| Organic matter (OM) plays an important role in governing soil properties and nutrient cycling in forest ecosystems. I investigated the chemistry of soil and dissolved organic matter (DOM) in clear-cut and undisturbed site at the Hubbard Brook Experimental Forest (HBEF) in New Hampshire using chemical methods and solid-state 13C nuclear magnetic resonance (NMR) spectroscopy. On average, extractable humic substances accounted for nearly 50% of soil organic matter, with alkyl C and O-alkyl C (carbohydrate) being the dominant C fractions in soils, humic substances, and dissolved organic matter. Alkyl C ranged from 30–61% of total C, while O-alkyl C comprised 20–45% of total 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 acid was more acidic than humic acid. 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 forest floor of these conifer-rich sites. Mineral soils in conifer-rich sites contained organic matter that was more aromatic than in hardwood sites. O-alkyl C decreased from 45% to 40% after clear-cutting in the Oa horizon, while soil solutions collected from the clear-cut watershed were more aromatic than the uncut watershed. Variations in humification processes, source materials, flowpaths and in-stream processes, and transport of organic matter could account for variations in the structure and chemistry of organic matter in soils, soil solution, and streams at Hubbard Brook. The sorption behavior of organic carbon (OC) was related to the content of carboxylic functional groups in the added solution. Hydrophobic OM had greater affinity to soils than hydrophilic OM, and Bh-horizon OM exhibited greater sorption than O-horizon OM. Maximum adsorption occurred at pH 4, and decreased with either an increase or decrease in pH. The pH-dependence of OM sorption probably reflects a balance between lower charge density of carboxyl groups at low pH and lower positive charge of adsorption sites at higher pH. |
