Solution-state 1H Nuclear Magnetic Resonance Research Articles

Assessment of salinity and heat impacts on natural organic matter composition in bentonite for used nuclear fuel storage

Bentonite clay (MX-80, Wyoming, USA) is a proposed sealing material for the safe and long-term management of Canada's used nuclear fuel in a deep geological repository (DGR). Due to the physiochemical properties of bentonite, this buffer layer can limit radionuclide mobilization, and minimize air and water circulation, and hence suppress microbial activity near the used fuel container. However, natural organic matter (NOM) in mined Wyoming-type bentonite (MX-80) may serve as microbial substrates and cause further corrosion to the used fuel container. In the proposed DGR, bentonite will be exposed to high salinity due to saline groundwater and heat released from radioactive decay of radionuclide inventory of the used nuclear fuel. There is limited knowledge about how NOM quantity and quality in MX-80 may change after being exposed to the anticipated levels of salinity and heat in a DGR, especially at the molecular-level. To explore this, organic carbon contents and NOM composition (both soluble- and solid-phase) in MX-80 after exposure to different saline (NaCl) solutions (1 M, 2 M, 3 M, 4 M and 5 M) with and without heat exposure (90 °C) were analyzed. Dissolved organic carbon (DOC) solubility decreased with increasing salinity but these differences were not statistically significant when compared to the control (P < 0.05). DOC solubilization was not significantly altered with heat (P < 0.05). Dissolved organic matter (DOM) composition was analyzed with solution-state 1H nuclear magnetic resonance (NMR) and ultraviolet–visible (UV–Vis) spectroscopy. The NMR results indicated that DOM samples with varied salinities and heat exposure had similar chemical signatures. The results from UV–Vis analysis revealed a small decrease in less unsaturated components in DOM with NaCl solution compared to deionized water. The chemistry of solid-phase NOM that remained in MX-80 after DOM isolation was similar and suggests an insensitivity to either heat and/or salinity. The solubilized DOC contributes less than 1% to the total organic carbon in MX-80, thus, the anticipated level of salinity and heat in a DGR is unlikely to enhance NOM solubilization and change NOM chemistry in the bentonite markedly. As the bentonite in this study was only exposed to the heat and salinity in a short-term experiment, further studies examining NOM composition and reactivity under long-term saline and heat conditions are recommended.

Assessment of heat and radiation impacts on natural organic matter composition in bentonite for used nuclear fuel disposal

Bentonite clay (Wyoming, USA) is proposed as a buffer layer for the engineered barrier system for the deep geologic repository to store Canada's used nuclear fuels in the long-term. The presence of natural organic matter (NOM) in the mined bentonite may support microbial activity which may promote the corrosion of used fuel containers. The NOM found in Wyoming-type bentonite is diagenetically altered, however, there is limited knowledge about how NOM chemistry may change with heat and radiation when exposed to the proposed deep geologic repository conditions. To investigate this further, molecular-level techniques were used to analyze NOM compositional differences in bentonite with different heat (30–105 °C for 3 days, and 90 °C for 14, 28 and 42 days) and gamma-radiation exposure (100 kGy for 3 days). Elemental analysis results indicated that all bentonite samples exhibited low total organic carbon concentrations (0.15–0.27%) and 1.6–3.3% of these were in the form of extractable dissolved organic carbon. Solution-state 1H nuclear magnetic resonance (NMR) and solid-state 13C NMR analyses revealed that chemical signatures of dissolved organic matter and overall NOM composition did not vary between samples. Slightly increased concentrations of short-chain (C9-C19) n-alkanoic acids were observed with heat (14, 28 and 42 days at 90 °C), which is likely due to the oxidation of long-chain (≥C20) n-alkanes or n-alkanols. However, the broader assessment of NOM composition via principal component analysis of forty-four targeted NOM compounds indicated no significant differences after heat or radiation exposure. Collectively, the molecular-level analyses indicate that heat and radiation only imparted nanogram-level differences in some targeted compound concentrations, but the overall NOM chemistry was not altered. These results expand the understanding of NOM chemistry in bentonite clay and suggest that under the conditions tested, NOM composition in bentonite was largely insensitive to heat and radiation exposure. Further work is warranted to investigate longer exposure durations and the overall stability of NOM in a deep geologic repository setting.

Identification of compounds from terrestrial dissolved organic matter toxic to cyanobacteria

There is emerging evidence for the phytotoxicity of terrestrial dissolved organic matter (DOM), however its sources, transformations and ecological effects in aquatic ecosystems are poorly understood. DOM characterization by Nuclear Magnetic Resonance (NMR) spectroscopy has typically involved solid-state techniques, but poor resolution has often precluded identification of individual components. This study is the first to directly identify individual phytotoxic components using a novel combined approach of preparative HPLC fractionation of DOM (obtained from leaves of two common riparian trees, Casuarina cunninghamiana and Eucalyptus tereticornis). This was followed by chemical characterization of fractions, using one-dimensional (1D) and two-dimensional (2D) solution-state 1H NMR analyses. Additionally, the phytotoxic effect of the fractions was determined using cultures of the cyanobacteria Raphidiopsis (Cylindrospermopsis) raciborskii. The amino acid, proline, from Casuarina leachate was identified as phytotoxic, while for Eucalyptus leachate, it was gallic acid and polyphenols. These phytotoxicants remained in the leachates when they were incubated in sunlight or the dark conditions over 5 days. Our study identifies phytotoxic compounds with the potential to affect algal species composition, and potentially control nuisance R. raciborskii blooms.

Dissolved organic matter characteristics in soils of tropical legume and non-legume tree plantations

Dissolved organic matter (DOM) drives many fundamental biogeochemical processes (e.g., carbon storage, nutrient cycling, and soil development) in forest soil. However, the molecular-level characteristics of DOM derived from different types of tropical forest soils are poorly understood. Here, water samples at different soil depths (0, 20, and 40 cm) from tropical legume (Acacia auriculiformis, AA) and non-legume (Eucalyptus urophylla, EU) tree plantations were analyzed using absorption and fluorescence spectroscopy, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), and solution-state 1H nuclear magnetic resonance (NMR) spectroscopy. The FT-ICR MS results indicated that DOM persisted in the soil, but its molecular composition notably shifted from low-mass (150–300 Da) and more-aromatic molecules to middle- (300–450 Da) and high-mass (>450 Da) and less-aromatic molecules with increasing soil depth. This was primarily mediated by consumption and mineral sorption of low-mass plant-derived DOM (e.g., low-mass carbohydrates and polyphenols) and further formation of larger microbial products (e.g., protein-like and lipid-like compounds). In addition, a higher abundance of microbial-derived molecules (e.g., protein-like and carboxyl-rich alicyclic molecules) was found at the legume plantation relative to the non-legume plantation, which suggests a faster microbial turnover of DOM. Also, the legume plantation had greater enrichment of middle- and high-mass and condensed aromatic-like DOM components in soils. These findings improve our understanding of the drivers that mediate the response of DOM to soil depth and tree species in tropical plantations.

Tunable ionization degree in cationic polyurethanes and effects on phase separation

A linear non-ionic hard-soft segmented polyurethane (PU0) was prepared by the prepolymer synthesis method. The isocyanate terminated prepolymer, formed by a short and rigid aromatic diisocyanate and a long flexible poly(tetramethylene oxide) (PTMO), was chain extended with a piperazine (PIP) diol derivative. The non-ionic polyurethane was then ionized by a post-modification method, i.e. alkylation of the piperazine nitrogen atoms, thus obtaining cationomers with three different ionization degrees, named PU10, PU50, PU80. The polymers were characterized by means of FT-IR spectroscopy, solution and solid state Nuclear Magnetic Resonance (NMR) spectroscopies, Differential Scanning Calorimetry (DSC) and ThermoGravimetric Analysis (TGA). The chemical structure of the obtained polymers and their cationic content was confirmed by solution state 1H NMR. It was found that the methylation occurs quantitatively up to 50% of the PIP nitrogen atoms and that, even if a large excess of methylating agent is used, in our operating conditions a maximum of 80% of the PIP nitrogen atoms are ionized. All the characterization techniques used suggested that an increasing ionization degree induces a better phase separation between hard and soft domains, with a lower amount of diisocyanate units dispersed in the soft phase. In particular, FT-IR showed that the introduction of ionic sites modifies the hydrogen bond network and induces a better aggregation of the rigid units. In addition, DSC showed that (i) the glass transition temperature values of the polymers decrease and tend to that measured for pure PTMO upon increasing ionization; (ii) a crystalline phase due to PTMO packing in ordered structures appears at the highest ionization degrees, similarly to pure PTMO. 1H Time-Domain NMR and 13C solid state NMR revealed an increase in PTMO mobility with increasing ionic content, in agreement with DSC results.The measurements of proton spin lattice relaxation times in both the laboratory (T1) and rotating (T1ρ) frames allowed limits on the size of the hard domains to be estimated. T1ρ data hint at a phase demixing effect upon increasing the ionization degree.

1D "Spikelet" Projections from Heteronuclear 2D NMR Data-Permitting 1D Chemometrics While Preserving 2D Dispersion.

Nuclear magnetic resonance (NMR) spectroscopy is a powerful tool for the non-targeted metabolomics of intact biofluids and even living organisms. However, spectral overlap can limit the information that can be obtained from 1D 1H NMR. For example, magnetic susceptibility broadening in living organisms prevents any metabolic information being extracted from solution-state 1D 1H NMR. Conversely, the additional spectral dispersion afforded by 2D 1H-13C NMR allows a wide range of metabolites to be assigned in-vivo in 13C enriched organisms, as well as a greater depth of information for biofluids in general. As such, 2D 1H-13C NMR is becoming more and more popular for routine metabolic screening of very complex samples. Despite this, there are only a very limited number of statistical software packages that can handle 2D NMR datasets for chemometric analysis. In comparison, a wide range of commercial and free tools are available for analysis of 1D NMR datasets. Overtime, it is likely more software solutions will evolve that can handle 2D NMR directly. In the meantime, this application note offers a simple alternative solution that converts 2D 1H-13C Heteronuclear Single Quantum Correlation (HSQC) data into a 1D “spikelet” format that preserves not only the 2D spectral information, but also the 2D dispersion. The approach allows 2D NMR data to be converted into a standard 1D Bruker format that can be read by software packages that can only handle 1D NMR data. This application note uses data from Daphnia magna (water fleas) in-vivo to demonstrate how to generate and interpret the converted 1D spikelet data from 2D datasets, including the code to perform the conversion on Bruker spectrometers.

Combination of NMR Methods To Reveal the Interfacial Structure of a Pharmaceutical Nanocrystal and Nanococrystal in the Suspended State.

The detailed structure of a pharmaceutical nanosuspension was investigated using three nuclear magnetic resonance (NMR) methods: solid-state, solution-state, and high resolution-magic angle spinning (HR-MAS) NMR. Carbamazepine (CBZ) and CBZ-saccharin (SAC) cocrystal nanosuspensions were prepared by wet-milling with hydroxypropyl methylcellulose (HPMC) and sodium dodecyl sulfate (SDS) as stabilizing agents. Solid-state 13C NMR indicated the presence of not only the crystalline drug substance but also solid-state HPMC, even though HPMC was used as an aqueous solution to prepare the nanosuspensions. Solution-state 1H NMR of the nanosuspensions with and without ultracentrifugation pretreatment indicated that a fraction of the CBZ, SAC, and SDS formed a solid or semisolid phase on the surface of the nanoparticles and was in equilibrium between the dissolved and undissolved states. 1H HR-MAS NMR was highly effective in detecting and quantifying the semisolid phase on the surface of the nanoparticles. From these comprehensive NMR studies, it was concluded that the nanosuspension was composed of crystalline drug core particles surrounded by a semisolid phase consisting of the drug and stabilizing agents. The semisolid phase on the nanoparticle surface was in equilibrium with the solution phase and contributed to the stabilization of the nanoparticle by steric hindrance and electrostatic repulsion.

Pretreatment of anaerobic digester samples by hydrochloric acid for solution-state 1H and 13C NMR spectroscopic characterization of organic matter

Pretreatment of anaerobic digester samples by hydrochloric acid (HCl) resulted in removal of Fe-based mineral and coordination compounds, attenuating their interferences with solution-state nuclear magnetic resonance (NMR) spectroscopic characterization of the solid phase organic matter. Substrate (influent) and digestate (effluent) samples from two full-scale anaerobic digesters, designated CD (co-digester) and SSD (sewage sludge digester), were investigated. Pretreatment of CD samples with 0.2–2.0 mol l−1 HCl and pretreatment of SSD samples with 1.0–3.0 mol l−1 HCl removed 96–100% and 76–80% of total Fe, respectively. Pretreatment declined overall paramagnetic characteristics of digestate samples, manifested by 50% (CD) and 70% (SSD) decrease in electron paramagnetic resonance signal intensities. As a result, meaningful solution-state 1H,13C heteronuclear single quantum coherence and 1H NMR spectra of DMSO-d6 soluble organic matter could be acquired. Sample pretreatment with the lowest concentration of HCl resulted in alteration of C:N ratios in solid phase, likely due to removal of labile organic and inorganic C- and N-containing compounds, while elevating the HCl concentration did not further change the C:N ratios. Furthermore, sample pretreatment increased the solubility of carbohydrates and proteins in DMSO-d6, enabling the detection of NMR resonances from certain structural units of carbohydrates (e.g. anomeric O2CH) and proteins (e.g. CHα in amino acids). Both attenuation of the paramagnetic matrix as well as an enhanced solubility of carbohydrate and protein fractions of the samples in DMSO-d6 solvent contributed to an improved molecular characterization of anaerobic digester samples by solution-state NMR analysis.

Solid-Phase Mediated Methodology To Incorporate Drug into Intermolecular Spaces of Cyclodextrin Columns in Polyethylene Glycol/Cyclodextrin-Polypseudorotaxanes by Cogrinding and Subsequent Heating

In this study, a new preparation method was developed to obtain drug/(polyethylene glycol/cyclodextrin-polypseudorotaxane (PEG/CD-PPRX)) complexes in which drugs were incorporated into the intermolecular spaces of CD columns in PEG/CD-PPRXs. This method was solid-phase mediated and used cogrinding and subsequent heating. Guest drug and CD in PEG/CD-PPRX were amorphized by cogrinding, and then crystallization of CD was promoted by subsequent heating. A previously reported sealed-heating method using the gas phase was not applicable for poorly sublimated and thermally unstable drugs such as piroxicam (PXC) and hydrocortisone, whereas this new method allowed these drugs to be incorporated into the intermolecular spaces of γ-CD columns. Furthermore, salicylic acid (SA) and salicylamide were successfully incorporated into the intermolecular spaces of CD columns using α-CD instead of γ-CD. Powder X-ray diffraction and solution-state 1H nuclear magnetic resonance measurements revealed that complexation followed ...

Composition-dependent structural changes and antitumor activity of ASC-DP/DSPE-PEG nanoparticles

