ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID

摘要:

In Vertisols and vertic intergrades, spatial and temporal variability remains a challenge in water flow and chemical transport studies. Infiltration measurements were made with tension infiltrometers operating at supply potentials ≥ −0.24 m in 42 clay-textured horizons from seven Vertisols and three vertic Alfisols. Variability of near saturated infiltration was shown to be linked closely to soil macroporosity and moisture. Data revealed negatively sloped relationships when soil macroporosities were plotted against initial gravimetric soil water contents and when apparent steady-state infiltration rates were plotted against initial water contents. A positive linear relationship was observed between apparent steady-state infiltration rate at 0-m supply potential and soil macroporosity. Because of well developed structure and the occurrence of various types of macropores, most of the vertic soils had enhanced low-tension infiltration rates compared with less structured clay soils. The change in soil macroporosity in the shrink-swell clay soils when change in water content occurred had a greater impact on low-tension flow processes than the change in water content itself. Soil cracking, aggregation, biopores, and the time scale involved in shrink-swell processes were important factors that provided a physical understanding of infiltration behavior in the field.

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID

摘要:

The process of rainfall infiltration into crusted soil has been the focus of many analytical and numerical studies. However, most of these studies have been supported by laboratory data because no field data were available. Following recent field experiments on crusted soils, which provided crust conductivity values using the disc infiltrometer technique, a simple two-layer Green and Ampt model is proposed. This model is of interest because it uses only measured variables-saturated hydraulic conductivity (K), pressure head, initial and saturated volumetric water content, and microscopic capillary length-as input data. Runoff in the model is calculated as the difference between cumulative rainfall and infiltration. Because actual runoff data is available and no parameter calibration is required, the comparison with model output serves as a validation test for the input data. Close agreement between measured and calculated runoff is found when the surface crust is considered, which demonstrates the importance of surface crusting and increases the confidence in the measured K values. It also shows that K measured at the 1-m2scale remains a relevant parameter, even for the study of a process that involves a scale two orders of magnitude larger.

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID

摘要:

The potential for groundwater contamination at a given location is influenced strongly by the hydraulic properties (K) of the soils. The objective of this study was to investigate the effect of field cultivation in a no-till soil on the variability of infiltration rates, fractional porosity distribution, and soil hydraulic conductivity during the growing season. Infiltration rates were measured at different tensions using tension infiltrometers in two no-till plots with cultivation (cultivated) and two no-till plots without cultivation (uncultivated) at two different depths (soil surface and 0.15-m depth). Tension infiltrometer readings were taken at four soil water tensions (0-, 30-, 60-, and 90-mm) and at three different times during the growing season (July, August, and September).Results of this study showed that infiltration rates at 0-mm water tension were significantly larger than infiltration rates at the three other tensions for all plots. The data on infiltration rates show that under saturated flow conditions, macropore flow is a significant pathway for water infiltration. For the cultivated plots, 70 to 80% of the saturated flux at the soil surface occurred though macropores. At 0.15-m depth for the same plots, there was a decrease in macropore flow during the growing season(from 69% in July to 44% in September). However, in the uncultivated plots, macropore flow increased (by almost 52%) at the soil surface during the growing season. In the uncultivated plots, macroporosity increased with time at the soil surface. During the latter part of the growing season, macroporosity values for no-till plots with cultivation were very similar to those of no-till plots without cultivation. The effect of cultivation disappeared with time during the growing season. Although there was great variability in the saturated K values, K values at the surface remained constant in the cultivated plots at the beginning of the growing season and increased during the latter part of the growing season. In the plots without cultivation, there was an increase in the surface-saturated K values from July through September.

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID

摘要:

For soils with low plant-available phosphorous(P) content, the kinetics of P sorption can be described adequately using the kinetic Freundlich equation (Q=atmCn), wherea, m,andnare fitting parameters. When this equation is used for soils that already contain a large amount of plant-available P as a result of previous P additions, the problem arises as to how the initial P content,Q0,(Qatt= 0) can be taken into account. In this study, we tested the applicability of the equation to soils with a high level of plant-available P and several ways to includeQ0, which was estimated using nonlinear regression. Adsorption isotherms for three Dutch soils (sandy soil, loamy sand, and sandy loam soil) were determined after 1, 3, 7, and 21 days. We have shown that for these soils, a higher value ofQ0must be used for a longer equilibration time. A description of the increase ofQ0with time of the form:Q0=x tmappeared to be applicable for the three soils tested. This leads to an extended kinetic Freundlich equation of the form:QΔC= (a Cn−x)tm, whereQΔCrepresents adsorbed P, determined from the amount that has disappeared from the shaking solution, andxis an additional fitting parameter.

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID

摘要:

It is often reported that mineralization of less stable forms of organic phosphorus (Po) in Histosols contributes to the P load in drainage waters entering the Water Conservation Areas of the biologically sensitive Everglades wetlands. Consequently, a detailed evaluation of the degree of Postability in Histosols is required, and for this purpose, a Pochemical fractionation scheme was developed. The proposed scheme entailed an acid-alkali sequence of extractions with inclusion of a microbial biomass determination. Experiments conducted to evaluate the validity of the proposed scheme included an assessment of the hydrolysis of selected Pocompounds [p-nitrophenyl phosphate (PNP), D-glucose 6-phosphate (GLU), glycerophosphate (GLY), and inositol hexaphosphate -phytic acid (PA)] in the extractants proposed for the fractionation scheme (0.5MNaHCO3, 1MHCl, and 0.5MNaOH). Minimal hydrolysis (<6%) of all four Pocompounds was caused by NaHCO3extraction, and approximately 40% of the PNP and the GLU was hydrolyzed during extraction with 1MHCl and 0.5MNaOH, respectively. However, using the proposed scheme, PNP and GLU would have been removed as readily labile Poby the preceding NaHCO3extraction. Significant effects of various soils extraction conditions on extractable P are also reported and include acid pretreatment and soil:extractant ratio. The proposed scheme was applied to three selected Histosols representing land cultivated for sugar cane, uncultivated land under pasture, and agricultural land converted to a wetland. The distribution of readily labile, moderately labile, and nonlabile pools of organic and inorganic P in the Histosols is discussed in relation to soil and land-use characteristics.

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID

摘要:

The mobility and persistence of alginate-encapsulated controlled release (CR) formulations of atrazine [6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine -2,4-diamine] were compared with commercial formulations of atrazine in field studies on a Sharkey clay soil. Commercial and CR formulations were applied at a rate of 2.24 kg active ingredient ha−1to 30-m−2plots planted to sugarcane (second stubble). Three replications of each treatment were included in a randomized complete block design. Soil samples were taken at several depths by extraction with methanol, and the atrazine content was determined by enzyme immunoassay (EIA). A subset of the samples was analyzed by a gas chromatograph-electron capture detector (GC-ECD) for comparison purposes. The results indicated that CR formulations provided greater retention of atrazine in the soil surface horizon than was observed for the commercial liquid formulation. Although all formulations exhibited a decrease in surface concentrations by the last sampling period (4.5 months after application), CR formulations were slightly more persistent than the commercial formulation. Neither the commercial nor the controlled release formulation increased the amount of atrazine leached into subsoils. Enzyme immunoassay results correlated well with GC-ECD (r= 0.76***,n= 27;r= 0.85***,n= 26). The EIA method predicted slightly higher atrazine concentrations than the GC method and also gave false-positive results. It was hypothesized that atrazine metabolites were responsible for these effects. The EIA antibody used for this work is known to cross-react with these compounds. Despite the cross-reactivity observations, significant differences were still observed between atrazine formulations and sampling dates.

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID

摘要:

The approach used most commonly to prevent pyrite oxidation in the field is the use of limestone. This approach, however, has a short span of effectiveness because the pyrite particle surfaces remain exposed to the atmospheric O2after treatment. The objective of this study was to examine the feasibility of controlling pyrite oxidation in bench-scale studies by creating a ferric hydroxide-silica coating that would prevent either O2or Fe3+from oxidizing pyrite further. Ferric hydroxide-silica coating formation involved leaching pyrite at room temperature using a 10-mm-i.d. chromatographic column with a solution containing H2O2, Na-acetate (NaOAC), and soluble silicate at a flow rate of 0.43 mL min−1. The results of this bench-scale study show that formation of a ferric hydroxide-silica coating was induced on the pyrite surface and that it inhibited pyrite oxidation under acid conditions.

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID

摘要:

Soils with stable surface aggregates resist water and wind erosion better than soils with unstable aggregates. From earlier studies, we had preliminary evidence that one to three freeze-thaw cycles (FTCs) increased soil aggregate stability when measured by wet sieving field-moist aggregates. In this study, we measured the stability of aggregates vapor-wetted to field capacity from the Ap horizons of four soils after undergoing either zero, one, two, or four FTCs, and we determined the number of FTCs at which aggregate stability would be greatest for each soil. Moist soil was packed to a dry bulk density of 1.15 Mg m−3by tapping it into 28-mm-diameter, 50-mm-tall brass cylinders. Each cylinder was then sealed in a polyethylene bag and inserted into a polystyrene foam tray. The soil in each cylinder was frozen convectively at −5°C for 48 h and then thawed at +6°C for 48 h for each FTC. Aggregate stability increased with the first one to two FTCs but changed little thereafter. Trend analysis revealed that aggregate stability would be greatest after two or three FTCs. When averaged across the four soils, FTCs stabilized aggregates more at 0 to 15 mm than at 15 to 30 mm. Near the surface of wet soils, two or three FTCs may be beneficial rather than detrimental to soil structure.

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID

摘要:

Iron and Mn redistribute in soil and saprolite during weathering. The geological weathering fronts of calcareous sedimentary rock were investigated by examining the bulk density, porosity, and distribution of Ca, Fe, and Mn. Core samples were taken of soil, saprolite, and bedrock material from both summit (HHMS-4B) and sideslope (HHMS-5A) positions on an interbedded Nolichucky shale and Maryville limestone landform in Solid Waste Storage Area 6 (SWSA-6). This is a low-level radioactive solids waste disposal site on the Dept. of Energy (DOE) Oak Ridge Reservation in Roane County Tennessee. This work was initiated because data about the properties of highly weathered sedimentary rock on this site were limited. The core samples were analyzed for pH, calcium carbonate equivalence (CCE), hydroxylamine-extractable (HA) Mn, and dithionite-citrate (CBD)-extractable Fe and Mn. Low pH values occurred from the soil surface down to the depth of the oxidized and leached saprolite in both cores. The CCE and HA-extractable Mn results were also influenced by the weathering that has occurred in these zones. Extractable Mn oxide was higher at a lower depth in the oxidized and leached saprolite compared with the Fe oxide, which was higher in the overlying soil solum. Amounts of Mn oxides were higher in the sideslope core (HHMS-5A) than in the summit core (HHMS-4B). Iron was more abundant in the deeper weathered summit core, but the highest value, 39.4 g kg−1, was found at 1.8 to 2.4 m in the sideslope core. The zone encompassing the oxidized and partially leached saprolite down to the unoxidized and unleached bedrock had higher densities and larger quantities of CaCO3than the soil solum and oxidized and leached saprolite. The overlying soil and oxidized and leached saprolite had lower pH and CCE values and were higher in Fe and Mn oxides than the oxidized and unleached saprolite. The distribution of Fe and Mn is important when evaluating soil and saprolite for hazardous waste disposal site assessment.

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID