ISSN:0038-075X    

出版商:OVID    

年代:2001

数据来源: OVID

摘要:

Humic substances (HS) are the major components of the mixture of materials that comprise soil organic matter, and these substances, which are by far the most abundant organic materials in the environment, are themselves complicated mixtures of biologically transformed organic debris. However, it is likely that many of the solubilization properties of the mixtures arise from the presence of nonhumic components that are intimately associated with the HS and that cannot be separated effectively from these components. Separation and fractionation techniques are improving, and most of the instruments needed to advance awareness of the composition and aspects of structures are now in place. There is a need, however, to review the operational definitions that now apply and to put in place a classification system that will take into account origins and some compositional characteristics. The broad based definition of humin is especially unsatisfactory. This umbrella term covers a mixture of materials that are insoluble in aqueous systems and that contain nonhumic components such as long chain hydrocarbons, esters, acids, and even relatively polar structures of microbial origin, such as polysaccharides and glomalin, that can be associated with the nonpolar moieties and with soil minerals, as well as plant components that are highly resistant to decomposition. Advances in the humic sciences in recent times have been impressive, and the questioning of ‘ingrained’ theories and concepts is opening vistas through which we are seeing new concepts of size, shape, and association; the remarkable developments in nuclear magnetic resonance are also allowing better interpretations of compositions and of aspects of structures. From these advances will emerge a more fundamental understanding of HS functions in such important roles as the stabilization of soil aggregates, the binding of anthropogenic organic chemicals, and the sequestration of C from atmospheric CO2. The latter function is especially important at this time because we need to know why some soils sequester more C than others and why the qualities (or composition and structure) of HS in some soils are different from those in others.

ISSN:0038-075X    

出版商:OVID    

年代:2001

数据来源: OVID

摘要:

Two principles are presented that define the molecular nature and ecological role of humic substances (HS). The First Principle (i) accounts for and organizes an extensive body of apparently disparate data relating to the inability to purify and establish a molecular structure for HS; (ii) offers a conceptual framework for dealing with HS and for evaluating the applicability and limitations of various experimental methods; and (iii) identifies molecular heterogeneity, in combination with pronounced chemical reactivity, as constituting the essence of HS. Five corollaries to the First Principle spell out its consequences in more specific detail. New definitions of HS that offer greater insight into the molecular nature of these materials arise from the First Principle. The inapplicability of the molecular structure concept to HS is explained. The concept of hypothetical pseudostructures is introduced to help visualize the chemical reactions and interactions of HS without the unjustified assignment of specific structures to the material as a whole. Constraints in the design of experiments and in the interpretation of experimental data caused by the heterogeneous nature of HS are discussed. The Second Principle makes a connection between the molecularly heterogeneous and chemically reactive nature of HS and the ecological need for a reactive and persistent medium for plant growth. Concepts presented herein have broad implications in many fields, including chemistry, geochemistry, environmental and soil sciences, and ecology.

ISSN:0038-075X    

出版商:OVID    

年代:2001

数据来源: OVID

摘要:

It is suggested that compounds with traditional structures for humic substances (HS) do not exist in soils but that soil organic matter consists instead of mixtures of plant and microbial constituents plus the same constituents in various stages of degradation. That is, HS are mixtures of plant carbohydrates plus microbial carbohydrates, plant proteins plus microbial proteins, plant lipids plus microbial lipids, etc., together with partially degraded lignins and tannins, etc., and with microbial materials such as melanins and other polyketides. Purely biological processes and abiotic reactions, such as the Maillard reaction, are discussed and rejected as major sources of HS. Factors such as the color of HS, their electron spin resonance spectra, fluorescence, molecular weight, the presence of mellitic acid, and others do not require compounds of the traditional type to be present in HS; they are entirely explicable on the basis of plant/microbial mixtures. Artifact formation is common in HS studies, and it can be seriously misleading.

ISSN:0038-075X    

出版商:OVID    

年代:2001

数据来源: OVID

摘要:

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.

ISSN:0038-075X    

出版商:OVID    

年代:2001

数据来源: OVID

摘要:

Multidimensional nuclear magnetic resonance (NMR) is a powerful and diverse tool for the elucidation of organic compounds and mixtures. This communication presents a discussion of the basic multidimensional liquid state NMR experiments that can be applied to humic substances. A range of experiments is discussed in detail regarding the information that can be obtained for the structural elucidation of humic materials and their solution interactions. In addition, for each experiment, a short review of previous applications is included along with practical tips and guidelines.

