摘要:

Predictive models for the solute diffusion coefficient, DS, dependency on volumetric soil-water content, &thetas;, are often applied in simulations of solute transport and fate in natural, undisturbed soils. However, all available DS(&thetas;) models have been developed from measurements on sieved, repacked soil. In this study, DSfor chloride was measured in both repacked and undisturbed loamy sands at different soil-water contents. The measurements on undisturbed soil were carried out using a modified half-cell method, where the source half-cell is a sieved and repacked soil core and the other half-cell is an undisturbed soil core. Thus, the problems of (i) incomplete contact area at the interface between undisturbed half-cells and (ii) potentially different diffusion properties in undisturbed half-cells can be avoided. The modified half-cell method requires that the diffusion coefficient in sieved, repacked soil is determined separately and that the experimental data is analyzed with a numerical solution to the diffusion equation. No significant difference in chloride DS(&thetas;) between undisturbed and sieved, repacked soil was observed for a Danish (Foulum) loamy sand and a Japanese (Hiroshima) loamy sand. A recently presented soil type dependent DS(&thetas;) model, derived from repacked soil data, shows it to be applicable also for predicting solute diffusion coefficients in natural, undisturbed soils.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

A novel, automated, miscible displacement system has been developed to study the fate and transport of volatile chemicals within soil. This system presents advantages over existing systems that utilize fraction collection, head space analysis, or single flow-through UV detectors because there is no volatilization, there is high chemical resolution, and there is simultaneous detection of multiple chemicals. This paper presents the design of the system and demonstrates its capabilities. The system's flow rate and chemical concentration detection stability were tested through time, as was its ability to detect multiple compounds. For a chemical detection stability test of 2.7 days, the pump was able to supply 10 mg L−1of toluene and measure its absorbency every 10 min with a coefficient of variation (CV) of 1.55%. The CV of the pumping rate was less than 1% for continuous flow over a 9-day period of time, during which the pressure more than tripled. A toluene breakthrough curve through an Ottawa sand resulted in 99.99% mass recovery of toluene, indicating no volatilization or sorption to the equipment. Finally, the system's ability to measure multiple compounds was demonstrated with simultaneous breakthrough curves of three chlorinated hydrocarbons through sand. This system makes possible a wide variety of experiments that previously could not be performed with acceptable precision on volatile organic compounds.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

To predict reversibility of soil and water acidification under conditions of decreasing S-input into forested ecosystems, the amount of reversibly bound SO2−4in the soil as well as its desorption behavior must be considered. This study investigated variability of sulfate isotherms and their spatial scaling.The Langmuir isotherm parameters in two forested catchments (one deciduous, one coniferous stand) and their spatial distribution at two different scales (site and catchment scale) were investigated. The soil samples (250 cm3) were taken in a systematic grid with spatial distances ranging from 20 × 20 m to 300 × 300 m. Isotherm parameters, soil pH, dithionite- and oxalate-extractable iron and aluminum, crystalline iron oxide, and Corgcontent were determined.Significant relationships were found between sulfate sorption isotherm parameters and soil chemical properties for each site. It would be useful to be able to replace the experimental determination of isotherm parameters with the less costly measurements of soil chemical properties such as pH, Al- and Fe-oxides, and Corgcontent of the soils. Regression analysis, however, resulted in different parameters for each site. Thus, at sites where this relation is unknown, isotherms have to be determined via soil extraction procedures and cannot be predicted by soil chemical properties. To determine the average isotherms with an accuracy of ± 0.2 μmol g−1SO2−4 sorb, a sample size of 55 for the site scale and up to 106 for the catchment scale is required.The semivariance of the isotherm parameters revealed no spatial patterns. The influence of isotherm variability on the prediction of SO2−4- fluxes with seepage was investigated using the chemical equilibrium model MAGIC. Forecast accuracy depends on proper consideration of the variability of sulfate sorption in a catchment.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

In soils containing vermiculite, fixed NH+4(Fix NH+4) levels are affected by exchange reactions with other cations, which may directly impact both NH+4availability to nitrifiers and mobility through the soil. Some previous soil suspension studies have shown that in certain soils, K+is highly effective at displacing adsorbed NH+4, whereas in other studies, K+induced high affinity NH+4adsorption in soils exhibiting vermiculitic behavior. The purpose of this study was to demonstrate the influence of K+on NH+4mobility in soils in a heterovalent ternary cation exchange system (K-NH4-Ca). For this study, we selected two Kentucky soils of mixed mineralogy with varying quantities of vermiculite: a Maury silt loam (fine, mixed, mesic Typic Paleudalfs) topsoil (0-6 cm) and an Eden clay loam (fine, mixed, mesic, Typic Hapludalfs) subsoil (15-30 cm). Ammonium breakthrough curves (BTC) were obtained by leaching packed soil columns with either 5 mM NH+4plus 1 mM Ca2+solution or 5 mM NH+4, 5 mM K+, plus 1 mM Ca2+solution. Ammonium BTC with and without added K+for both soils indicated, as expected, that NH+4movement through the column was impeded by soil retention. For the Maury soil in the presence of K+, the NH+4BTC appeared approximately 1 pore volume to the left of the NH+4curve in the absence of K+, signifying that K+competed with NH+4for soil surface exchange sites, whereas for the Eden soil in the presence of K+, the NH+4BTC appeared approximately 5 pore volumes to the left of the NH+4curve without K+. This signified an apparently strong competitive effect by K+, with respect to NH+4, for soil surface exchange sites. However, extraction of soil subsamples from the columns revealed that adding K+to the Maury soil reduced ExNH4but did not change Fix NH4levels, whereas for the Eden soil, adding K+reduced ExNH4and greatly increased Fix NH+4. FTIR analysis of the soils revealed NH+4deformation bands at 1454 and 1430 cm−1, suggesting that NH+4was held in the Maury and Eden soils in two chemically distinct binding environments, possibly representing two different NH+4sinks, i.e., exchangeable and fixed. Furthermore, this distortion of the tetrahedral molecule, evidenced by the shifts of the NH+4deformation bands to wavenumbers beyond 1399 cm−1for free or uncomplexed NH+4, led us to propose that the vermiculite interlayer was more stereospecific for the NH+4ion than for the physically and chemically distinct spherical K+ion. That the IR spectra were identical for both soils in the absence and presence of added K+implied agreement with the extraction data that K+was not able to affect interlayer NH+4complexation directly. We propose that K+fixation collapsed the interlayer around NH+4ions, thus seeming to induce NH+4fixation.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

