摘要:

Estimates of soil hydraulic conductivity (K) and air permeability (ka) at given soil-water potentials are often used as reference points in constitutive models for K and kaas functions of moisture content and are, therefore, a prerequisite for predicting migration of water, air, and dissolved and gaseous chemicals in the vadose zone. In this study, three modeling approaches were used to identify the dependence of saturated hydraulic conductivity (KS) and air permeability at −100 cm H2O soil-water potential (ka100) on soil physical properties in undisturbed soil: (i) Multiple regression, (ii) ARIMA (autoregressive integrated moving average) modeling, and (iii) State-space modeling. In addition to actual soil property values, ARIMA and state-space models account for effects of spatial correlation in soil properties. Measured data along two 70-m-long transects at a 20-year old constructed field were used. Multiple regression and ARIMA models yielded similar prediction accuracy, whereas state-space models generally gave significantly higher accuracy. State-space modeling suggested KSat a given location could be predicted using nearby values of KS, ka100and air-filled porosity at −100 cm H2O soil-water potential (ε100). Similarly, ka100could be predicted from nearby values of ka100and ε100. Including soil total porosity in the state-space modeling did not improve prediction accuracy. Thus, macro-porosity (ε100) was the key porosity parameter for predicting both KSand ka100in undisturbed soil.

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

摘要:

Diffusion processes in the soil water and air phases often govern transport and fate of nutrients, pesticides, and toxic chemicals in the vadose zone. This final paper in a 10-part series on diffusion-reaction processes in soils concerns the development of a unifying model platform for predicting solute and gas diffusion coefficients as functions of fluid-phase (water or air) content and pore-size distribution in unsaturated soils. We find that theBuckingham (1904)expression predicts solute diffusivities in water-saturated porous media more accurately than other classical expressions and, extended with a pore-size distribution-based term, yields a new and accurate model for solute diffusivity in unsaturated soil. The same was shown for gas diffusivity in undisturbed soil in Part IX of this series. Thus, the predictive diffusivity models can be rewritten in a common form with two model parameters that vary between solute and gas diffusivity and, in the case of gas diffusivity, also between undisturbed and repacked soil. It is suggested that the two parameters in this unified diffusivity model (UDM) represent porous media (solids-induced) tortuosity (T) and water-induced fluid phase disconnectivity (W), respectively, with W increasing with clay content for solute diffusion but being constant (repacked soil) or decreasing (undisturbed soil) for gas diffusion. Tested against data for 77 soils, the UDM model was markedly more accurate than commonly used soil-type independent models, with 35–50% (gas diffusivity) and 75% (solute diffusivity) reduction in root mean square error of prediction. The use of the new UDM to predict effective diffusion of sorbing chemicals in the soil water and air phases is illustrated. The UDM concept enables a new definition of the relative diffusion coefficient in soil, i.e. relative to the diffusion coefficient in a fluid-saturated porous media instead of in free water or air. This provides new possibilities for analyzing tortuosity phenomena in the soil water and air phases and their effects on diffusive and convective transport parameters in unsaturated soil.

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

摘要:

Macropores created by biological or physical processes can profoundly influence water movement through the soil. In cold regions, natural processes such as wetting/drying and freezing/thawing can influence macropore stability. Little has been documented, however, concerning the over winter stability of macropores. This study measured changes in the physical dimensions of artificial macropores, and the accompanying soil water content, from autumn to spring in the northern US Corn Belt. Macropores 30 cm deep having diameters of 5, 10, and 15 mm were created in a Barnes loam in October 1999 and October 2000. Soil water content was measured by neutron attenuation and Time Domain Reflectometry (TDR). After snowmelt the following spring, all of the artificial pores were at least partially filled with soil. In the spring all of the 5- and 10-mm pores and more than 95% of the 15-mm pores were undetectable at the soil surface. The extent of the smaller pores maintained over winter was generally greater than that of the larger pores. Over both winters the 5-mm pores maintained approximately 65 to 70% of their created length, and the 10-mm pores maintained approximately 50% of their created length. The length maintained by the 15-mm pores varied greatly over the two winters, 46% over the 1999–2000 winter and 4% over the 2000–2001 winter. The difference in the stability of the 15-mm pores over the two winters was attributed to the much wetter conditions of the 2000–2001 winter. No significant difference in the soil water content before or after freeze-up was measured as the result of the presence or size of the macropores. The results of this study show that macropores are not stable over winter, but the collapse of the macropores had no clear effect on soil water dynamics the following spring.

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

摘要:

The concentration of organic chlorine (Clorg) was determined in soil samples collected in the O-horizon in forest soils in southern Sweden to describe differences among stand types and elucidate the influence of chloride (Clinorg), pH, and organic carbon. The samples were collected within the Swedish National Survey of Forest Soils and Vegetation, with moraine as the dominating soil type and with granite and gneiss as bedrock. The concentration of organic chlorine (Clorg) found in the deciduous forest soils was significantly lower than that in the coniferous forest soils. The Clorgincreased with Clinorg, organic carbon content, and decreasing pH but was most strongly correlated to Clinorg. Crosswise comparisons among the variables showed that the influence of Clinorgoverrode the influence of pH and organic carbon. We concluded that the major driving force in the formation of Clorgin soils is deposition of Clinorgand that the difference among deciduous and coniferous stands is due primarily to a higher input of Clinorgin the canopy of coniferous trees as a result of a larger wet and dry deposition in coniferous compared with deciduous forests.The concentration of Clorgdecreased significantly from June to August. This follows the pattern of chloride deposition in the region and gives further evidence that Clinorgis a driving force in the formation of Clorgin soil. The results of the present study gives further evidence that the turnover of Clorgis closely related to the turnover of Clinorgand that it is necessary to include Clorgin the assessment of the biogeochemical cycling of chlorine in the environment.

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

摘要:

Organic C additions and changes in soil chemical properties caused by waste disposal, root exudation, and organic matter (OM) decomposition processes affect heavy metal solubility, mobility, and bioavailability. The solubility and exchangeability of zinc (Zn) were determined in two contrasting Maryland A horizon soils to evaluate the influence of an organic C amendment (citric acid) on Zn retention and cation exchange capacity as a function of pH, two properties that interact to control Zn speciation in soils. Water soluble Zn in the A horizons of Galestown loamy sand (Psammentic Hapludults) and Jackland silt loam (Typic Hapludalfs) soils, with and without citric acid addition, decreased with increasing pH from 4.0 to 7.0 and with increasing cation exchange capacity (CEC) from 4.7 to 96.7 mmolc/kg soil. At pH values < 5.0, the water soluble Zn fraction dominated and became exchangeable Zn as pH increased from 5.0 to 7.0 in the Galestown soil, whereas the exchangeable Zn fraction dominated from pH 4.0 to pH 6.0 and became nonexchangeable Zn as pH increased to 7.0 in the Jackland soil. Citric acid addition, compared with no citric acid addition, lowered (P< 0.05) the solubility of Zn and favored the exchangeability of Zn at pH < 5.0, and this effect of C addition on the solubility of Zn was more pronounced in the coarse-textured Galestown sandy soil than it was in the finer textured Jackland soil. The addition of citric acid at a Zn/citric acid molar ratio of 1:1 enhanced nonexchangeable Zn through chemisorption at pH > 6.0, whereas the effect of citric acid on levels of nonexchangeable form of Zn retention was not significant at pH < 6.0. The citric acid addition caused an abrupt transition from water soluble and exchangeable to nonexchangeable forms of retention and reduced the potential availability of Zn in the Jackland soil as soil pH approached 6.0. The results indicate that the pH influence on CEC was the dominant factor influencing the effect of citric acid on the redistribution of water soluble, exchangeable, and nonexchangeable Zn, especially in the finer textured soil. These results have implications for controlling soil pH to minimize Zn activity and bioavailability in soils used for organic waste disposal.

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

摘要:

Understanding phosphorus (P) sorption of calcareous soils is important for the development of successful fertilizer and manure management practices. This study was conducted to identify soil chemical properties controlling P sorption in semiarid calcareous soils of the Pacific Northwest. Sorption isotherms of 18 primarily calcareous soils ranging widely in soil physical and chemical properties were constructed by equilibrating 4 g of soil with 40 mL of 0.01MCaCl2containing between 0 and 700 mg P L−1for 24 h. The P sorption isotherms at low to medium P concentrations fit the Freundlich isotherm (r2≥ 0.93). The slope of the isotherm generally increased abruptly at high P concentrations, suggesting Ca-P precipitation. The maximum P sorption prior to Ca-P precipitation was closely related to organically complexed Fe and Mn (R2> 0.98), suggesting that such complexes may regulate P sorption in these soils. The equilibrium P concentration at the point where Ca-P precipitation begins to dominate was closely related to pH and organic carbon. A better understanding of the role of organically complexed metals in controlling P sorption in calcareous soils is needed to determine how P solubility is affected by organic matter additions and how this will ultimately impact the plant availability and potential off-site transport of P from these soils.

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

摘要:

Alfisols are the predominant soil resource in Ohio. An understanding of the relationships among their soil and site variables will help in planning judicious future land use to maintain soil quality and sustain agriculture. The objective of this study was to identify associations and their significance between dependent variables of surface horizons of Alfisols (consisting of bulk density, base saturation, CEC, pH, Munsell value and chroma) and descriptive variables (including drainage class, elevation, sand, slope gradient, soil organic carbon) using canonical correlation analysis (CCA). The association of SOC concentration with bulk density was found to be the most important relationship among the selected variables, followed by the association of soil clay content with CEC. The association of drainage class and Munsell chroma was identified as being dominant and significant from the third pair of canonical variates. Despite pronounced variations in the means of all selected variables among land uses, these associations and their significance were consistently confirmed, regardless of land use, and were responsible for about 60% of the variance within each set of variables. Additional associations could be identified from other canonical variates but were less explicit and varied widely among land uses. As a means of identifying associations and the order of their importance among multivariates, the CCA has become a promising technique for simplifying, and simultaneously analyzing, two sets of complex data consisting of biotic and abiotic variables.

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID