Non-targeted Explorations in the Compositional and Structural Space of Natural Organic Matter
Natural organic matter (NOM) occurs in soils, freshwater and marine environments, in the atmosphere and in the form of prebiotic organic matter and represents an exceedingly complex mixture of organic compounds that collectively exhibits a nearly continuous range of properties (size-reactivity continuum). The fate NOM in the bio- and geosphere is governed according to the rather fundamental restraints of thermodynamics and kinetics. In these intricate materials, the "classical" signatures of the (geogenic or ultimately biogenic) precursor molecules, like lipids, glycans, proteins and natural products have been attenuated, often beyond recognition, during a succession of biotic and abiotic (e.g. photo- and redox chemistry) reactions. Because of this loss of biochemical signature, these materials can be designated non- repetitive complex systems. The most informative, "bottom-up" approach to molecularly characterize these complex materials necessarily relies upon spectroscopic methods which translate high-precision frequency measurements into very significant molecular-level information. Frequencies can be measured with an accuracy of 15 digits. This extent of accuracy in frequency measurements translates directly into high resolution, itself a very useful and even indispensable feature to produce information-rich data with sufficient resolution to overcome the otherwise common and detrimental effects of intrinsic averaging, which deteriorate spectral resolution to the degree of a bulk-type characterization rather than to a molecular resolution analysis. High-precision frequency measurements, which can be translated into isotope-specific molecular resolution detail of unprecedented significance and richness, define the core of the two most influential methods of organic structural spectroscopy for the investigation of complex materials, namely NMR spectroscopy (provide unsurpassed insight into close-range molecular order to assess the structural space) and FTICR mass spectrometry (provide unsurpassed resolution to explore the compositional space). The quality of this stand-alone de novo molecular-level resolution data is of unparalleled mechanistic relevance and sufficient to fundamentally advance our understanding of structure and function of NOM, which at present are poorly amenable to meaningful target analysis. This presentation will provide an evaluation of state-of-the-art concepts and applications of molecular level structure elucidation to NOM materials of various origin. According to these findings, NOM is a rather active participant of the global carbon cycle, and the current perception of NOM being considered refractory can be regarded as a consequence of insufficient resolution of methods commonly used in its characterization. References: N. Hertkorn, et al., Geochim. Cosmochim. Acta, 70 (2006) 2990-3010. E. M. Perdue, et al., Anal. Chem., 79 (2007) 1010-1021. F. Einsiedl, et al., Geochim. Cosmochim. Acta, 71 (2007) 5474-5482. N. Hertkorn, et al., Anal. Bioanal. Chem., 389 (2007) 1311-1327. N. Hertkorn, et al., Anal. Chem., 80 (2008) 8908-1919.
The Critical Role of Dissolved Organic Matter in Colloidal Stability of Manufactured Nanomaterials
Nanomaterials (1-100 nm) are found in increasing number of products and applications due to the rapid development of nanotechnology. As a result, nanomaterials will be eventually introduced into the environment from intentional applications and accidental release. Recent toxicological data raise concerns over the environmental and health risks of these nanomaterials which will be largely determined by their fate, mobility, and bioavailability of in the environment. In this research, colloidal behavior of carbon nanotubes (CNT) and aluminum oxide nanoparticles (Al2O3 NP) was examined in the presence of dissolved organic matter (DOM) and mechanistic discussion will be presented with structural consideration of DOM. Tannic acid greatly increased the stability of CNT suspension at environmental-relevant DOM concentrations. The suspension stability of CNT was also strongly affected by pH, cation type, and ionic strength (I). Zeta potential data clearly showed that humic acid (HA) stabilized the Al2O3 NP suspension when added at pH near or above its zero point of charge (ZPC) by lowering the zeta potential through ionization of polar functional moieties of adsorbed and/or free HA in suspension. However, in acidic conditions Al2O3 NPs had strong aggregation in the presence of free long chain polymeric materials present in less polar HA, even at very low I. AFM imaging displayed coiling of long chain hydrophobic fractions of HA followed by entrapment of Al2O3 NPs leading to aggregation. Aggregation kinetics of three structurally different HA-coated Al2O3 NPs was also investigated in the presence of Ca2+ in both acidic and alkaline conditions. Critical coagulation concentration (CCC) of Ca2+ was determined for each HA-coated Al2O3 NP system. The CCC increased with decreasing polarity of the adsorbed HA on the nanoparticle surface. The CCC values also increased in alkaline condition compared to acidic condition for less polar HA-coated Al2O3 NPs. Long chain polymeric fraction of less polar HA could significantly stabilize Al2O3 NP suspension when bounded to the nanoparticle surface through steric stabilization. This study demonstrates the critical role of DOM in determining the mobility, fate, and bioavailability of manufactured nanomaterials. Therefore, ecological exposure and risk of nanomaterials should be evaluated with an understanding of their interactions with DOM.
Role of lipids in the structural organization of humic acid
The role of lipid components in the structural organization of humic acid in the solution- and the solid-state has been investigated using surface tensiometry and differential scanning calorimetry. A combination of aqueous alkaline and organic solvent extractions was used to isolate two humic-like fractions (HA1 and HA2) and one lipid-like fraction from humic acid. Fraction HA1 represents approximately two-thirds of the total organic carbon of the original humic acid and under alkaline conditions is a very weak surfactant. Fraction HA2 represents up to one-third of the humic acid and significantly lowers the surface tension of water. It is also intimately associated with the lipid fraction. Unlike the original humic acid samples, HA2 does not show micelle-like aggregation over the concentration range studied. The solid-state structural organization was examined by directly comparing the organization of the humic acid as isolated with a physical mixture of the same chemical composition composed of its three fractions using differential scanning calorimetry. Comparative measurements of the specific heat capacity as a function of temperature of the original humic acid reveal differences when compared to a mixture of the components with the same chemical composition. These differences provide direct evidence that humic acid's solid state structure is more than just a mixture of components and is determined by specific interactions between its components. This study indicates that humic acid has a hierarchical or "structure-within a structure" architecture. The lower-level structure is determined by the self-assembly of amphiphilic components of humic acid with lipids into a nanostructured composite material. A higher-level structure is formed by the association of this composite material with the remainder of humic acid's components.
The Charcoal Component of Soil Organic Matter in the Boreal Forest of Western Quebec (Abitibi Region)
Despite high interest in pyrogenic carbon (PyC) as a stable and possibly major component of soil organic matter (SOM), there is surprisingly little information on production, stocks, longevity, chemical properties or ecological role of PyC in boreal forests. We define the whole range of fire-transformed biomass and SOM as pyrogenic C (PyC), black carbon (BC) as the fraction resistant to laboratory oxidation, and charcoal as that determined visually. Fire is the major disturbance in boreal forests, with panboreal production estimated at 12.7 Tg/y as solid PyC and 0.38 Tg/y as atmospheric soot (estimated as 5% and 0.15% of emissions, respectively). PyC is considered a highly-stable component of SOM, and thus should contribute to long-term C sequestration by partially offsetting C losses due to fire. Forest floor charcoal is considered to enhance N availability after fire, partly by sorbing phenolics and providing microsites for microbial activity, while other studies have indicated that BC enhances mineral soil fertility, mainly by enhancing cation exchange capacity as it oxidizes. However, studies of its ecological role in boreal forests have not sufficiently isolated charcoal effects per se from direct effects of fire. Without fire disturbance many boreal forests undergo gradual paludification, with increasing thickness of organic horizons and dominance of sphagnum moss and ericaceous shrubs. As part of extensive studies of fire history and paludification in the Abitibi region of Quebec, charcoal fragments (>2mm) were separated at 1 cm depth increments (organic horizons plus varying depths of mineral soil, 2-3 monoliths per plot) in 31 plots of black spruce (Picea mariana) and 19 of jack pine (Pinus banksiana). Plots included stands originating after high- and low-severity fires, the former defined as leaving <5 cm of organic horizon. Plots from low-severity fires (mainly black spruce, up to 229 y) generally had multiple layers of charcoal in the organic horizons, whereas for plots from high-severity fires (including the oldest plots, 710 to 2355 y black spruce) charcoal was mainly concentrated at the organic-mineral interface or in mineral soil. Preliminary results show total charcoal ranging from approximately 50 to 5500 kg/ha, often with wide variation between monoliths, but generally in keeping with other studies using visual identification. Selected samples will be analysed for total C, N and ash and characterized by solid-state C-13 NMR. We are also exploring requirements, approaches and available data for integration of PyC into the Carbon Budget Model of the Canadian Forest Sector 3 (CBM-CFS3). Using this complete ecosystem carbon dynamics model and available data, we investigate the relative contribution of loss rates and oxidation in subsequent fires to the dynamics of BC in boreal forest ecosystem. Preliminary results based on a medium-severity fire every 125 years indicate a PyC pool in the order of 4000-7000 kg C/ha, constituting 5-10% of total soil C and thus reasonably consistent with field data for charcoal.
Molecular-Level Transformations of Lignin During Photo-Oxidation and Biodegradation
As the second most abundant component of terrestrial plant residues, lignin plays a key role in regulating plant litter decomposition, humic substance formation, and dissolved organic matter (OM) production from terrestrial sources. Biodegradation is the primary decomposition process of lignin on land. However, photo- oxidation of lignin-derived compounds has been reported in aquatic systems and is considered to play a vital role in arid and semiarid regions. With increasing ultraviolet (UV) radiation due to ozone depletion, it is important to understand the biogeochemical fate of lignin exposed to photo-oxidation in terrestrial environments. This study examines and compares the transformation of lignin in a three-month laboratory simulation of biodegradation and photo-oxidation using molecular-level techniques. Lignin-derived monomers extracted by copper oxidation were analyzed by gas chromatography/mass spectrometry (GC/MS) from the water-soluble and insoluble OM of 13C-labeled corn leaves. Biodegradation increased the solubility of lignin monomers in comparison to the control samples, and the acid-to-aldehyde (Ad/Al) ratios increased in both the water-soluble and insoluble OM, indicating a higher degree of side-chain lignin oxidation. Photo-oxidation did not produce a significant change on the solubility or Ad/Al ratios of lignin from corn leaves. However, the ratios of trans-to-cis isomers of both cinnamyl units (p-coumaric acid and ferulic acid) increased with photo-oxidation and decreased with biodegradation in the insoluble OM. We also investigated the role of photo-oxidation in lignin transformation in soils cropped with 13C-labeled corn. Interestingly, the organic carbon content increased significantly with time in the water-soluble OM from soil/corn residues under UV radiation. An increase in the concentration of lignin monomers and dimers and the Ad/Al ratios was also observed with photo-oxidation. Iso-branched fatty acids of microbial origin remained in a similar concentration in the water-soluble OM from the UV-radiated and control soils, indicating little microbial contribution to the observed increase in water-soluble carbon. These observations suggest that photo-oxidation may increase the solubility of soil organic matter (SOM) through the oxidation of lignin-derived compounds. Mechanisms of lignin oxidation (demethylation or side-chain oxidation) and molecular size distribution changes of the water-soluble and NaOH-soluble OM during photo-oxidation and biodegradation will also be examined using solution-state nuclear magnetic resonance (NMR) spectroscopy. Collectively, our experiment demonstrates that while biodegradation predominates in the decomposition of lignin in plant litter, photo- oxidation may play an important part in destabilizing lignin-derived compounds in the soil.
Long-term, Trans-Canada Decay of 13C-labelled Crop Residues
The balance between soil C inputs and outputs has important implications for agricultural sustainability and atmospheric composition. While considerable information is available on the short-term (2 to 20 months) decomposition of soil C inputs, the long-term decomposition and persistence remains a major gap in our understanding of carbon and nitrogen cycling in agroecosystems. In many biogeochemical models, assumptions about long-term decomposition are largely unverified. Many of the data available for long-term crop residue decomposition were collected before 1970 when radiocarbon-enriched materials were used. To address these gaps, we implemented a long-term, trans-Canada decay study to measure the decomposition (10 to 20 years) of barley (Hordeum vulgare) residues at ten sites across Canada's agricultural region. The barley residues were uniformly and highly enriched with the stable 13C isotope so that small amounts can be distinguished from background soil carbon. In this presentation we will discuss the rationale for the study, and explain how it was implemented and will be maintained. Because the study was initiated in the fall of 2007, we will present initial results on residue persistence during the early stages of crop residue decomposition. We will also discuss the potential for exploiting the 13C tracer to investigate the structural chemistry of stabilized soil organic matter, and the functional groups of organisms within the detrital community.
Elucidation of Specific Binding Sites for Organofluorine Compounds in Peat Humic Acid using Reversed Heteronuclear Saturation Transfer Difference NMR Spectroscopy
In this presentation, a modified version of the NMR technique of Saturation Transfer Difference (STD) NMR is used to identify the distinct structural components of a peat humic acid mixture to which organic contaminants may bind. In our version of this experiment the direction of saturation transfer is reversed, from small molecules to macromolecules, and the transfer is between heteronuclei, from 19F nuclei on organofluorides to 1H nuclei on humic acid components, hence our term: reversed-Heteronuclear Saturation Transfer Difference (r-HSTD) spectroscopy. In the r-HSTD spectra of a humic acid mixture, only those components in contact with an organofluorine species will be observed, the identity of which can be inferred from the 1H chemical shift. This approach is used here to show that aromatic organofluoride compounds interact preferentially with lignin- derived material, whereas an aliphatic perfluorocarboxylic acid interacts nearly exclusively with protein-derived material. These findings have direct implications in the greater understanding of soil sorption phenomenon, as we present a powerful tool for the elucidation of the specific interactions through which organic pollutants bind to natural organic matter.