This study examines the relationship of soil solution composition, water table, and redox potential (Eh) in three rice-growing Ultisols of the Chungli Terrace toposequence in northern Taiwan. The three soil series were Houhu (Typic Plinthaquult), Hsinwu (Typic Plinthaquult), and Lungchung (Plinthaquic Paleudult). Water tables and redox potentials were monitored in 1996 and 1997. Soil solutions were collected in 1997 at different depths by the tension free method. Redoximorphic features were consistently found in the Btv horizons of the three pedons. Sodium, calcium, and sulfate were the dominant ions in soil solutions at different depths. Decreased concentrations of Na, Mg, Mn, and Cl were associated with increasing elevation in this toposequence, and this trend was then related to the soil moisture regime and the distance from the sea. There was much more Mn than Fe in the soil solution. The low concentration of Fe in the soil solution was the result of low Fe activity ratio (Fe0/Fed) associated with low organic carbon content in the soils; it was also caused by dilution of the irrigation water utilized for rice production. Manganese is more highly correlated than Fe with the concentration in the soil solution and the redox regime.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Changes in soil properties caused by forest-to-pasture conversion and subsequent livestock grazing may contribute to the observed slow rates of tropical forest regeneration in abandoned pastures. As part of a larger study of forest regeneration in abandoned premontane tropical pastures in southern Costa Rica, we studied soil phosphorus characteristics and other soil chemical properties in pastures and adjacent forests to determine if conversion to pasture had measurable effects. We sampled soil at 30 points across five sites in ridge and slope topographic positions and at 25 m into the forest, the forest-pasture edge, and 50 m into the pasture. We quantified plant-available phosphorus, soil phosphorus retention capacity, P fixation rates, water pH, NaF pH, organic carbon, organic nitrogen, and effective cation exchange capacity (ECEC). Although anion resin strips were used to determine plant-available phosphorus, P levels extracted from the resins were below instrumental detection limits. There was a trend of increasing pH and P retention capacity with change from forest to pasture cover, but there were no significant differences between topographic positions or between forest and pasture for any measured soil properties. No differences in soil chemical characteristics across forest-pasture interfaces indicate that fundamental soil properties in the upper 15 cm of pasture soils have not changed since conversion from forest and, therefore, that the soil chemical properties we measured do not hinder recruitment and survival of forest species in abandoned pastures.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Soil water evaporation (E) measured by small, weighable lysimeters is affected by their size, construction materials, and replacement frequency. Steel and plastic are commonly used materials, but they have different thermal characteristics. Our objective was to investigate how wall material affects E and soil temperature of small lysimeters filled daily with undisturbed soil. Research was conducted during 5 days in August 1995 at Bushland, Texas, where the soil is a Pullman silty clay loam (fine, mixed, thermic Torrertic Paleustoll, 30% clay, 53% silt). Plastic and steel lysimeters 76 mm long, with inside diameters of 82 and 86 mm, respectively, were filled each morning by pressing them into undisturbed soil, and E was determined by mass change. Soil temperatures inside additional steel and plastic lysimeters were measured by thermocouples. No significant differences in E due to wall material were measured. For lysimeters of both wall materials, daily E ranged from 2 to 5 mm, daytime and nighttime E averaged 2.7 mm and 0.5 mm, respectively, and total cumulative E was 15.5 mm. Evaporation from small lysimeters was within 5% of E from a nearby large, precision, weighing lysimeter. Steel lysimeters were warmer at night near the surface, with significant differences from 0.5 to 0.9 °C, and warmer during the day at the bottom, with significant differences from 0.5 to 2.8 °C. Plastic lysimeters had greater vertical soil temperature differences than steel lysimeters. Significant differences inside plastic lysimeters ranged from about 2.0 to 3.5 °C greater than those of steel lysimeters during the daytime, and 0.5 to 1.0 °C greater during the nighttime. Measured temperature differences were consistent with greater thermal conductivity and enhanced heat transfer in steel sidewalls compared with plastic. Wall material affected temperature distribution, but not evaporation, of small lysimeters that were replaced daily.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Electricity generation from biomass is becoming a more environmentally sound option than nonrenewable fuels. Pilot studies have demonstrated that alfalfa (Medicago sativaL.) stems are a suitable feedstock for energy generation via gasification. Developing beneficial uses for ash produced as a byproduct will enhance the economic viability of bioenergy. A greenhouse study was conducted to evaluate the potential use of alfalfa ash as a fertilizer and/or liming agent using corn (Zea maysL.). Two soils, a Hubbard sand (low pH, low K, and high P), and a Barnes clay loam (high pH, high K, and low P) were used. Treatments included a control, K and/or P fertilizers, and six ash rates ranging from 0.2 to 6.4 g kg−1(450 to 15,000 kg ha−1). Ash application significantly increased K and decreased Mg concentrations in corn. However, neither plant P nor trace elements were affected by ash application. Elemental uptake by plants paralleled their respective concentrations. Soil pH increased linearly in the Hubbard soil with ash but was not affected in the Barnes soil. Ash application significantly increased salinity, Cl, extractable P, exchangeable K, Ca, Mg, and Na in both soils. Ash from gasified alfalfa stems seems to be a potential source of K and a potential liming agent for acid soils. When used at agronomically reasonable rates, alfalfa ash did not pose any risk of excessive accumulation of metals tested in soil or plants.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID