ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Increasing agronomic productivity and improving the quality of the environment are among the important goals of soil physical management in the tropics. Several perceptions, created by insufficient scientific data and misunderstanding of the basic processes, are being resolved by an improved data base and better understanding of the dynamics of soil physical properties and processes. Principal issues in the tropics for the 21st century include: (i) achieving food security; (ii) curtailing soil degradation and restoring degraded soils; and (iii) improving environment quality. These issues can be addressed by identifying and prioritizing research needs in soil physical management. Important among these are: (i) assessing soil physical constraints at farm scale; (ii) managing soil structure and tilth to minimize the risks of crusting, compaction, and hard-setting; (iii) quantifying and controlling soil erosion by water and wind, developing a cause-effect relationship between soil and climate factors, and evaluating the effects on productivity; (iv) managing soil-water, controlling soil salinity in irrigated agriculture, and developing water harvesting techniques in rain-fed agricultural systems; (v) studying the dynamics of soil physical properties in puddled soils of rice-based cropping systems; (vi) developing conservation tillage and residue management methods to improve soil tilth; (vii) understanding soil moisture retention characteristics in relation to plant available water capacity; and (viii) developing indicators of soil physical quality. Important environmental issues relevant to soil physical management are transport of agricultural chemicals into surface and ground waters, emission of greenhouse gases from soils to the atmosphere, and disposal of urban and industrial wastes. Soil physical management in the tropics must be based on a holistic approach to solve practical problems. It is also important to make the public aware of the contributions of soil science to society's well-being. In addition to enhancing food production, soil physical management needs to address environmental, engineering, social, legal, and archeological issues. Achieving these goals necessitates soil scientists working in close collaboration with engineers, climatologists, geologists, biologists, and specialists in GIS and geostatistics.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Soil heat and mass transfer are important processes in nature The effects of soil solution concentration on soil heat and mass transfer have not been investigated thoroughly for freezing/thawing conditions. The objective of this study was to evaluate the SHAW model predictions of heat, water, and chemical transfer in saline and solute-free soils. Heat and mass transfer was studied in closed soil columns. Included were loam soil materials with two salinity levels. Three derivatives of benzoic acid were used as tracers for liquid water movement. Soil was packed into PVC columns that were buried vertically in a field soil profile and exposed to ambient weather conditions. After 72 days, the soil columns were removed from the field and sectioned into 0.02-m increments in order to measure water content, chloride, and benzoic acid tracer distributions. In addition to the observed heat and mass transfer, the SHAW model was used to predict heat and mass transfer in the soil columns. The model described the temperature distributions accurately for both salinized and solute-free soils. Both observed and predicted values of water distribution showed water accumulation in the upper 0.4 m in the solute-free soil. The water diminished slightly in the upper 0.20-m region in the salinized soil columns. Within the salinized soil columns, the final solute distribution did not indicate appreciable net solute movement. The observed and predicted frost depths in the solute-free soil were between 0.45 m and 0.65 m. Freezing was not significant in the salinized soil. The SHAW model should be improved if it is expanded to include the effects of temperature and osmotic potentials on liquid water flow in soil.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Sorption of organic vapors on soil increases dramatically as soil-water content decreases in a dry region. Equations for calculating organic vapor partition coefficients in unsaturated soils as a function of soil-water content are proposed. The equations were based on the hypothesis that organic vapor in soils are found adsorbed onto water-solid and air-solid interfaces and dissolved in the soil solution. In the dry range, where water in soils can be considered the sorbate, water vapor and organic vapor compete for sorption sites with water vapor adsorbed preferentially at the air-solid interfaces because of the higher polarity of water molecules. The air-solid interfaces that are not covered by water molecules and are available for sorption of organic vapor can be estimated according to the Brunauer-Emmett-Teller adsorption theory. The predictions made by the proposed equations were compared with partition coefficients of three volatile organic compounds (VOCs)-benzene, toluene, and trichloroethylene-and a pesticide, diazinon (O,OdiethylO-(2-isopropyl-4-methyl-6-pyrimidinyl) phosphorothioate) in Yolo silt loam, and with partition coefficients of the same three VOCs for two clay minerals. The measured and predicted partition coefficients agreed reasonably well. All parameters of the proposed equations are measurable, and no curve-fitting is needed.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