Ascorbyl 2,6-dipalmitate (ASC-DP) and distearoyl phosphatidylethanolamine polyethylene glycol 2000 (DSPE-PEG) formed stable nanoparticles at a molar ratio of less than or equal to 2:1 after dispersing the solvent-evaporated film in water. The mean particle sizes measured by dynamic light scattering were within the range of ca. 100–160nm. Composition-dependent changes of the ASC-DP and DSPE-PEG molecular states within the film were analyzed by wide-angle X-ray diffraction and infrared (IR) and solid-state nuclear magnetic resonance (NMR) spectroscopy. Transmission electron microscopy (TEM) of nanoparticles revealed that ASC-DP/DSPE-PEG changed from a micelle to a disk and tubular structure as the molar ratio increased. Quantitative solution-state 1H NMR measurements elucidated the structure of nanoparticle in water; the core could be composed of ASC-DP and hydrophobic acyl chains of DSPE, whereas the hydrophilic PEG chains of DSPE-PEG on the surface form the hydration shell to stabilize the nanoparticle dispersion in water. Cytotoxicity of ASC-DP against cancer cell lines was observed by using ASC-DP/DSPE-PEG nanoparticles, and no cytotoxicity against normal cells was found. Thus, the ASC-DP/DSPE-PEG formulation, with tumor cell specific cytotoxicity, can be applicable for cancer monotherapy or in combination with other anticancer drugs.

Biochar amendment altered the molecular-level composition of native soil organic matter in a temperate forest soil

Environmental contextBiochar amendment in soil can sequester carbon but may also stimulate microbial activity, potentially enhancing soil organic matter degradation. We incubated biochar in a temperate forest soil and characterised the soil organic matter composition using molecular-level biomarker and nuclear magnetic resonance techniques. Biochar amendment altered the native soil organic matter composition and decreased the concentration of easily degradable soil organic matter components. AbstractBiochar amendment in soil can sequester carbon and improve soil water and nutrient retention, fertility and plant productivity. However, biochar may stimulate microbial activity, leading to priming or accelerated soil organic matter (OM) degradation, which could alter the native soil OM molecular composition. To investigate this, we amended sugar maple wood biochar (pyrolysed at 500°C) at four concentrations (0, 5, 10 and 20 metric tons per hectare) in a temperate forest soil for 32 weeks. Solvent extraction and CuO oxidation were used to characterise free compounds and lignin-derived phenols respectively at 8 week intervals, while base hydrolysis was used to examine plant wax, cutin and suberin components at the end of the incubation. Stimulated soil microbial activity following an adaptation period (16 weeks) resulted in increased inputs of microbial- and plant-derived soil OM components including solvent-extractable short-chain n-alkanols and n-alkanoic acids, long-chain n-alkanes and n-alkanols, and sugars. Degradation parameters for base-hydrolysable cutin- and suberin-derived compounds did not show any significant degradation of these plant biopolymers. Analysis of lignin-derived phenols revealed lower concentrations of extractable phenols and progressive oxidation of syringyl and vanillyl phenols at higher biochar application rates over time. Solution-state 1H nuclear magnetic resonance analysis of base-extractable soil OM after 32 weeks showed a decrease in the proportion of labile OM components such as carbohydrates and peptides and a relative increase in more recalcitrant polymethylene OM constituents in the amended soils. The biochar-mediated shifts in soil OM composition and reduction in labile carbon may reduce soil fertility in biochar-amended systems with long-term amendment.

Characterization of natural organic matter in bentonite clays for potential use in deep geological repositories for used nuclear fuel

