摘要:

Stability of aggregates at the immediate soil surface affects the detachment, transport, and deposition of soil particles. The aggregate stability (expressed as mean weight diameter, MWD) and erosion rate were tested for clayey surface soils at two different water contents. The soils consisted of six Oxisols, one Ultisol, three Vertisols, one Alfisol, and one Aridisol. The MWD was determined by wet sieving of 4.76 to 8.00 aggregates, which were either air-dried (MWDd) or capillary prewetted at −0.5 kPa for 24 h (MNDw). Air-dried and prewetted (saturated from the bottom up for 2 h) soils were subjected to a 110-mm h−1rain until a steady-state infiltration was reached. Runoff, sediment loss, and infiltration (for prewetted soil only) were monitored throughout the rain. The MWDdranged from 0.18 to 4.80 mm, and the MWDwranged from 0.44 to 5.66 mm. Prewetting increased the MWD for the oxidic or kaolinitic Oxisols and Ultisols and for one smectitc/kaolinitic Vertisol. The MWDwwas positively related to CEC for highly weathered soils and negatively related for swelling soils, whereas base saturation was negatively related for both groups of soils. The ΔMWD, which expresses the increase in stability caused primarily by a decrease in slaking, was greatest for soils rich in kaolinite and Fe and Al oxyhydroxides and low in organic carbon. The steady-state infiltration rate (Is) ranged from 1.8 (illitic soil) to 54.8 mm. h−1(sandy loam kaolinitic soil). The Iswas consistently less for swelling than for highly weathered soils, and it was related to the sand/clay ratio, sand, and water retained between −33 and −1500 kPa. The increased aggregate stability with prewetting was generally not reflected in smaller amounts of erosion and runoff, possibly because of the faster rate of wetting than for the stability test, a higher water table for the prewetted soil than for the air-dried soil, and the inclusion of smaller aggregates in the erosion tests. Prewetting decreased the time to ponding for three highly weathered Oxisols and the illitic Alfisol, increased steady-state runoff rate (Qw) for five highly weathered soils, increased sediment loss rate (Qs) for the illitic and for one highly weathered soil, and increased Qson a stable Oxisol. Soil textural properties (usually clay and sand/clay) and “binding” agents (organic carbon and Fe and Al oxyhydroxides) were the significant independent variables included in the stepwise multiple linear regressions for Is, Qs, and Qw

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

The objective of this study was to develop a procedure to predict the maximum potential amount of clay dispersible from soils during wetting. Information in the literature on the relationship between soil water content at sampling (Θ) and dispersible clay (DC) and that between Θ and wet aggregate stability for 10 different soils was used to develop extrapolation equations. The Θ at which DC became maximum (defined Θmax) ranged from 0.34 to 0.54 kg kg−1, and it increased with increasing clay and organic matter contents (R2= 0.56). The DCmax, defined as the amount of clay that will disperse at Θmax, ranged from 3.22 to 20.05 g clay 100 g−1soil. The DCmaxincreased with increasing clay and decreasing organic matter contents (R2= 0.92). Decreases in organic matter content brought about larger increases in DCmaxin a clay soil than in a loam

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

The variation of percent gypsum with depth in gypsiferous soils requires a special calibration curve for each layer in order to obtain accurate results in soil-water studies using a neutron probe. To overcome the tedious work of the field calibration for each layer, a correction factor for percent gypsum and bulk density was suggested. The effects of gypsum and bulk density on the neutron probe calibration curve were studied in the laboratory, using a special drum that can be bolted together in two halves, and in the field. The effect of bulk density determined on core samples was negligible for the soil studied in the laboratory, whereas it was significant for one of the field calibrations carried out on gypsiferous soils. An increase in the slope of moisture content on a volume basis vs. count ratio with increasing gypsum content determined by precipitation with acetone method of the soil was observed in the laboratory calibration. A simple method for correction of the calibration curve for gypsum content was adopted to obtain a specific curve for each layer. The adopted method requires the gypsum fraction to be estimated for each layer and then incorporated in the calibration curve to improve the coefficient of determination. The field calibrations showed improvements of the determination coefficient by introducing gypsum fraction, calcium carbonate fraction determined by the calcimeter, and bulk density in one calibration in addition to count ratio using moisture content on a volume basis as a dependent variable in multilinear regression analysis. The presence of gypsum in the soil increases count ratio in that some of the water is water of hydration, unavailable to plants. The effect of bulk density is not as clear in the case of organic soils and when hydrogen exists in the soil in forms other than free water

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

Bentonite clay liners are commonly employed to mitigate the movement of contaminants from waste disposal sites. In order to assess the ability of clay liner material to restrict the mobility of amine compounds under a variety of chemical conditions and to further elucidate amine adsorption characteristics, the adsorption of aniline ando-, m-, andp-toluidine on Ca2+- and K+-saturated Wyoming bentonite (SWy-1) was investigated. Adsorption experiments were performed under conditions of varied pH and ionic environment. Amine adsorption on montmorillonite is pH dependent. Maximum amine adsorption occurs when solution pH is approximately equal to the pKaof the anilinium ion deprotonation reaction (pH 4.45–5.08). An amine adsorption envelope results from the combined influence of increasing anilinium ion and anilinium-aniline complex formation (as pH decreases to the pKa) and amine competition with H+for surface sites, decreasing anilinium-aniline complex concentration, and decreasing aniline available for water bridging with exchangeable Ca2+and K+(as solution pH decreases below the pKa). For any given amine, maximum adsorption increases with decreasing ionic strength. Maximum amine adsorption is greater in the Ca2+systems than in the K+systems at equivalent cation charge and reflects the formation of an amine water bridge with the exchangeable Ca2+. Amine adsorption is also greater in chloride systems compared with sulfate systems at comparable cation concentrations, possibly due to the formation of aqueous anilinium-sulfate complexes. The amine compounds are retained mainly by bentonite through a cation exchange process, the capacity of the clay to adsorb the amine compounds being a significant percentage of the exchange capacity at the pKa. However, amine retention decreases with increasing pH and is minimal at solution pH values greater than 7

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

The chemical composition of organic matter was studied in the whole spodosol Bh horizon soil and in five particle-size fractions (clay, fine silt, medium silt, coarse silt, sand) by doing organic carbon (Corg) and total nitrogen (Nt) analyses and pyrolysis-field ionization mass spectrometry (Py-FIMS). Compared with the whole soil, the silt and clay fractions were increasingly enriched in Corgand Nt, with decreasing particle size, whereas the sand-size fraction was depleted in these elements. This distribution of soil organic matter (SOM) was significantly reflected by the proportions of volatilized matter (r= 0.998***) and the total ion intensities (r= 0.878**) per mg sample.Assuming a complete removal of adsorbed water, it was calculated that 85% of the organic matter in the whole soil, 40% each in silt and clay fractions, and 100% of the organic matter in the sand was volatilized during Py-FIMS. On the basis of these amounts of volatilized SOM, the following order could be computed for the proportions of assigned biomarkers of SOM in the samples: whole soil (48%) > sand (43%) > fine silt (26%) > medium silt (17%) = coarse silt (17%) > clay (14%). In the whole soil, silts, and clay, the accumulated organic matter showed characteristic signals of lipids, alkylaromatics, carbohydrates, phenols and lignin monomers, and lignin dimers. N-containing compounds were of lesser abundance. Relatively intensive signals of dehydroergosterol and ergosterol referred to soil fungi in the spodosol Bh-horizon. For the sand fraction, the divergent total ion intensity per mg corg, the lower average molecular weight of the pyrolysis products, and the Py-FI mass spectrum indicated a different composition of organic matter compared with the finer particle sizes. Thus, higher relative abundances of carbohydrates, phenols and lignin monomers and lower relative abundances of dimeric lignin units and N-containing compounds were observed. The high accumulation of SOM in clay, fine silt, and medium silt in combination with the thermal evolution profiles of the biomarkers in these fractions indicated a higher importance ofintramolecularSOM bonds compared withintermolecularorganomineral bonds for the stabilization of organic matter in the particle-size fractions <20 μm. With decreasing particle size, carbohydrates, phenols and lignin monomers, and N-containing compounds were stabilized increasingly in thermolabile bonds. Except for sand, more thermostable bonds were observed for lignin dimers and alkylaromatics in all particle fractions. The bimodal curves of lipids referred to two bond types for this biomarker in the organic matter of the spodosol Bh-horizon.

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

Allophane-free and -adsorbed humic acids were extracted from Ando soil with aqueous ammonia, and three kinds of fractions were sequentially isolated with 90% aqueous acetonitrile, 90% aqueous dioxane, and aqueous ammonia from the respective humic acids. The electron spin resonance spectra of each fraction was measured at pH 13.0 under argon atmosphere in the presence of various concentrations of Na2S2O4or K3[Fe(CN)6]. Spin concentrations of all the fractions increased as time elapsed after dissolution under argon. When air was bubbled into the solutions for 5 min, the concentrations were decreased by the oxidation with oxygen molecules. However, in time, the original signals were recovered. The spin concentrations were increased by the reduction with a small amount of dithionite, but decreased on increasing the concentration of dithionite. The spin concentrations were decreased by the oxidation with hexacyanoferrate(III), and new signals were observed with a large amount of hexacyanoferrate(III). When the amount of hexacyanoferrate(III) was not enough to oxidize all the electron donor moieties in the humic acids, the original signals were recovered over time. The data suggest that intramolecular or intermolecular electron transfer occurs in humic acids

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

Soils developed from mica schist in Eastern Nepal (The Himalayas) were investigated. All soil horizons were characterized by high organic carbon and extractable aluminum contents. Chemical and physical analyses indicated andic soil properties. No short-range order minerals could be detected. It is assumed that aluminum is linked to the organic matter, which leads to insolubilization and stabilization of the organic material against microbial degradation. The formation of Al-humus complexes instead of short-range order minerals is preferred if enough Al and water-soluble organic compounds are present. Al-humus complexes are considered responsible for the formation of Andisols from nonvolcanic parent material. The soil is classified as Dystric Haplustand, medial, mesic (US Soil Taxonomy)

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

Peds bounded by undulating surfaces having a luster or shininess constitute the subsoil of many major soil types worldwide. Characteristics of ped surfaces, such as presence or absence of luster, color, texture, and porosity, are often used as diagnostic criteria for clay coatings at a high category level in soil classification. There is, however, little agreement on either the formation or the physical nature of undulating lustrous ped surfaces. In particular, soil micromorphological studies have demonstrated that clay coatings apparent on ped faces during field examination are often not detectable when using soil thin sections and optical microscopy. Scanning electron microscopy of fractures across the undulating lustrous surfaces of peds from 30 layers of a wide variety of soil types (Alfisols, Mollisols, Vertisols, Ultisols and Oxisols) indicates that these peds were covered by depositional clay coatings. In 12 of the thirty soil layers, these clay coatings were

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

The accurate analysis of soil pore structures in soil thin sections depends on achieving sufficient contrast between soil pores and minerals. The objective of this research was to develop a method that requires a minimum number of crossed polarizer images to distinguish minerals from pores in soil thin sections. Five undisturbed soil samples were taken from each of the three soils used in the experiment. One thin section was made from each sample. Images of the thin sections were captured with a digital video camera through a microscope at X3.2 and saved for later analysis. Two crossed polarizer images at 0 and 45 degrees combined with a plane light image could distinguish more than 90% of the minerals. The combination of three crossed polarizer images at 0, 30, and 60 degrees was more efficient than Ismail's method, which used six crossed polarizer images

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID