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 nm (E4/E6) 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), E4/E6 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.

Selenium (Se) concentrations exceeding ecological guidelines for sediments and suspended particulate matter (SPM) have been observed in the northern reach of the San Francisco Bay estuary. Longterm availability of elevated Se in wetland sediments depends in part on the fluxes controlling Se distribution. The relative contribution of sedimentary vs. post-depositional Se fluxes in two San Francisco Bay intertidal wetlands was estimated. Selenium concentrations on surface wetland sediments were compared with levels on SPM, and with previously established background levels in San Francisco Bay sediments. Sediment Se fluxes to the wetlands were measured directly using sediment traps. Although dissolved Se concentrations are higher than particulate Se concentrations in San Francisco Bay water, sediment input into the system provides the major flux of Se. Strong correlation between Se and C on SPM (r2 = 0.81) indicates the importance of organic particulate deposition. Dependence on sediment texture was qualitatively established by measuring Se on particle-size separates. Normalization to Al showed that 65% of Se spatial variability is related to sediment texture. Selenium is further enriched in the marsh via post-depositional inputs, probably due to in situ adsorption from overlying water and chemical reduction. According to sediment flux measurements, enrichment in the marsh is equivalent to 20 to 25% of the particulate Se flux, thereby defining the marsh as a Se sink. These findings highlight the need for more intensive monitoring of SPM as the major source of Se to intertidal wetlands.

Characteristics of dissolved organic matter (DOM) in aqueous extracts of litter may provide early indications of alteration of soil organic matter due to disturbances. Aqueous extracts of litter (pine and understory components) were prepared from two young ponderosa pine (Pinus ponderosa Doug. ex Laws.) plantations in California. Treatments were vegetation control (understory removal by herbicides), full nutrient fertilization, and control (untreated). Early changes due to management practices shown by spectroscopic techniques were not revealed by standard chemical analysis of DOM concentrations, pH or C/N ratios. Fourier‐transform infrared spectroscopy (FTIR) showed that DOM from both treatments at the warmer site (Whitmore) had relatively more COOH, phenolics, and aliphatics compared with controls, for both pine and understory components. At the cooler site (Chester) similar trends were observed only for understory DOM. Ultraviolet‐visible spectroscopy (UV‐VIS) showed that the ratio of absorptions at 465 vs. 665 nm (E4/E6 ratio) was higher for vegetation‐control DOM at both sites (6.7), compared with controls (5.9). Fertilization increased E4/E6 ratios at Whitmore only. The E4/E6 ratio was higher at Whitmore (6.7) than at Chester (5.1). The DOM from treated plots had smaller or less condensed chromophores, and relatively higher COOH, phenolics, and aliphatics, compared with the control. These effects were not as pronounced at Chester. At Whitmore warmer temperatures and increased water availability resulting from understory removal, and increased nutrient supply resulting from fertilization, may enhance the activity of decomposers. Total nutrients in litter increased with fertilization and decreased with understory removal. However, at Whitmore, understory removal increased the nutrients stored specifically in pine litter.

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