The Nuclear Waste Management Organization (NWMO) is developing a Deep Geological Repository (DGR) to contain and isolate used nuclear fuel in a suitable rock formation at a depth of approximately 500m. The design concept employs a multibarrier system, including the use of copper-coated used fuel containers, surrounded by a low-permeability, swelling clay buffer material within a low permeability, stable host rock environment. The natural organic matter (NOM) composition of the bentonite clays being considered for the buffer material is largely uncharacterized at the molecular-level. To gain a better understanding of the NOM in target clays from Wyoming and Saskatchewan, molecular-level methods (biomarker analysis, solid-state 13C NMR and solution-state 1H nuclear magnetic resonance (NMR)) were used to elucidate the structure and sources of NOM. Organic carbon content in three commercially available bentonites analyzed was low (0.11–0.41%). The aliphatic lipid distribution of the clay samples analyzed showed a predominance of higher concentration of lipids from vascular plants and low concentrations of lipids consistent with microbial origin. The lignin phenol vanillyl acid to aldehyde ratio (Ad/Al) for the National sample indicated an advanced state of lignin oxidation and NOM diagenesis. The 13C NMR spectra were dominated by signals in the aromatic and aliphatic regions. The ratio of alkyl/O-alkyl carbon ranged from 7.6 to 9.7, indicating that the NOM has undergone advanced diagenetic alteration. The absence lignin-derived phenols commonly observed in CuO oxidation extracts from contemporary soils and sediments as well as the lack of amino acids suggests that the material corresponding to the aromatic signal is not composed of lignin or proteins but may be derived from another source such as black carbon or some other non-extractable aromatic-rich NOM. The aliphatic signal appears to correspond to long-chain compounds with little side branching based on the results of the one-dimensional (1D) and two-dimensional (2D) solution-state 1H NMR analyses. Overall, the organic geochemical analyses suggest that the NOM is composed mainly of plant-derived waxes and highly aromatic carbon with low contributions from small molecules. The compounds identified by the molecular-level analysis of NOM in the clay samples are hypothesized to be recalcitrant but future studies should examine if these compounds may serve as a microbial substrate to further test the observations of this study. Furthermore, our study suggests that the NOM has undergone diagenesis and that marine NOM signatures are no longer recognizable or detectable. As such, future work may also examine the diagenesis of these deposits to further understand the NOM geochemistry and paleoenvironmental conditions in bentonite deposits.

Comparison of cryoconite organic matter composition from Arctic and Antarctic glaciers at the molecular-level

Glacier surfaces are reservoirs that contain organic and inorganic debris referred to as cryoconite. Solar heating of this material results in the formation of water-filled depressions that are colonized by a variety of microbes and are hypothesized to play a role in carbon cycling in glacier ecosystems. Recent studies on cryoconite deposits have focused on their contribution to carbon fluxes to determine whether they are a net source or sink for atmospheric CO2. To better understand carbon cycling in these unique ecosystems, the molecular constituents of cryoconite organic matter (COM) require further elucidation. COM samples from four glaciers were analyzed by targeted extraction of plant- and microbial-derived biomarkers in conjunction with non-targeted NMR experiments to determine the COM composition and potential sources. Several molecular proxies were applied to assess COM degradation and microbial activity using samples from Greenland, the Canadian Arctic, and Antarctica. COM from Canadian (John Evans glacier) and Greenlandic (Leverett glacier) locations was more chemically heterogeneous than that from the Antarctic likely due to inputs from higher plants, mosses and Sphagnum as suggested by the solvent-extractable alkyl lipids and sterols and the detection of lignin- and Sphagnum-derived phenols after cupric oxide chemolysis. Solid-state 13C nuclear magnetic resonance (NMR) experiments highlighted the bulk chemical functional groups of COM allowing for a general assessment of its degradation stage from the alkyl/O-alkyl proxy whereas solution-state 1H NMR highlighted both microbial and plant contributions to base-soluble extracts from these COM samples. The dominance of 1H NMR signals from microbial protein/peptides in base-soluble extracts of COM from Antarctica (Joyce glacier and Garwood glacier), phospholipid fatty acid (PLFA) biomarker detection and the absence of plant-derived biomarkers in both the solvent and cupric oxide extracts suggests that this COM is dominated by microbial-derived material. These results indicate that COM carbon composition is dependent on the local glacier environment which may have a profound impact on carbon cycling and sequestration on glacier surfaces.

Comparison of soil organic matter composition after incubation with maize leaves, roots, and stems

As plants are major contributors to soil organic matter (OM), the chemical composition of plant tissues entering the soil should be related to the soil OM composition. We examined the OM composition of maize leaves, stems, and roots, to determine if differences in plant tissue composition altered the soil OM composition during a 36-week degradation experiment. Lignin phenols were measured by gas chromatography/mass spectrometry (GC/MS), and soil OM and humic substances were characterized using solid-state 13C and solution-state 1H nuclear magnetic resonance (NMR) spectroscopy respectively. Lignin phenol composition, oxidation, and depletion relative to cutin-derived OH-acids changed less in leaf- compared to stem- and root-amended soils over time, and may be due to greater vanillyl concentrations in leaves. Soil amended with stems had higher concentrations of carbohydrates in soil OM. Humic substances from leaf-amended soils had higher concentrations of aliphatic components, likely due to higher concentrations of aliphatic compounds in leaf tissues, which suggests that compounds derived from leaves are potential contributors to the stable pool of soil OM. After 36-weeks of incubation, the contribution of microbial-derived OM was greatest in humic extracts from root-amended soils, and increased contribution from these compounds was detected earlier in these soils than stem- and leaf-amended soils. This indicates that root amendment may enhance contributions from microbial-derived OM. Our study suggests that changes in soil OM composition over time was related to the chemical composition of the plant tissue, and demonstrates the important link between plant chemistry and soil OM turnover.

Comparison of nuclear magnetic resonance methods for the analysis of organic matter composition from soil density and particle fractions

Environmental contextThe association of specific organic matter (OM) compounds with clay mineral surfaces is believed to protect these compounds from degradation and thus result in long-term protection in soil. The molecular-level composition of soil OM associated with soil fractions was measured and compared using solid-state 13C nuclear magnetic resonance (NMR) and solution-state 1H NMR methods. Combining these methods allowed more detailed characterisation of OM associated with different soil fractions and will improve the understanding of OM dynamics in soil. AbstractOrganic matter (OM) associated with fine soil fractions is hypothesised to be protected from complete biodegradation by soil microbes. It is therefore important to understand the structure and stage of decomposition of OM associated with various soil fractions. Solid-state 13C nuclear magnetic resonance (NMR) spectroscopy has been used extensively to investigate the OM composition of soils and soil fractions. Solution-state 1H NMR spectroscopy has not been used as much but is an emerging tool for analysing soil OM because 1H NMR spectra are often better resolved and provide information that complements the structural information obtained from solid-state 13C NMR experiments. This study compares one-dimensional solution-state 1H NMR and solid-state 13C NMR methods for assessing the degradation and composition of OM in three different soils, and their light and clay-size fractions. The alkyl/O-alkyl degradation parameter was consistent across all NMR methods and showed that OM associated with clay-size fractions were at more advanced stages of degradation as compared to that in light density soil fractions. Solution-state 1H and diffusion edited (DE) 1H NMR results showed the presence of high concentrations of microbial-derived peptidoglycan and peptide side-chains in clay-sized fractions. Lignin was also identified in clay-sized fractions using solid-state 13C and solution-state 1H NMR techniques. The combination of solid-state 13C and solution-state 1H NMR methods provides a more detailed analysis of OM composition and thereby facilitates a better understanding of the fate and preservation of OM in soil.

Association of specific organic matter compounds in size fractions of soils under different environmental controls

Inherent chemical recalcitrance and association of organic matter (OM) with minerals are mechanisms responsible for the long term preservation of OM in soils. The structural characteristics of OM are also believed to control specific interactions between OM and soil minerals. However, the extent of the relationship between recalcitrance and mineral protection and the specificity of these chemically driven interactions are not clearly understood at the molecular level. To measure chemical patterns of OM sequestration in sand-, silt-, clay-size and light fractions, we analyzed three soils, which mainly differed in carbon content and overlying vegetation, but have similar clay mineralogy, using biomarker analysis and nuclear magnetic resonance (NMR). Despite differences in environmental controls, long chain aliphatic compounds generally accumulated in the fine fractions of all soils. This accumulation is likely due to the strong interaction between recalcitrant forms of OM and soil minerals. For example, polymethylene and >C 20 organic acids accumulated in fine fractions, while lignin-derived phenols were protected from oxidation in silt-size fractions. Diffusion edited solution state 1H NMR suggested that contributions from microbial-derived OM was greater in finer fractions, which is likely due to the accumulation of microbial-derived compounds or higher microbial activity in clay micro-sites. Our data suggest that, for these Prairie soils, the specific structure of OM and not environmental factors is responsible for long term preservation of OM in mineral fractions. Further research is necessary to understand the interplay between these preservation mechanisms such that the long term fate of OM can be further elucidated.

