[Show abstract][Hide abstract] ABSTRACT: In this study, we examined the temporal and spatial variability of dissolved organic matter (DOM) abundance and composition in the lower Mississippi and Pearl rivers and effects of human and natural influences. In particular, we looked at bulk C/N ratio, stable isotopes (delta 15N and delta 13C) and 13C nuclear magnetic resonance (NMR) spectrometry of high molecular weight (HMW; 0.2 mum to 1 kDa) DOM. Monthly water samples were collected at one station in each river from August 2001 to 2003. Surveys of spatial variability of total dissolved organic carbon (DOC) and nitrogen (DON) were also conducted in June 2003, from 390 km downstream in the Mississippi River and from Jackson to Stennis Space Center in the Pearl River. Higher DOC (336-1170 muM), C/N ratio,% aromaticity, and more depleted delta 15N (0.76-2.10/00) were observed in the Pearl than in the lower Mississippi River (223-380 muM, 4.7-11.50/00, respectively). DOC, C/N ratio, delta 13C, delta 15N, and % aromaticity of Pearl River HMW DOM were correlated with water discharge, which indicated a coupling between local soil inputs and regional precipitation events. Conversely, seasonal variability in the lower Mississippi River was more controlled by spatial variability of a larger integrative signal from the watershed as well as in situ DOM processing. Spatially, very little change occurred in total DOC in the downstream survey of the lower Mississippi River, compared to a decrease of 24% in the Pearl River. Differences in DOM between these two rivers were reflective of the Mississippi River having more extensive river processing of terrestrial DOM, more phytoplankton inputs, and greater anthropogenic perturbation than the Pearl River.
Journal of Geophysical Research Atmospheres 05/2007; 112(G2). DOI:10.1029/2006JG000206 · 3.43 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Here we report on temporal changes in the composition of dissolved organic carbon (DOC) and nitrogen (DON) collected in the tidal freshwater region of the lower Mississippi and Pearl Rivers (MR and PR) (USA). Bulk stable carbon isotopes and 13C nuclear magnetic resonance (NMR) spectrometry were used to examine the composition of high molecular weight ( 1 kDa) dissolved organic matter (HMW DOM). Monthly water samples were collected at one station in each river from August 2001 to July 2003. Surveys of spatial variability (225 km downstream in the MR and from Jackson to Stennis Space Center in the PR) in total DOC and DON were also conducted in both rivers in June 2003. Higher total DOC (336 to 1156 uM), DON (9.3 to 59.5 uM), % HMW DOM (25 to 47 %), ultraviolet (UV) absorption (0.13 to 0.70 /m), and more depleted delta-15N (0.76 to 2.16 per mil) delta-13C (-25.1 to -28.0 permil) were observed in the PR than in the lower MR (223 to 380 uM, 6.1 to 13.4 uM, 16 to 38 %, 0.08 to 0.17 /m, 0.76 to 2.16 permil, -25.7 to -27.1 permil, respectively). 13C-NMR spectra revealed that alkyl and carbohydrate carbons were dominant in HMW DOC in both rivers. However, a significantly lower percentage of aromatic C (13.2 to 16.6 %) and higher carboxyl C (17.1 to 25.8 %) were observed in the lower MR than in the PR (16.9 to 21.3 % and 12.3 to 20.9 %). Total DOC, DON, HMW DOM, and percent aromaticity of HMW DOM were higher in the PR during local flooding events, and lower during low discharge, indicating a coupling between local carbon inputs (soil and wetlands) and regional precipitation events in the PR. Conversely, seasonal variability of total DOC, DON, and HMW DOM in the lower MR was controlled by spatial variability of an integrative signal from watershed inputs and in-situ production from upriver sources, resulting in a more phytoplankton-derived 13C-NMR signature of HMW DOM. Spatially, very little change occurred in total DOC (259 to 282 uM) and DON (8.85 to 13.3 uM) in the downstream survey of the lower MR, compared to decreases of 24 % and 50 % in DOC and DON, respectively, in the PR. Once again local inputs are more important in the PR compared to the MR and likely account for higher variability. Recent lab incubation experiments also suggest that photochemical oxidation, coupled with bacterial degradation of DOM, accounts for significant alteration of DOM in these lower floodplain rivers.
[Show abstract][Hide abstract] ABSTRACT: The properties of aquatic dissolved organic matter (DOM) isolated by solid phase extraction (SPE) C-18 cartridges, ultrafiltration, and XAD chromatography are compared. Samples taken from the Suwannee River, Georgia, USA and McDonalds Branch in the Pine Barrens, New Jersey, USA were chosen to represent waters where DOM originates from predominantly terrestrially-derived (allochthonous) precursors. Pony Lake, Antarctica represented an exclusively algal/microbially-derived (autochthonous) DOM. Fluorescence, UV absorption, 13C NMR spectroscopy, and high-pressure size exclusion chromatography (HPSEC) were employed to discern differences and similarities between the DOM isolated by these three methods. Only subtle differences between isolation methods were observed for the terrestrially derived DOM samples when assayed by light and fluorescence spectroscopy. Conversely, the Pony Lake DOM isolates exhibit greater variability when analyzed by these methods. 13C-NMR analyses showed structural differences between the methods for all samples. HPSEC analysis also revealed differences with the C-18 isolates exhibiting the highest molecular weights. Thus, it appears that each method isolates sufficiently different fractions of DOM that can only be delineated when a consortium of analytical methods are used to assay the samples. Nonetheless real differences between autochthonous and allochthonous derived DOM were observed with the algal-derived samples exhibiting high fluorescence ratios and lower aromaticity relative to the terrestrially derived materials. These results demonstrate that caution must be exercised when interpreting DOM reactivity data that rely upon the use of specific fractions.
