Chemical characteristics and acidity of soluble organic substances from a northern hardwood forest floor, central Maine, USA

Our understanding of the chemistry, structure, and reactions of organic substances in forest floor leachates is limited and incomplete. Therefore, we examined the organic and inorganic chemistry of forest floor leachates collected from a hardwood forest in central Maine over a two-year period (1987–1989), including detailed study of dissolved organic carbon (DOC). Seasonal variations in NH4+, NO3−, K+, and total Al were believed due to organic matter decomposition and release. Leaching of other base cations closely followed that of NO3−. Snowmelt resulted in NO3− levels that decreased in time due to flushing of mineralization/nitrification by-products that had accumulated during the winter months. Total DOC ranged from 2228 to 7193 μmol L−1 with an average of 4835 μmol L−1. Monosaccharides and polyphenols constituted 3.9% (range of 3.4 to 4.4%) and 3.0% (2.2 to 3.7%) of the DOC, respectively, which suggests DOC may contain partially oxidized products that are possibly of a lignocellulose nature. Fractionation of the forest floor DOC indicated high organic acid contents (hydrophobic and hydrophilic acids) that averaged 92% of the total DOC. Organic acids were isolated and analyzed for elemental content (C, H, N, and S), and determination of UV absorptivity (E4E6) ratios, CuO oxidation products, FT-IR and 13C-NMR spectra, and acidity by potentiometric titration. Results from these analyses indicate the organic acids in the forest floor leachates are similar to fulvic acids. Hydrophobic and hydrophilic acids had average exchange acidities of 0.126 and 0.148 μeq μmol−1 C, respectively, and pKa, of 4.23 and 4.33. Their FT-IR and 13C-NMR spectra suggest they are primarily carboxylic acids, with aliphatic and aromatic structure. An organic charge contribution model was developed using titration data, DOC fractionation percentages, and the total DOC in the forest floor leachates. Application of the model to all solutions accounted for 97% of the charge balance deficits. Adjusted values for the flux of C and organic acidity due to organic solutes in forest floor leachates indicated translocation of 112 to 260 kg C ha−1 yr−1 and 460 to 1330 eq ha−1 yr−1, respectively, to the underlying mineral subsurface horizons.

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