more conducive to decom- position than those formed under conifers. For example, Prescott et al. (2000)observed faster decomposition of broadleaf litter compared to needle litter. They attributed this to the lower lignin and higher N concentrations in deciduous leaf litter. Also, the degree of decomposition and vegetation type have been shown to influence the chemical composition of SOM (Mathur and Farnham, 1985; Kögel et al., 1988; Krøsshavn et al., 1992). Humic substances from similar geographic locations but different depositional environments often have different chemical structures (Rasyid et al., 1992).