[Show abstract][Hide abstract] ABSTRACT: Soil organic matter (SOM) is the largest terrestrial C and N store. Microbial yand abiotic processes that control the transformation of protein nitrogen in litter and ysoils into macromolecular humic materials play an important role in organic matter ystorage and soil productivity. There are major gaps, however, in our understanding of ythese processes and behaviors. Abiotic reactions of amines, phenols and sugars derived yfrom forest leachates or present in detrital and litter organic matter are known to be ykey processes in the formation of complex organic nitrogen. We present here the yresults from a study designed to investigate how the inherent chemistry of lignin, leaf ylitter, and progressively advanced brown-rot wood decay impact the chemical reaction yof amino acids with this organic matter. Additionally, experiments in the presence of ybirnessite (MnO2) were also conducted to investigate the role of mineral induced phenol yoxidation on specific amino acid chemical humifcation processes. Solid and liquid state yNMR, 13C-labelled tetramethyl ammonium hydroxide thermochemolysis and stable ycarbon and nitrogen isotope ratio mass spectrometry were used to track the alteration yof litter material and document uptake of 13C and 15N labeled amino acids. yPreliminary results from birnessite-containing experiments suggest that the metal-ypromoted oxidation of the lignin, leaf litter, and, in particular, demethylated brown rot ywood residues, is necessary to convert the phenols to quinones of some type permitting yamine addition. This relationship is particularly true for the production of soluble yfractions after two and six weeks of reaction in the presence of the manganese oxides. yAdditionally, the production of leachable organic matter with incorporated N was ypromoted in the soluble fractions. Ongoing NMR studies will elucidate the nature of ythe chemical binding in these experiments. y
[Show abstract][Hide abstract] ABSTRACT: Dissolved organic matter (DOM) leaching from forest floor coarse woody debris is an yimportant mediator in metal cycling, microbial activity, and mineral dissolution in the yassociated soils. Much research has been directed at characterizing the chemical ycomposition and molecular weight range of DOM to better assess and predict its yreactivity. Unfortunately, these chemical and structural properties of DOM are extremely ydifficult to assess and consequently a wide variety of indirect analytical techniques are ytypically employed. Direct analysis of DOM by High-field Fourier Transform Ion yCyclotron Resonance Mass Spectrometry combined with Electrospray Ionization (ESI yFT-ICR MS) has only recently been applied to a limited number of environmental yorganic matter samples. With a resolving power of greater than 200,000, this is currently ythe only analytical technique capable of fully resolving individual molecules in such ycomplex mixtures. y To simulate leachates from coniferous woody debris samples of red spruce (Picea yrubens) wood were degraded with brown rot fungi over a period of 43 weeks and yextracted with water. Water-soluble fractions of the degraded residue were analyzed yusing a 9.4 Tesla ultra-high resolution FT-ICR MS. Supporting information on the yleachate and solid residue chemistry was obtained using on-line 13C-labelled ytetramethylammonium hydroyide (TMAH) thermochemolysis GC-MS, solid-state NMR yand elemental analysis. y The FT-ICR mass spectra reveal structures at every nominal mass from approximately yy250 to 1200 Daltons as well as molecular families containing ions that differ from each yother in degree of saturation or number of functional group substitutions. There is a yprogressive increase in the number of individual compounds within a nominal mass unit, yfrom 2 compounds at 4 weeks decay to 6 compounds at 32 weeks, resulting in thousands yof discrete extractable compounds at the conclusion of decay. Degradation of wood over ytime does not change the overall molecular weight range of products, however, a shift in ylocation of the data distribution on the Kendrick plot demonstrates changes in yhomologous series with degradation. This data is currently being analyzed using the van yKrevelen diagram to gain additional information.y
[Show abstract][Hide abstract] ABSTRACT: Here we report on the temporal changes in the composition of dissolved organic carbon (DOC) collected in the tidal freshwater region of the lower Mississippi River. Lignin-phenols, bulk stable carbon isotopes, compound-specific isotope analyses (CSIA) and 13C nuclear magnetic resonance (NMR) spectrometry were used to examine the composition of high molecular weight dissolved organic matter (HMW DOM) at one station in the lower river over 6 different flow regimes in 1998 and 1999. It was estimated that the annual input of DOC delivered to the Gulf of Mexico from the Mississippi River was of 3.1 × 10−3 Pg, which represents 1.2% of the total global input of DOC from rivers to the ocean. Average DOC and HMW DOC were 489 ±163 and 115 ± 47 μM, respectively. 13C-NMR spectra revealed considerably more aliphatic structures than aromatic carbons in HMW DOC. Lignin phenols were significantly 13C-depleted with respect to bulk HMW DOM indicating that C4 grass inputs to the HMW DOM were not significant. It is speculated that C4 organic matter in the river is not being converted (via microbial decay) to HMW DOM as readily as C3 organic matter is, because of the association of C4 organic matter with finer sediments. The predominantly aliphatic 13C NMR signature of HMW DOM suggests that autochthonous production in the river may be more important as a source of DOC than previously thought. Increases in nutrient loading and decreases in the suspended load (because of dams) in the Mississippi River, as well as other large rivers around the world, has resulted in significant changes in the sources and overall cycling of riverine DOC.
[Show abstract][Hide abstract] ABSTRACT: In this study we used multidimensional solution-state NMR to elucidate the differences in the chemical composition of solid phase extracted and ultrafiltered DOM isolates. DOM was isolated from water sampled from an oligotrophic river, the River Tagliamento (Italy). The recovery of total DOM was up to 42% with both isolation techniques. In addition to 1- and 2-D solution-state NMR, we also applied 1-D solid-state 13C NMR spectroscopy for DOM characterization. 13C NMR spectroscopy only produced broad overlapping resonances, thus allowing a bulk characterization of DOM composition. However, it demonstrated that the bulk chemical composition of the two DOM fractions exhibited minor spatial-temporal changes. The 2-D experiments (TOCSY, HMQC) showed that the solid phase extracted hydrophobic DOM contained predominantly aliphatic esters, ethers, and hydroxyl groups, whereas the ultrafiltered DOM was comprised partially of peptides/protein, with further evidence for a small amount of aliphatic/fatty acid material. Sugars were present in both DOM fractions. The results show the two isolation techniques selected for different suites of compounds within the bulk DOM pool.
[Show abstract][Hide abstract] ABSTRACT: Use of solid-state 13C nuclear magnetic resonance (NMR) spectroscopy has become commonplace in studies of humic substances in soils and sediments, but when modern high-field spectrometers are employed, care must be taken to ensure that the data obtained accurately reflect the chemical composition of these complex materials in environmental systems. In an effort to evaluate the quality of solid-state 13C NMR spectra obtained with modern high-field spectrometers, we conducted a series of experiments to examine spectra of various humic acids taken under a variety of conditions. We evaluate conditions for obtaining semiquantitative cross polarization magic angle spinning (CPMAS) 13C NMR spectra of humic acids at high magnetic field and spinning frequency. We examine the cross polarization (CP) dynamics under both traditional and ramp CP conditions on Cedar Creek humic acid. Fitted equilibrium intensities from these CP dynamic studies compare to within 3.4% of the intensities determined from a Bloch decay spectrum of the same sample. With a 1-ms contact time, ramp CP and traditional CP spectra were acquired on this sample and were found to compare to within 5.4% of the Bloch decay spectrum; however, the signal-to-noise ratio per hour of data acquisition was found to double under ramp CP conditions. These results demonstrate the power of applying modern solid-state NMR techniques at high magnetic field strengths. With these techniques, high-quality, semiquantitative spectra can be quickly produced, allowing the application of solid-state NMR techniques to more environmentally relevant samples, especially those where the quantity is limited.
[Show abstract][Hide abstract] ABSTRACT: Analytical chemistry has played a pivotal role in soil science, providing an avenue for advances in knowledge and understanding of the transformation, reactivity, and occurrence of chemical compounds in soil. Organic matter, perhaps the least known area of soil, has generally suffered from a lack of suitable techniques for its characterization, but that is changing rapidly as new analytical methods, used primarily in the biochemistry field, are being applied. This review is intended to highlight some of the techniques that are used routinely in advanced studies of soil organic matter, primarily the complex humic substances. Our focus is on modern methods of pyrolysis gas chromatography-mass spectrometry, thermochemolysis with tetramethylammonium hydroxide coupled to gas chromatography-mass spectrometry, modern soft-ionization mass spectrometry, and solid-state nuclear magnetic resonance spectroscopy. Although these methods have been used in past studies, there are now new developments in need of review.
[Show abstract][Hide abstract] ABSTRACT: Recent work suggests that nitrogen in humic acids exists primarily as amide functional groups that mirror those in protein. However, the mode for the existence of such labile materials as protein still remains unclear. With the combined applications of NMR spectroscopy, tetramethylammonium hydroxide (TMAH) thermochemolysis, pyrolysis/GC/MS, and elemental analysis, we propose that the survival of proteins in humic acids is carried out by encapsulation into hydrophobic domains of humic acids. To test this hypothesis, we simulated encapsulation of 15N-labeled protein extracts into humic acids and demonstrated that complete hydrolysis of the protein is prevented by the encapsulating humic acid. Results from this study constitute evidence to support the encapsulation mechanism involved in the formation of refractory organic nitrogen during sediment diagenesis.