An alternative procedure for calculating root water extraction from different depths of a soil profile is proposed. The procedure is based on the concept that the root water extraction entails energy expenditure by the plant and that the plant seeks to minimize the total rate of energy expenditure during water uptake. The model, therefore, considers root water extraction to be a minimization problem whose solution can be sought using a dynamic programming framework. We tested the model by simulating the variations of the soil water content using the time and depth of a maize-sorghum intercrop experiment reported for an 8-day drying cycle by Ozier-Lafontaine et al. (Plant and Soil 204:183-201, 1998). Simulated patterns follow the observed water content distribution quite well. Furthermore, results from numerical experiments show that the model is capable of simulating a range of water extraction patterns in a realistic manner. Patterns of water extraction from uniformly wet soil profiles follow those of the root distribution with depth. The extraction rate is highest in the section where the root length density is also highest. Once the soil profile dries out, water extraction patterns cease to bear any similarity to root distribution. Model simulations also show increased root activity at greater depths when the top sections of the soil dry out. Generally, the model avoids the need to make any prior assumptions about the pattern of the root water extraction.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Soil microbial denitrification is a significant source of atmospheric nitrous oxide (N2O), a trace gas important in global climate change and stratospheric ozone depletion. In this paper we describe a mechanistic submodel, which is incorporated in the model NLOSS, designed to predict the soil biogenic source and efflux of N2O and N2during denitrification. NLOSS simulates transient soil moisture and temperature, decomposition, soil anaerobicity, denitrifying bacterial biomass, rates of soil nitrogen transformations, soil trace-gas transport, and gas efflux to the atmosphere. Uncertainty in predicted N gas effluxes is computed using a Monte Carlo approach. We test NLOSS's denitrification estimates by comparing predictions with results from a15N tracer experiment in a Mexican agricultural system. The model accurately predicted the measured soil moisture and denitrified N2O and N2fluxes during the experiment. We also apply NLOSS to compute denitrified N trace-gas speciation curves as a function of soil hydrologic properties and moisture content. These speciation curves will be used in future work to extrapolate the plot-scale modeling results presented here to field and regional estimates of N trace-gas emissions. The results presented here suggest that NLOSS can be used to identify the processes most important for trace-gas losses and to facilitate efforts to scale plot-level modeling results to regional estimates of N trace-gas emissions.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

The effect of organic matter (pig manure,Sus scrofa) addition on solubility and free Cd(II) and Zn(II) speciation was studied in two mineral soils. The soils were extracted with ultra pure 0.01MKNO3, and the extracts were analyzed for total dissolved Cd and Zn by graphite furnace AAS and ICP, respectively, and for labile Cd and Zn by differential pulse anodic stripping voltammetry (DPASV). Based on the assumption that the non-ASV-labile fraction of the total dissolved Cd and Zn was organically bound to fulvic acid (FA), the relative fraction of labile to total dissolved Cd and Zn was used to estimate conditional stability constants (log K) for the formation of Cd-Fulvate (CdFA) and Zn-Fulvate (ZnFA). Species of organically and inorganically associated Cd and Zn, as affected by the addition of pig manure to the two different soil types, were calculated. The addition of organic matter increased the solubility of Cd and Zn in both soils by the formation of organo-metallic complexes. The lability of Zn was, however, reduced substantially, whereas for Cd it was unaffected. The conditional log K values calculated indicate that the stability of organo-metallic complexes with Cd and Zn may be more important than reported previously. This implies that increasing concentrations of dissolved organic acids can increase their solubility, thus leading to the leaching of Cd and Zn into ground water.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Various metals are added to poultry diets to facilitate weight increase and disease prevention. The large amounts of poultry waste produced annually are dispersed intensively over relatively small areas of land, resulting in accumulations that pose potential environmental risks to the surface and groundwater. The focus of this study was to assess the distribution of heavy metals among various solid-phase fractions in soil profiles from a 25-year poultry waste-amended soil. Copper and Zn accumulated close to the soil surface where the total amounts of Cu and Zn in waste-amended soils were significantly higher than in nonamended soils. The total metal concentrations in amended soils were not critically high. Copper in the amended soil was present mostly in the organic matter (OM) fraction (46.9%), whereas Zn was found in the easily reducible oxide (ERO) fraction (47.3%). This suggests that the Cu and Zn in this long-term amended soil are potentially bioavailable and mobile. We observed the mobility of Zn through much of the soil profile of the long-term waste-amended soil. Zinc in this soil profile was found primarily in forms of the residual (RES) and crystalline iron oxide bound (CryFe) fractions, followed by the organic matter-bound and exchangeable (EXC) fractions.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Soil moisture-monitoring equipment is difficult to install in poorly consolidated sand or sediments using hand tools because the loose material tends to collapse. The technique described herein uses a 5.5-hp wet/dry vacuum cleaner, powered by a portable gasoline generator, to remove the soil while an operator pushes a conductor pipe or casing into the profile. After initiating the hole using a hand bucket auger, an open-ended metal pipe or polyvinyl chloride (PVC) casing is inserted vertically into the shallow hole. A smaller tube, or stinger, attached to a wet/dry vacuum is inserted into the pipe to extract loose material while downward pressure is applied on the pipe. Once the casing is installed, instrumentation such as lysimeters, gypsum blocks, or tensiometers can be placed at the desired depth and backfilled with native soil. The casing is then raised and the soil allowed to collapse around the equipment, or the pipe can be left in place for neutron probe access. Measurements of soil water content after an infiltration experiment demonstrated uniform downward movement with minimal preferential flow or soil disturbance as a result of the vacuum installation of gypsum blocks and a neutron access tube.

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