ISSN:0038-075X    

出版商:OVID    

年代:2001

数据来源: OVID

摘要:

The scientific understanding of the molecular size and shape of humic substances (HS) is critically reviewed. The traditional view that HS are polymers in soil is not substantiated by any direct evidence but is assumed only on the basis of laboratory experiments with model molecules and unwarranted results produced by incorrectly applying either analytical procedures or mathematical treatments developed for purified and undisputed biopolymers. A large body of evidence shows an alternative understanding of the conformational nature of HS, which should be regarded as supramolecular associations of self-assembling heterogeneous and relatively small molecules deriving from the degradation and decomposition of dead biological material. A major aspect of the humic supramolecular conformation is that it is stabilized predominantly by weak dispersive forces instead of covalent linkages. Hydrophobic (van der Waals, π-π, CH-π) and hydrogen bonds are responsible for the apparent large molecular size of HS, the former becoming more important with the increase of pH. This innovative understanding of the nature of HS implies a further development of the science and technology for the control of the chemistry and reactivity of natural organic matter in the soil and the environment.

ISSN:0038-075X    

出版商:OVID    

年代:2001

数据来源: OVID

摘要:

Extensive knowledge about refractory biomacromolecules has been accumulating for 20 years. The components, characterized by a conspicuous resistance to drastic base and acid laboratory hydrolyses, also exhibit a relatively high resistance to degradation under natural conditions. These refractory biomacromolecules, identified in vascular plants and microalgae, probably play a major role in living organism protection, and they are thought to be important for organic matter sources, composition, and turnover in soils and sediments as well. In addition, some refractory macromolecules are formed in natural environments as the result of condensation/aromatization processes. This review is concerned with six families of refractory macromolecules: lignins, sporopollenins, aliphatic macromolecules (algaenans, cutans, suberans), tannins, black carbon, and proteins. The origin and composition of each family and its contribution to organic matter in soils and sediments are discussed, focusing on recent advances and on questions that are still pending.

ISSN:0038-075X    

出版商:OVID    

年代:2001

数据来源: OVID

摘要:

The literature of humin is reviewed. Humin is described chemically as having a significant aliphatic nature that is probably attributable to the lipids that comprise a significant portion of its organic components. The humic component of humin consists of aromatic and carbohydrate carbon, with an aliphatic component less significant than the corresponding humic acid or fulvic acid isolated from the same sample. Based on NMR relaxation time measurements, the aliphatic carbon and aromatic/carbohydrate carbons seem to represent distinct domains that may represent the different sorption domains of humin described in several recent contaminant sorption models. Several possible areas requiring further research are identified for future study of the environmental chemistry and geochemistry of this material.

ISSN:0038-075X    

出版商:OVID    

年代:2001

数据来源: OVID

摘要:

Soil carbon is a major component of the terrestrial carbon cycle. The soils of the world contain more carbon than the combined total amounts occurring in vegetation and the atmosphere. Consequently, soils are a major reservoir of carbon and an important sink. Because of the relatively long period of time that carbon spends within the soil and is thereby withheld from the atmosphere, it is often referred to as being sequestered. Increasing the capacity of soils to sequester C provides a partial, medium-term countermeasure to help ameliorate the increasing CO2levels in the atmosphere arising from fossil fuel burning and land clearing. Such action will also help to alleviate the environmental impacts arising from increasing levels of atmospheric CO2. The C sequestration potential of any soil depends on its capacity to store resistant plant components in the medium term and to protect and accumulate the humic substances (HS) formed from the transformations or organic materials in the soil environment. The sequestration potential of a soil depends on the vegetation it supports, its mineralogical composition, the depth of the solum, soil drainage, the availability of water and air, and the temperature of the soil environment. The sequestration potential also depends on the chemical characteristics of the soil organic matter and its ability to resist microbial decomposition. When accurate information for these features is incorporated in model systems, the potentials of different soils to sequester C can be reliably predicted. It is encouraging to know that improved soil and crop management systems now allow field yields to be maintained and soil C reserves to be increased, even for soils with depleted levels of soil C. Estimates of the soil C sequestration potential are discussed. Inevitably HS are the major components of the additionally sequestered C. It will be important to know more about the compositions and associations of these substances in the soil if we are able to predict reasonably accurately the ability of any soil type to sequester C in different cropping and soil management systems.

ISSN:0038-075X    

出版商:OVID    

年代:2001

数据来源: OVID