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

摘要:

This paper presents a simple method for estimating the soil-water sorptivity (S) from vertical infiltration experiments at ponding conditions. The proposed estimation method is based on the Haverkamp et al. (R. Haverkamp, J.-Y. Parlange, J. L. Starr, G. Schmitz, and C. Fuentes. 1990. Infiltration under ponded conditions. 3. A predictive equation based on physical parameters. Soil Sci. 149:292–300.) predictive infiltration equation. This equation is simplified, so that a simple analytical solution is obtained for the sorptivity. The new S estimation method requires measurement of cumulative infiltration versus time and independent measurement of the saturated hydraulic conductivity on the same soil cores as used for the infiltration experiment. Criteria for avoiding numerical instability when using the new estimation method are given. Use of the proposed method is illustrated on infiltration data measured on undisturbed sand cores. The method gave good and robust estimates of the soil-water sorptivity (coefficient of variation > 25% for all S estimates). Horizontal and vertical infiltration tests for homogeneous packed soil columns showed good agreement between the new S estimation method and the classical Philip method for horizontal infiltration at nonponding conditions.

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

Reduction-dissolution kinetics and apparent activation energies (Ea) of reagent grade β-MnO2(Fisher Scientific, Fair Lawn, NJ), β-MnO2(Ward's Earth Science, Rochester, NY pyrolusite), and MnOOH (Ward's Earth Science, Rochester, NY manganite) were determined by continuous gasometric analysis employing an isothermal automanometric apparatus. This method was based on pseudo-first-order kinetics of oxygen gas (O2) evolution by reducing the Mn(IV) and/or Mn(III) to Mn(II). The manganese oxide reduction was carried out employing a solution mixture of 2.4% H2O2and 9.7% concentrated H2SO4on a volume basis. The pseudo-first-order constant (k) at 21°C was 0.66 min-1 for reagent grade β-MnO2, 1.68 min-1 for pyrolusite (β-MnO2), and 0.009 min-1 for manganite (MnOOH). The apparent activation Eawere determined to be 35.56 kJ/mol-1 for reagent grade β-MnO2, 29.95 kJ/mol-1 for pyrolusite (β-MnO2), and 44.75 kJ mol-1 for manganite (MnOOH). The most kinetically stable manganese oxide species of the three minerals tested was MnOOH (manganite).

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

We examined particle size fractions from the surface horizons of a native untilled grassland site and an adjacent field cultivated for 65 years in the Peace River region of British Columbia, Canada. While cultivation resulted in the loss of approximately 50% of the C and N in the soil, there was little change in the distribution of particle sizes, in their C/N ratios, and in the fraction of C found as carbohydrate. Changes in the nature of the organic C were generally small, but obvious differences were seen in the 13C CPMAS NMR spectra of two of the size fractions. Compared with the native grassland, the sand fraction from the cultivated field had a lower proportion of 0-alkyl C and a loss of resolution. The clay fraction from the cultivated field had lower 0-alkyl and higher alkyl C than its grassland counterpart. Subtraction of linear combinations of spectra of the whole soils from the two sites showed a 10% increase with cultivation in the proportion of humified to decomposable organic C. All of these trends are consistent with a greater degree of decomposition in the cultivated site. The changes are small and do not suggest a serious degradation in the quality of the soil organic matter, compared with the large change in quantity. We also examined factors that might affect the quality and quantitative reliability of the 13C CPMAS NMR spectra. Dithionite treatment was not very effective in improving resolution or fraction of observable C for these samples. The latter was in the range of 10–30%, largely a result of the association of organic matter with paramagnetic iron. For this reason, comparison of relative areas must be interpreted with caution and confined to samples with reasonably similar contents of C and iron.

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

There has been very little study of the P adsorption and buffer capacities of semiarid soils of northeast Brazil. The objectives of this study were to assess the effect of soil properties on P adsorption, buffer capacities, and requirements of some benchmark soil profiles. Phosphorus adsorption data of the soils were adequately described by the Toth and Redlich-Petersen isotherms. Phosphorus adsorption maxima varied from 117 to 350 mg kg-1. Averaged across the horizons of the profiles, adsorption capacities followed the order: Non-Calcic Brown > Lake sediment > Cambisol > Planosol > Lithosol > Alluvial. Adsorption capacity increased with soil depth in the Non-Calcic Brown, Planosol, and Cambisol, but decreased with depth in the Alluvial and Lake sediment. The soil P buffer capacities followed the trend of P adsorption capacity, and the external P requirements were between 3.4 and 109 mg kg-1 in comparison with 700 mg kg-1 reported for most Cerrado soils. The most important soil properties affecting P adsorption and buffer capacities were oxalate and dithionite Al, silicate Al, clay, and bicarbonate inorganic and organic P (HCO3-Pi and HCO3-Po) which explained 81 % of the variance in adsorption maxima, 70% of P affinity index, and between 66 and 90% of P buffering indices. The effect of silicate Al on P adsorption by the soils was attributed to ligand exchange with hydroxyl groups on the clay edges and the interaction with adsorbed cations in the Stern and diffuse double layer. It was, therefore, concluded that Al oxides played a more dominant role than Fe oxides on P adsorption and buffer capacities of these soils, even though dithionite-citrate extractable Al levels were three to ten times lower than dithionite-citrate Fe.

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

Ultramafic materials weather rapidly to maintain relatively high saturation with exchangeable Mg, and they have very high Fe and high Mn contents. These features retard the translocation of humus compounds, which is essential for podzolization. Three soils in a range of ultramafic materials were sampled to relate their morphology and spodic properties to parent material differences. Pedon 1 in residuum from weathered dunite is a coarseloamy, oxidic Dystric Cryochrept with no E horizon. Pedon 2 in ultramafic till with a cemented substratum is a loamy, serpentinitic, frigid, shallow Aquic Durochrept. It has E and Bs horizons, but the pH in the Bs horizon is too high (pH > 6) for spodic materials. Pedon 3 in mixed (partially ultramafic) till with compact substratum is a loamy, mixed, shallow Typic Cryaquod. It has an E and a spodic B horizon. The B horizons of all three pedons have low acid oxalate extractable Al (Alo) and very high iron (Feo) contents. They have low pyrophosphate-extractable to dithionite-extractable Fe ratios (Fep/Fed< 0.5) as a result of very high Fedcontents. The highest Feoand Fedin each pedon are 92 and 284 g/kg in the Bw horizon of Pedon 1, 98 and 84 g/kg in the Bs horizon of Pedon 2, and 34 and 38 g/kg in the Bhs horizon of Pedon 3. Feo/Fedratios > 1, as in the Bs horizon of Pedon 2 and in the C horizons of all three pedons, are due primarily to high magnetite contents in the soils. Weathering and leaching in the perhumid climate of southeast Alaska have not been sufficient to form a Podzol or a Spodosol in the soil on dunite. The free Mn and exchangeable Mg contents are lowered sufficiently to form a Podzol in the ultramafic till, but the pH remains too high for a Spodosol. A soil with Podzol morphology and Spodosol chemistry has developed in the mixed till.

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID

摘要:

A thick grayish to light olive brown (2.5Y 5/3, 7/3 dry) hardpan was found in till in a cold perhumid climate. It was sampled to determine the nature and conditions of cementation. The hardpan is very gravelly, has a mean particle density of 3.0, a bulk density of 2.6 Mg m-3, including pebbles, and a dry compressive strength about 20–50 MPa. Olivine, clinopyroxene, serpentine, and magnetite are the dominant minerals in the hardpan. Only serpentine was detected in the clay fraction. The nature of the cement is not discernible in thin-sections. Silicon, Mg, and Fe are the main elements in the cemented matrix. Dissolution studies indicated that basic (NaOH) and reducing (dithionite) solutions effected more disintegration of the hardpan than acid (Na-acetate, pH 5) solution. All of these solutions dissolved comparable amounts of Si, but NaOH solution dissolved little Fe. Therefore, the key element in the cement is probably Si. Na-acetate (pH 5) is more effective in dissolution of the ferromagnesian silicate minerals than in dissolution of the cement. Si-cemented hard-pans, or duripans, occur where weathering releases Si more rapidly than it is incorporated into secondary silicate minerals or is leached from the soil. These conditions are most extensive in seasonally dry soils with volcanic ash. Weathering may release Si from ultramafic materials rapidly too, because olivine and serpentine are more readily weathered than other common silicate minerals. The cement in the ultramafic till may be either silica or some combination of serpentine and silica.

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

ISSN:0038-075X    

出版商:OVID    

年代:1994

数据来源: OVID