摘要:

Most numerical calculation schemes are either unstable or fairly complicated to solve and program when used for calculating water and solute transport in unsaturated soils. A solute transport model that is both numerically stable and easy to program will represent a useful alternative to existing models. We present an accurate and easily programmed numerical model for one-dimensional transport of solutes in unsaturated or saturated soils at steady or transient water flow. The new approach is based on the classical convection-dispersion concept of solute transport. The model is labeled the moving concentration slope (MCS) model because it uses the slope of the curve representing the natural log of the solute concentration (c) times the ratio of the hydrodynamic dispersion (D) to the water flux (v) plotted versus the convective solute flux (vc) as a governing, time-dependent parameter. The MCS model is a modification of the recently presented moving mean slope (MMS) water flow model and was developed using the mathematical equivalence between the basic flux and continuity equations for water and solute transport. In the MCS model, an integrated version of the solute flux equation is used together with a simple, forward-time discretization of the continuity equation to calculate solute transport. In case of a depth increment larger than 1 cm and a ratio ofDto the pore water velocity (u) smaller than 2, it is necessary to correct the MCS model for numerical errors of second order (numerical dispersion), but the corrections are easy to make. Analytical equations for the second and third order numerical errors inherent in the MCS model were derived using Taylor series and validated using method of moments analysis. The magnitude of numerical errors inherent in the MCS model is generally small compared to explicit finite difference (FD) models. Also, the MCS model is very simple to program compared to finite element (FE) and implicit FD models. The MCS model was validated against analytical solutions in the case of steady water flow and against FE models in the case of transient water flow. The MCS solute transport model used together with the MMS water flow model represents a convenient tool for easy and accurate modeling of one-dimensional transport of water and solutes in soils.

ISSN:0038-075X    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

It is generally believed thatDe/D0, which is the ratio of the gas diffusion coefficient in soils to that in free air, does not depend on the kind of diffusing gases if the gases do not react with the soil. This hypothesis was examined using N2, CH4, and H2gases. Obtained values ofDe/D0were almost the same for these three gases. Next, the relationship betweenDe.D0andnawas investigated experimentally in dry and wet sands, wherenais the air porosity. The sand samples were prepared by compaction. The percentage of fines contained in the sand was adjusted in the range from 0% to 50% by weight. The experimental results and the data from the literature show that the relationship in dry sand is described by the equationDe/D0=naμ. The constant μ is dependent on the percentage of fines. The relationship in wet sand is more appropriately described by the equationDe/D0= α(na− β) in the air porosity range less than 0.3 where α and β are constants.

ISSN:0038-075X    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

We present a model for predicting soil temperatures in arid regions using current above surface conditions (temperature or temperature and solar radiation) and the annual sinusoidal behavior of temperature in the deeper soil profile. The model is based on that of Parton (Parton, W. J. 1984. Soil Sci. 139:93–101) and provides considerable temperature detail with a daily time step: maximum, minimum, and mean temperature at any depth; hourly temperature detail if desired. The error in predicted temperatures is about the same as those in models that require soil temperature inputs from at least one depth. This model, however, can be used for predicting soil temperatures in arid sites for which there are no soil temperature data or for predicting soil temperatures under future climates or environmental conditions.

ISSN:0038-075X    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

Activated carbon can be used to trap volatile selenium (Se) in long-term field experiments. Volatile Se is principally in the form of dimethyl selenide (DMSe). An extraction method was developed to oxidize alkylated Se compounds to inorganic selenate form with alkaline H2O2and wash out from the charcoal filters.Alkaline H2O2and water were compared as extractants in shaker and drip methods. Water extracted only 14.9% and 48.8% in shaker and drip methods, respectively in three consecutive extractions from a known quantity of DMSe. Both peroxide only and peroxide/water treatments extracted an average of 90% DMSe in shaker and drip methods. Therefore, it is concluded that using the drip method with alkaline H2O2, at least for the first extraction, is superior to extracting with water alone. To expedite the extraction procedure, all filters from each chamber were extracted together and tested with two different flow rates. It was found that the slow flow rate of 2.2 ml min−1for a syringe of capacity 60 ml with an area of cross section of 5.7 cm2extracted nearly 96% of DMSe and was slightly better than the flow rate of 3.3 ml min−1.

ISSN:0038-075X    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

Size fractions (2000–20 μm, 20–2 μm, 2–0.2 μm, and 0.2–0 μm) were obtained after Na-resin treatment of five top horizons from low-activity clay soils. Their specific surface areas (SSA) were measured by the N2-BET method. The same method was applied after organic matter removal by H2O2treatment to two of the bulk soils and their size fractions. The weighted sum of the SSA of the fractions fitted well to the SSA of the bulk soils, indicating that the association of particles, destroyed by the fractionation procedure had no effect on the SSA. The SSA of the coarsest fractions were greater than theoretical values from a sphere model, because of the presence of microaggregates. A good relationship was found between the SSA of the soils and subfractions and their total contents in R2O3(R2O3= Al2O3+ Fe2O3), i.e., an estimation of the importance of the secondary minerals present. Organic matter removal from two of the soil samples and their subfractions resulted in systematical but weak increase in SSA (≤ 8 m2·g−1). This increase was not correlated to the amount of C removed from the samples. It was mainly due to changes in SSA occurring in the clay fractions where amorphous organic matter was assumed to act as a glue binding some clay plates together.

ISSN:0038-075X    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

Deformation-stress-moisture relationships of cultivated sphagnum peat materials were studied in the laboratory using the Ottawa Texture Measuring System, an apparatus developed by Agriculture Canada. Impact of compaction on hydrologic characteristics was also investigated to determine the maximum stress which may be applied in the field during seed-bed preparation without limiting aeration for crop production. The study revealed that the compactibility of peat is relatively unaffected by moisture content at compactive pressure ≤13 kPa. However, under higher compactive pressure, the peat reaction was very similar to that of mineral soils, in which the compactibility increases with increasing moisture content until it reaches a maximum bulk density at saturation. The deformation-stress-moisture content relationships may be expressed by a simple third order polynomial equation. In response to compaction, the saturated hydraulic conductivity of the peat material reduced more than three orders of magnitude as bulk density increased from 0.1 to 0.25 g/cm3. A major increase in moisture retentivity was found in the compacted peat. Changes in hydrologic characteristics are primarily attributed to the reduction in large pores. Based on this information, it is concluded that compactive pressure exceeding 50 kPa may cause adverse effects on soil aeration.

ISSN:0038-075X    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

Corn (Zea maysL.) production practices have significant effects on soil organic nitrogen (N) and carbon (C) levels. The effects of plant population, irrigation, and fertilization rate on soil organic C, total N, and organic N fractions after 3 and 6 years of continuous corn on a Chicot sandy clay loam soil (fine-loamy, mixed, frigid Typic Hapludalf) were studied.Soil organic C content increased by 7% after 3 years of continuous corn and by 18% after 6 years of continuous corn. The C/N ratio increased after the first 3-year period. No change in C/N ratio was found after the second 3-year period. The greater rate of fertilization (400 kg N ha−1, 132 kg P ha−1, 332 kg K ha−1) compared to the normal rate of fertilization (170 kg N ha−1, 44 kg P ha−1, 141 kg K ha−1) had a greater level of soil total N over 3 years. This increased soil total N was still found after 6 years of corn production.After 3 years of corn production, the relative amount of hydrolyzable NH4+-N and amino acid-N in acid hydrolyzates of soil organic N decreased. High rates of fertilization reduced the relative amounts of total hydrolyzable-N and unidentified-N and increased non-hydrolyzable-N compared with the normal fertilizer rate.

ISSN:0038-075X    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

The variability of soil sampling and surface nutrient loading was investigated at Okeechobee, FL, on a Myakka fine sand (sandy, siliceous, hyperthermic Aeric Haplaquod) and Immokalee fine sand (sandy, siliceous, hyperthermic Arenic Haplaquod), where both sites were used as dairy animal holding areas. Soil pH and nutrient variability of P, K, Ca, Al, Fe, and Na were high across both sites. Soil nutrient variability was dependent on localized animal intensity and surface topography. Accumulation of total soil P in surface soils was related to the level of organic matter loading, but was also associated with other soil characteristics that influence the release of P such as total Al, Ca, and soil pH. The number of soil composite cores within animal holding areas necessary to obtain a specified confidence in soil test and laboratory results was determined. Depending upon the soil parameter measured, the number of samples required to obtain a high level of confidence (α = 0.10) with a 10% relative error in results ranged from 1 to 336 samples. Under guidelines generally recommended and used for soil sampling (≤20 samples per area), high relative errors (>50%) can be expected from soil test results due to high variability in diary animal holding areas. The soil testing methodology used in this study to generate point sources of data reflected the surface accumulation of selected nutrients in soil. But due to high spatial variability among point sources, traditional methods of sampling may not be of practical value for monitoring or determining accurate levels of nutrient loading.

ISSN:0038-075X    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

The influence of initial water content upon soil compaction by tractor traffic was investigated in a fine-loamy, siliceous, thermic Plinthic Paleudult soil. A fallow field area was thoroughly wetted using sprinkler irrigation, and the soil surface was exposed to tractor passes 1, 3, and 9 days thereafter in order to impose different soil-water contents. Distributions of penetrometer resistance (PR) in the top 0.5 m of the soil profile were determined 11 days after irrigation when the soil was relatively dry. In the upper 0.1 m of the profile, the mean PR for the compacted soil (1.6 MPa) was significantly greater than for the control plots (1.4 MPa). PR data from the control soil clearly indicated the initial presence of a traffic pan in the soil horizon between 0.1 and 0.2 m.Mean field-saturated hydraulic conductivity (K0) determined with a constanthead permeameter for the compacted soil (4.5 × 10−6m s−1) was significantly (P< 0.05) less than the control plots (13.9 × 10−6m s−1). Water release curves for control and compacted soils were distinct at 0.1 m depth but not at 0.3 m. Control and compacted soils were not significantly distinct for bulk density and macroporosity in this sandy soil. Thus, under a moderate level of compaction, the effects were primarily restricted to the upper levels of the soil and did not appreciably affect soil bulk density, even in the topsoil.

ISSN:0038-075X    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

Because Cd, Cu, Ni, Pb, and Zn coexist in soil solution, competitive adsorption isotherms of these metals were generated for two soils, each selected under continuous corn from different long-term cropping systems. Competitive adsorption of the metals as a function of pH was evaluated by equilibrating soil with 200, 1,000, or 2,000 μMeach of Cd, Cu, Ni, Pb, and Zn perchlorate in 10 mMCa(CIO4)2after adjusting the pH with NaOH or HCIO4solution to range from pH 4 to 8. In general, metal affinity by both soils was: Pb > Cu > Ni ≥ Cd = Zn. Competition among metals for exchange sites was enhanced as the initial metal concentration was increased. The metal affinity sequence of the soils was compared with the predicted affinity sequences obtained from metal properties. Poor agreement was found between observed metal affinity sequence and metal affinity sequence predicted by ionic potential, indicating that bonding of metals by these soils was not predominantly electrostatic. The observed affinity of Pb and Cu for the soils was consistent with that based on metal hydrolysis, but the affinity sequence of Cd, Ni, and Zn could not be explained by metal hydrolysis. With the exception of Cd, affinity sequence based on the hard and soft acids and bases theory of Pearson agreed with the metal affinity sequence for these soils. Comparison of results in this work with previous findings suggested that metal adsorption by these soils could be described by: (i) Adsorption of Cu as CuOH+and Pb as PbOH+and/or (ii) replacement of indigenous exchangeable cations (Ca, Mg) by transition- and heavy-metal cations.

ISSN:0038-075X    

出版商:OVID    

年代:1992

数据来源: OVID