Organic compounds with surface-active properties reduce the surface tension of water and can change soil water contact angles depending on their atueous concentration. The presence of such compounds in the unsaturated zone affects soil moisture characteristics and unsaturated hydraulic conductivity relations, and can cause'flow as a result of induced soil water pressure gradients. Some studies have used horizontal column experiments to measure the surfactant-driven movement of water in unsaturated media; however, they have all used destructive sampling methods to determine water contents and did not measure soil water pressures. We attempted to gain better insight into this unsaturated soil water flow process through continual monitoring of water contents with time domain reflectometry and soil water pressures with pressure transducer equipped tensiomerers. One half of the horizontal one-dimensional flow cell was prewetted (but unsaturated) ~vith water while the other half contained the same fluid content of 7% butanol solution. The temporal and spatial water content and soil water pressure information improved our understanding of the surfactant- induced flow, including perturbations associated with solute~gradients. Backflow in the flow cell was observed at later times as the water-content-induced component of the hydraulic gradients became more important. Hysteretic numerical simulations of the one-dimensional horizontal flow cell using a modified version of HYDRUS 5.0 including coupled flow and transport through concentration-dependent surface tension~ were performed for the case in which butanol is the surface-active compound. The numerical simulations, which used inde- pendently measured flow and transport parameters, provided a good fit to the experimental data and provided further insight into the induced flow behaviors. [ABSTRACT FROM AUTHOR]
Soil Science Society of America Journal. Nov/Dec2007, Vol. 71 Issue 6, p1813-1821. 9p. 1 Diagram, 1 Chart, 1 Graph.
TRICHLOROETHYLENE, ORGANIC compounds, HUMIC acid, SIMULATION methods & models, MOLECULAR dynamics, ORGANIC chemistry, HUMUS, BIOLOGY experiments, and ARABLE land
Gas transport of volatile organic compounds through soils is important for understanding the fate of these organic compounds. Soil organic matter plays a key role in the transport of organic compounds in soils; however, studies have not been performed at the molecular level. A gravimetric experimental method and a computer simulation method were used to study the sorption-desorption of organic contaminants in humic substances. The average apparent diffusivities of trichloroethylene (TCE) in soil humic acid are 1.1 x 10-8 cm²/s for sorption and 3.0 x 10-8 cm²/s for desorption. The activation energies are 29.3 and 59.5 kJ/mol for sorption and desorption, respectively. The molecular simulation results of the kinetics and the activated energy of TCE sorption in humic acid are in good agreement with the experimental data. Both results indicate that the sorption rate of TCE to humic acid increases with the environmental temperature. The sorption of TCE into humic acid is mainly diffusion controlled. Molecular dynamics of chlorinated volatile organic compounds in natural humic substances does yield meaningful results, which can help with understanding the sorption mechanism of organic chemicals in soils at the molecular level. [ABSTRACT FROM AUTHOR]
Chemical extractions of soil organic matter (SOM) have not been widely used to elucidate the dynamics of SOM in field settings, especially to address issues of nutrient cycling. To illustrate potential applications of chemical extractions to nutrient issues, this report reviews studies in which the extraction of SOM fractions was based on their binding to polyvalent soil cations. Radiocarbon ages and cycling rates of 13C and 15N indicated that the unbound mobile humic acid (MHA) fraction cycled faster than did the cationic-bound calcium humate (CaHA) fraction. Analyses for C, N, H, and 0 concentrations and for biochemical groups including carboxyl, phenol, amino, diester P, and free radicals demonstrated that the MHA consisted of more labile and less humified materials than did the CaHA. Quantities and chemical natures of both fractions responded to recent crop management, especially those of the MHA. Three case studies are described in which characterization of the MHA and CaHA contributed toward a process-level understanding of nutrient cycling: (i) a phenol accumulation in the MHA fraction was linked to an inhibition of N mineralization in tropical lowland soils under continuous rice (Oryza sativa L.) cropping, (ii) addition of the MHA to California cotton (Gossypiurn hirstum L.) soils