摘要:

Experimental methods are needed to measure infiltrations at several locations simultaneously during rainfall or irrigation. The objective of this study was to test the feasibility of using a time domain reflectometry (TDR) probe installed vertically into the soil to track the propagation of the wetting front during infiltration. We used a numerical method to simulate wave traces. The dielectric constant above the wetting front and the probe characteristics were known. The trace simulation method was coupled to a nonlinear optimization program to fit the apparent lengths of the TDR probe above and below the wetting front and the dielectric constant of the soil below the wetting front. The optimization program employed a genetic algorithm. The progression of the wetting front into the soil was recorded as a function of the apparent length of the section of the TDR probe above the wetting front. Direct measurements of the wetting front advance were obtained from observations of infiltration in a clear acrylic cylinder packed with soil and ponded with water. The root mean square errors of the predicted wetting front depths did not exceed 0.4 cm. The method shows promise in estimating wetting front depth as a function of time.

ISSN:0038-075X    

出版商:OVID    

年代:2002

数据来源: OVID

摘要:

One of the most sensitive parameters in hydrological models, the saturated hydraulic conductivity (Ks), is also one of the most problematic measurements at field scale in regard to variability and uncertainty. The performance of a new type of simple and inexpensive field permeameter, the Philip-Dunne permeameter (PD), is compared with two established alternatives, the laboratory constant head permeameter (LP) and the field Guelph permeameter (GP). A PD prototype, a protocol of usage, and a numerical routine to findKswere developed and tested on a 70-point array laid out on an 850-m2volcanic soil plot. A power transformation was applied to the raw data using the three methods, and the transformed data were shown to be normally distributed. The LP and GP data were better described by a log-normal distribution, whereas the PD data could also be approximated with a power-normal distribution. A factor of 3 was found to relate PD, LP, and GP hydraulic conductivity estimates, E[Ks], such thatE[Ks-PD] ≌ 3E[Ks-LP];E[Ks-LP] ≌ 3E[Ks-GP]. Such differences may be explained by the different water infiltration geometries and sample wetted volume for the three methods. The PD has advantages over the other two methods in terms of personnel involved, preparation time, and ease of operation. Additionally, the PD methodology required a smaller number of samples (41% less than GP and 69% less than LP) to estimate the population meanKs. Both PD and GP also give the suction at the wetting front, an important parameter that characterizes the unsaturated flow properties of the soil.

ISSN:0038-075X    

出版商:OVID    

年代:2002

数据来源: OVID

摘要:

Hydrophobic organic chemicals labeled with14C are often used as tracers to study chemical fate and transport in soil. Wet oxidation is the recognized but labor-demanding method used most often to measure the concentration of radioactive organic tracers in the soil. In this study, we test an alternative, simpler method for quantification of14C-labeled hydrophobic organic chemicals in soil samples. The soil samples were extracted directly with scintillation fluid in glass scintillation vials (Scintillation Fluid Extraction, SFE method) and were subsequently analyzed by liquid scintillation counting. Application of internal standards showed no significant quenching or reduced counts of the14C-labeled chemical caused by the presence of settled soil particles in the scintillation vials. Hence, the scintillation fluid could be used simultaneously as both scintillation and extraction media. Decreasing the amount of soil containing14C-labeled chemicals added to the scintillation vial was shown to increase the extraction efficiency of the SFE method. Aging (contact time) and concentration were shown to affect the results of the SFE method. However, in a naphthalene diffusion experiment, where aging varied between almost no contact time and 20 days and concentration of naphthalene varied between ∼0 to 40 μg g−1, the estimated naphthalene diffusion coefficient was shown to be influenced only minimally when using the SFE method compared with the wet oxidation method. We emphasize that the SFE method is applicable only to sterile systems with no degradation or assimilation of14C-labeled compounds. If this condition is met and there is appropriate consideration of the effects of chemical aging and concentration, the SFE method seems to be a useful and labor-saving alternative to the traditional wet oxidation method for determination of the concentration of14C-labeled organic chemicals in soil samples.

ISSN:0038-075X    

出版商:OVID    

年代:2002

数据来源: OVID

摘要:

Once emitted to the atmosphere, nitric oxide (NO) and nitrous oxide (N2O) regulate several important processes in the troposphere and stratosphere. Understanding controls over N oxide gas emissions from fertilized soils is often complicated by highly heterogeneous and dynamic field conditions. An intact soil core method is described here for examining multiple subsurface controls concurrently with N oxide surface fluxes under hydrostatic and isothermal conditions dominated by Fickian diffusive transport. The method allows for simultaneous measurement of inorganic N levels, pH, water content, and soil-gas concentrations of NO, N2O, O2, and CO2and for subsequent calculation of N mass balances, net nitrification rates, CO2respiration rates, O2uptake rates, fraction of nitrified N lost as gas, and anaerobic microsite fraction profiles. The method is applied to moderately acidic soil fertilized with anhydrous ammonia. Steady nitrification and persistent NO2−levels resulted in NO gaseous losses representing 22–37% of the inorganic N initially present and >50% of the nitrified N. Of the initial N mass present, 96–119% was accounted for as N solutes and gases. High N2O soil-gas concentrations (5–20 mg N m−3) and fluxes (1 mg N m−2h−1) persisted despite anaerobic fractions of ≤ 10−10m3anaerobic soil m−3soil, suggesting that N2O sources other than denitrification were important.

ISSN:0038-075X    

出版商:OVID    

年代:2002

数据来源: OVID

摘要:

The development of process-based models describing N oxide trace gas emissions from agricultural soils will assist in the assessment of present sources, prediction of future trends, and development of mitigation strategies. We compare the predictions of a transport and transformation model with data from intact soil core experiments and then use the model to evaluate the relative importance of individual processes after application of anhydrous ammonia to moderately acidic soil. Soil-gas concentrations of NO and N2O are adequately described using previously obtained kinetic expressions. Net NO surface fluxes are shown to be the result of high rates of subsurface gross NO production (>100 mg N m−2h−1) balanced by gross consumption rates equivalent to 92–97% of gross production. The overall temperature dependency of NO emissions is described adequately as the net result of individual temperature-dependent processes. Denitrification was estimated to contribute only 8–14% of the total N2O source under primarily aerobic conditions, with the balance caused by abiotic nitrous acid decomposition. The bulk soil reactivity of NO2was estimated to be at least 500 times greater than the reactivity of NO. The potential for nonbiological nitrate formation is examined, and the effectiveness of pH control and subsurface fertilizer injection for minimizing gas losses are evaluated.

ISSN:0038-075X    

出版商:OVID    

年代:2002

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:2002

数据来源: OVID

摘要:

Java Island in Indonesia is dominated by volcanic ash soils. A study was conducted to characterize the soil’s physico-chemical properties in order to assess the influence of change of parent material on differing soil characteristics along an E-W sequence. All of the soils are Andisols of varying chemical composition. Soil pH and exchangeable Ca decreased from East to West Java, attributable to the parent ash becoming more acid from east to west. However, the Si, Al, and Fe extracted by acid-oxalate and the estimated amounts of allophane (11–35%) and ferrihydrite in the fine earth increased from east to west, showing that development of active forms of Al and Fe is favored by strongly leaching environments. This trend holds true for the point of zero net charge (PZNC) and for pHo. Samples rich in allophane showed relatively high PZNC. The relatively high pHovalues for the Central and West Java soils indicate development of net positive variable charges at pH < pHo. East Java and Kopeng soils are capable of holding more basic cations than the other soils at the actual soil pH. Therefore, these soils are less likely to be deficient in Ca and Mg under yearly application of acidifying nitrogenous fertilizers.

ISSN:0038-075X    

出版商:OVID    

年代:2002

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:2002

数据来源: OVID