摘要:

Soil air permeability in undisturbed soil is closely related to soil structure and heterogeneity and hydraulic properties. Knowledge of air permeability behavior for different types of soils is, therefore, valuable. In this study, we investigated the variability and spatial correlation structure of soil air permeability and soil textural properties in an undisturbed constructed field at Hiroshima University, Hiroshima, Japan. Air permeability was measured on undisturbed soil samples along two 70-m-long transects in the top 10 cm of a field constructed approximately 20 years ago from sandy loam soil collected from a nearby mountain. Air permeability (ka) showed spatial correlation along both transects, with a range of approximately 20 m. Measurements of kaon large (3140 cm3) samples were similar, showing a soil with little small-scale heterogeneity. Local-scale measurements of kawithin a 1.6- by 1.6-m grid taken 4 months after the transect sampling suggested that soil structure and pore size distribution had changed significantly over time, as a result of tilling and precipitation, causing an increase in ka. No spatial drift in soil physical parameters within the 1.6- by 1.6-m grid was found. On-site kameasurements after 1- to 2-day rainy periods compared well with laboratory measurements at a soil-water potential of −100 cm H2O, suggesting that natural field capacity occurs at this potential. Measurements of kaas a function of air-filled porosity (&epsis;) at the mid-point of the two transects were compared with predictions by two recently presented ka(&epsis;) models, and good agreement between simulated values and measurements was found.

ISSN:0038-075X    

出版商:OVID    

年代:2001

数据来源: OVID

摘要:

Sensors for landmine detection are often affected by soil water content, temperature, electrical conductivity, and/or dielectric constant. The most important of these is water content because it influences the three other properties directly. Using HYDRUS-2D, we modeled water distributions around antitank mines buried in six soil textures varying from sandy loam to clay loam under the climatic conditions of Bosnia and Kuwait. The modeling results demonstrate that soil water content regimes around landmines are strongly affected by the interaction between climate, soil type, and landmine geometry. The occasional short-term accumulation or loss of soil water around landmines depends greatly on weather conditions and soil types. Results also show that steady-state analysis of water flow around buried objects and time averaging of observed water contents may lead to unrealistic conclusions regarding the transient behavior of soil water distributions around land-mines.

ISSN:0038-075X    

出版商:OVID    

年代:2001

数据来源: OVID

摘要:

The commonly accepted polymeric nature of humic substances is still a matter of debate. In this work, both humic matter of different origin and undisputed neutral (polysaccharides) and negatively charged (polystyrenesulphonates) polymers of known molecular weight were studied by high performance size exclusion chromatography after dissolution in mobile phases of different composition but constant ionic strength. Modification of the control mobile phase with methanol, hydrochloric acid, and acetic acid showed a progressive alteration of the chromatographic behavior of humic materials, but it did not alter that of polymers. Absolute size reduction of bulk humic material revealed by a refractive index detector was accompanied by a substantial decrease in absorbance of peaks indicated by the UV detector. Reduction of molecular absorptivity of humic substances with changes in solution composition was confirmed by UV spectroscopy over a wide range of wavelengths. Differences in chromatographic behavior between undisputed polymers and humic samples suggest that humic matter reflects, rather than a polymeric nature, a supramolecular association of heterogeneous molecules held together mainly by weak hydrophobic forces. The content of hydrophilic and hydrophobic carbons in humic substances appears to control the intermolecular hydrogen and dispersive bondings that ultimately stabilize humic conformations in different mobile phases.

ISSN:0038-075X    

出版商:OVID    

年代:2001

数据来源: OVID

摘要:

Molecular characterization of soil lipids often provides valuable biogeochemical information about the impact of vegetation, microorganisms, and abiotic factors on the soil C sequestration proccess. The total lipid extracted with petroleum ether from nine soils developed under three types of Mediterranean forest (stone pine (Pinus pineaL.), evergreen oak (Quercus rotundifoliaL.), and Spanish juniper (Juniperus thuriferaL)) has been analyzed by high-resolution13C nuclear magnetic resonance (13C-NMR) under quantitative acquisition conditions. Tentative assignments of the spectral peaks are presented, and the spectra of soil lipids are compared with those from the lipids extracted directly from leaves of the corresponding trees. This comparison evidenced that soil lipids behaved as biomarker soil fractions when analyzed by13C-NMR as a whole. Analysis by gas chromatography-mass spectrometry (GC/MS) reveals that the volatile fraction of the lipid extract (46%, on average, as estimated by internal reference) consisted mainly of free alkanes, alkanoic acids (<C30), and diterpene resin acids.We observed some differences between the chemical structures suggested by13C-NMR and GC/MS. This was interpreted as a portion of soil and plant lipids consisting of extractable material that cannot be detected by standard GC methods. The complex signal pattern in the 0 to 30 ppm chemical shift range showed typical signals for carbons in acyl polymethylene chains, which overlapped with a pattern suggesting isoprenoid-like branching in long-chain structures (major signals at ca. 22, 26, and 32 ppm). In addition, periodic unsaturations suggested by signals at ca. 124 and 135 ppm are also compatible with polyprenoid-type backbones. The alkyl region coincided with those of mono- to triacyl glycerol fatty esters. It seems evident that13C-NMR allows us to characterize structures present in nonvolatile complex material.

ISSN:0038-075X    

出版商:OVID    

年代:2001

数据来源: OVID

摘要:

We studied (i) the competitive adsorption of arsenate and phosphate on goethite as affected by pH, anion concentration, and order of anion addition; (ii) the effect of increasing amounts of arsenate or phosphate on the desorption of phosphate or arsenate from the surfaces of goethite; and (iii) the influence of oxalate on the desorption of arsenate and phosphate from goethite.Similar amounts of phosphate and arsenate were adsorbed on goethite samples at pH 3.0-8.5. By adding arsenate and phosphate as a mixture (As+Psystems; arsenate/phosphate molar ratio of 1), the amounts of arsenate and phosphate adsorbed on goethite were similar at pH > 6.0, but at pH < 6.0 slightly more arsenate than phosphate was adsorbed. When arsenate was added before phosphate (As before Psystems), the efficiency of arsenate in inhibiting the adsorption of phosphate on goethite was higher than that of phosphate in inhibiting the adsorption of arsenate, when phosphate was added before arsenate (P before Assystems). Furthermore, we found that the decrease in adsorption of phosphate in the presence of increasing concentrations of arsenate was greater than that of arsenate in the presence of increasing concentrations of phosphate.There is much evidence indicating that arsenate and phosphate compete primarily for a similar set of surface sites on goethite, but they are adsorbed on some sites uniquely specific for arsenate and phosphate.In the pH range of 3.0 to 8.5, much more phosphate previously adsorbed on goethite was desorbed by arsenate (mainly at pH < 6.0) than arsenate by phosphate. The desorption of both the ligands was influenced by time.Finally, the presence of oxalate ions in solution desorbed higher amounts of phosphate than arsenate ions, adsorbed previously on goethite, mainly in acidic systems.

ISSN:0038-075X    

出版商:OVID    

年代:2001

数据来源: OVID

摘要:

Soil genesis involving felsic rocks frequently involves understanding the roles of climate and time on the transformations of the primary minerals into secondary products. This study focuses on the weathering of peralkaline felsics in a semiarid climate. Soils and fresh parent materials were collected from different microclimate regimes in the Chisos Mountains of Big Bend National Park, Texas. Soils in the Chisos mountains generally have either a mollic-argillic-rock or a mollic-rock horizon sequence, whereas other sites at lower and drier locations have an ochric-rock horizon sequence. Atmospheric deposition of Ca and Sr and wind deflation of clay are important attributes of soil genesis. Elemental loss rates of Si, Al, K, Na, and other elements are appreciable, suggesting that leaching has been important to the soil formation. Iron oxide formation, which is related to the moisture regime, is greater in soils having wetter microclimates. Solution chemistry suggests that Mg concentrations are appropriate for smectite synthesis and maintenance, potassium concentrations reflect the presence of feldspar, sodium concentrations reflect their loss rates because of leaching, and calcium reflects the presence of calcite. The rare earth elements similarly demonstrate loss rates consistent with chemical weathering. In general, chemical weathering of feldspars, atmospheric deposition of Ca and Sr, and wind deflation are important pedogenic processes.

ISSN:0038-075X    

出版商:OVID    

年代:2001

数据来源: OVID