in laboratory studies resulted in increased K availability and plant K uptake, reproducing the benefit of animal manure application in field conditions, and (iii) effects of straw management and winter flooding on N cycling in California rice soils were elucidated by studying a fraction comparable to the MHA fraction. This fractionation is well suited for studying N dynamics, especially in soils enriched in phenolic compounds, and it enabled the linkage of SOM function with chemical nature. It worked well in C-rich flooded soils but needs further evaluation in upland aerobic soils. Further insight into chemical structure and function relations might be achieved by its integration with physical and biological extractions. [ABSTRACT FROM AUTHOR]
Contaminant interactions with soil organic matter (SOM) are central to understanding the fate and transport of chemicals in soil environments. Elucidation of sorption processes will facilitate the efficiency of passive remedial methods and improve the accuracy of risk assessment models. Early studies in the 1960s identified a relationship between SOM and the sorption of chemicals and laid the foundation for an area of research which is still active today. The onset of analytical instrumentation assisted the characterization of SOM chemical fractions, namely the fulvic acid (FA) and humic acid (HA) fractions. The employment of SOM chemical fractions in contaminant sorption studies has produced many empirical relationships between contaminant sorption behavior and SOM structure. More recently, molecular-level techniques such as nuclear magnetic resonance (NMR) spectroscopy have been applied to examine specific interactions between contaminants and SOM fractions. These methods enable direct studies and are likely to further improve the fundamental understanding of contaminant interactions with SOM in the near future. For instance, NMR techniques should produce mechanistic information that will enable the accurate explanation of sorption phenomena at the macroscopic and landscape level. In addition to SOM chemical structure, researchers must consider the organic matter physical conformation at the soil-water interface because chemical methods provide structural information of the whole sample but do not provide detail about their physical architecture within the soil. This manuscript highlights studies which have examined contaminant interactions at the macroscopic- and molecular-level and demonstrates the common themes stemming from different levels of investigation. [ABSTRACT FROM AUTHOR]
The classical methods for the isolation of soil organic matter (SOM) components use aqueous base or neutral salt solutions, and combinations of aqueous base and pyrophosphate. Organic solvents have been rarely used, largely because of difficulties in recovering solutes. This review provides the relevant chemistry of aqueous and organic chemicals relevant to extracting and fractionating the components of SOM that are bound and are not bound by the soil mineral surfaces. Uses of aqueous media to separate the SOM components on the basis of charge density differences are described. Combinations of aqueous base and urea enhance the isolations of the SOM components that have a high degree of polarity [humic and fulvic acids (HAs and FAs), polysaccharides, peptides]. However, the nature of the associations between the solute molecules has prevented the isolation of any purified SOM component. Properties are listed of organic solvents that have potential for the isolation of SOM components, and novel procedures are described for the recovery of the SOM components dissolved in organic solvents. The procedures include the uses of resins with varying degrees of polarity and the recovery of the SOM components in aqueous media. No satisfactory solvent system has been found that can isolate all of the humin materials sorbed by the inorganic colloids. However, procedures are outlined that can extract much of the material classified as humin in the classical definitions. Because the polar components of the SOM can now be removed, it is likely that the compositions of the nonpolar humins strongly held by the mineral colloids will be resolved using procedures such as pyrolysis-mass spectrometry. [ABSTRACT FROM AUTHOR]
Physical fractionation methods are based on the premise that soil organic matter (SOM) can be divided into pools of functional relevance. Physically uncomplexed organic matter (OM) is isolated on the basis of particle size and/or density. Our objective here is to review research on the biological and chemical characteristics of physically uncomplexed OM that demonstrates its value (or otherwise) as a meaningful pool of SOM. Chemical characterization indicates that fractions isolated by size are not identical to those separated by density; even materials separated using variations of a particular fractionation method (i.e., different sizes or different densities) have different chemical and biological properties. Physically uncomplexed OM often contains a substantial portion of whole soil carbon (C) and nitrogen (N) and, compared with the whole soil or heavy fraction, has a wide C/N ratio and high 0-alkyl (i.e., carbohydrates) and low carbonyl (i.e., proteins) C contents. The response of physically uncomplexed OM to changes in land use and management practices is greater than that of other labile OM fractions or the whole soil C and N. Studies to elucidate the nutrient availability of physically Uncomplexed OM suggest that it is not an immediate source of nutrients. That the quantity of physically uncomplexed OM is not always related to the amount of plant residue inputs suggests that other factors may control its accumulation in soil. Thus the quantity and the biological and chemical properties of physically uncomplexed OM are affected by the amount, composition, and accessibility of plant residues entering the soil; environmental conditions that may enhance or constrain decomposition; and the fractionation technique used. [ABSTRACT FROM AUTHOR]
Nonparametric approaches are being used in various fields to address classification type problems, as well as to estimate continuous variables. One type of the nonparametric lazy learning algorithms, a k-nearest neighbor (k-NN) algorithm has been applied to estimate water retention at -33- and -1500-kPa matric potentials. Performance of the algorithm has subsequently been tested against estimations made by a neural network (NNet) model, developed using the same data and input soil attributes. We used a hierarchical set of inputs using soil texture, bulk density (Db), and organic matter (OM) content to avoid possible bias toward one set of inputs, and varied the size of the data set used to develop the NNet models and to run the κ-NN estimation algorithms. Different ‘design-parameter’ settings, analogous to model parameters have been optimized. The κ-NN technique showed little sensitivity to potential suboptimal settings in terms of how many nearest soils were selected and how those were weighed while formulating the output of the algorithm, as long as extremes were avoided The optimal settings were, however, dependent on the size of the development/reference data set. The non-parametric κ-NN technique performed mostly equally well with the NNet models, in terms of root-mean-squared residuals (RMSRs) and mean residuals (MRs). Gradual reduction of the data set size from 1600 to 100 resulted in only a slight loss of accuracy for both the κ-NN and NNet approaches. The κ-NN technique is a competitive alternative to other techniques to develop pedotransfer functions (PTFs), especially since redevelopment of PTFs is not necessarily needed as new data become available. [ABSTRACT FROM AUTHOR]
Soil Science Society of America Journal. May/Jun2004, Vol. 68 Issue 3, p802-808. 7p. 2 Diagrams, 3 Charts, 9 Graphs.
PHOSPHORUS in soils, PHOSPHORUS, PHOSPHATES, ORGANIC compounds, ORGANIC chemistry, and CHEMISTRY
A large proportion of the organic P in softs can occur as scylloinositol phosphates. These compounds are rarely detected elsewhere in nature and remain poorly understood, partly because conventional procedures for their determination are lengthy and erroneous. We report a straightforward procedure for the determination of scylloinositol phosphates in soil extracts using solution 31P nuclear magnetic resonance (NMR) spectroscopy. Solution 31P NMR chemical shifts of a range of synthetic scyllo-inositol phosphate esters were determined in alkaline solution. Of these, only the signal corresponding to scylloinositol hexakisphosphate at approximately 4.2 ppm was identified in soft NaOH-EDTA extracts, constituting between 6.5 and 9.8% of the NaOH-EDTA extracted P. This signal has been previously assigned to choline phosphate, but we confirmed it to be an inositol phosphate using hypobromite oxidation, a procedure that destroys all organic matter except inositol phosphates. Lower order scylloinositol phosphate esters were not identified in the extracts studied here, and literature reports suggest that they probably occur in insufficient concentrations to be detected by this procedure. The identification of scyllo-inositol hexakisphosphate in soils and other environmental samples will allow its quantification in a range of environments, and facilitate research into the origins and function of this enigmatic compound. [ABSTRACT FROM AUTHOR]