The role of biodegradation and photo-oxidation in the transformation of terrigenous organic matter

Microbial and photochemical decomposition are two major processes regulating organic matter (OM) transformation in the global carbon cycle. However, photo-oxidation is not as well understood as biodegradation in terms of its impact on OM alteration in terrigenous environments. We examined microbial and photochemical transformation of OM and lignin derived phenols in two plant litters (corn leaves and pine needles). Plant litter was incubated in the laboratory over 3 months and compositional changes to OM were measured using nuclear magnetic resonance (NMR) and gas chromatography–mass spectrometry. We also examined the susceptibility of soil organic matter (SOM) to ultraviolet (UV) radiation. Solid-state 13C NMR spectra showed that O-alkyl type structures (mainly from carbohydrates) decreased during biodegradation and the loss of small carbohydrates and aliphatic molecules was observed by solution-state 1H NMR spectra of water extractable OM from biodegraded litters. Photochemical products were detected in the aliphatic regions of NaOH extracts from both litter samples by solution-state 1H NMR. Photo-oxidation also increased the solubility of SOM, which was attributed to the enhanced oxidation of lignin derived phenols and photochemical degradation of macromolecular SOM species (as observed by diffusion edited 1H NMR). Overall, our data collectively suggests that while biodegradation predominates in litter decomposition, photo-oxidation alters litter OM chemistry and plays a role in destabilizing SOM in soils exposed to UV radiation.

Chemical characterization of microbial-dominated soil organic matter in the Garwood Valley, Antarctica

Despite its harsh environmental conditions, terrestrial Antarctica contains a relatively large microbial biomass. Natural abundance carbon and nitrogen stable isotope signatures of organic materials in the dry valleys indicate mixed provenance of the soil organic matter (SOM) with varying proportions of contributions from lichens, mosses, lake-derived algae and cyanobacteria. Here we employed two complementary analytical techniques, biomarker measurements by gas chromatography/mass spectrometry and solution-state 1H nuclear magnetic resonance spectroscopy, to provide further information at a molecular-level about the composition and possible source of SOM in the Garwood Valley, Antarctica. The predominance of branched alkanes and short-chain lipids in the solvent extracts indicates that the primary contribution to the SOM was microbial-derived. Chemical structures in the NaOH extracts from soils were also dominated by amide, peptides, and a CH 3-dominating aliphatic region that were characteristic of microbial signatures. Furthermore, the SOM in the Garwood Valley contained compounds that were different from those in the cyanobacteria-dominated mat from a nearby lake (including monoethyl alkanes and enriched side-chain protons). This observation suggests that easily degradable carbon sources from the nearby lake did not dominate the SOM, which is consistent with a fast turnover of the mat-derived organic matter found in the valley. This study highlights the important role of native soil microbes in the carbon transformation and biogeochemistry in terrestrial Antarctica.

Chemometric prediction of alginate monomer composition: A comparative spectroscopic study using IR, Raman, NIR and NMR

The potential of using infrared (IR), Raman and near infrared (NIR) spectroscopy combined with chemometrics for reliable and rapid determination of the ratio of mannuronic and guluronic acid (M/G ratio) in commercial sodium alginate powders has been investigated. The reference method for quantification of the M/G ratio was solution-state 1H nuclear magnetic resonance (NMR) spectroscopy. For a set of 100 commercial alginate powders with a M/G ratio range of 0.5–2.1 quantitative calibrations using partial least squares regression (PLSR) were developed and compared for the three spectroscopic methods. All three spectroscopic methods yielded models with prediction errors (RMSEP) of 0.08 and correlation coefficients between 0.96 and 0.97. However, the model based on extended inverted signal corrected (EISC) Raman spectra stood out by only using one PLS component for the prediction. The results are comparable to that of the experimental error of the reference method estimated to be between 0.01 and 0.08.