SPECTROSCOPY OF AQUEOUS EXTRACTS OF FOREST LITTER .1. SUITABILITY OF METHODS

Structural characteristics of dissolved organic matter (DOM) have not been closely examined until recent analytical techniques became available. We examined the applicability of ultraviolet-visible (UV-VIS), Fourier-transform infrared (FTIR), and electron spin resonance (ESR) spectroscopies using aqueous extracts of pine and broadleaf understory leaf litter. The ratio of UV-VIS absorbances at 465 nm and 665 mm (E(4)/E(6)) were 7.8 to 8.5 for extracts of pine and 6.3 to 7.7 for the understory between pH 3.0 and 6.0. Using the standard method for humic substances (in 0.05 M NaHCO3 solution, pH 8.0), E(4)/E(6) measurement produced varying results, and is not recommended for DOM. Well-resolved FTIR spectra had features similar to those of humic substances; peak-height ratios were used for semiquantitative analysis of DOM functional groups. Two FTIR sample-preparation procedures were also compared: dried extracts on AgCl disks, and freeze-dried DOM in KBr pressed pellets. The first allowed better analysis of COOH functional group deprotonation; the second preparation procedure produced spectra with identical absorbance ranges as those of AgCl disks, with similar peak-height ratio trends as a function of pH, except for those associated with COOH/COO-. Contact between DOM and the salt in pressed pellets may have aided deprotonation of COOH, even at low pH. Interference from water occurred with KBr pressed pellets. Free radicals identified by ESR increased with pH, and shifts from 2.0036 to 2.0050 in ESR g values indicated that oxidation also increased with pH. We conclude that these spectroscopic techniques, modified from those developed for analysis of humic substances, are complementary and together provide useful information about the structure of the DOM of forest litter.