摘要:

Soil water retention is an important hydraulic property in the study of water flow and solute transport in soils. However, soil water retention measurements are costly and time-consuming. In this study, a procedure was developed to estimate this function based on soil particle-size distribution and the fractal theory. A relationship between the fractal dimension and cumulative particle-size distribution was derived. Based on the relationship, the fractal dimension was determined using particle-size distribution data. The fractal dimension was incorporated as the exponent parameter of retention models. The procedure was tested using particle-size distribution data and soil water retention data of 110 soils from a data base. Good agreement between the experimental data and estimations of soil water retention from the procedure was obtained for most of the soils. The results suggest that the procedure is a sufficiently accurate and economic method to predict the soil water retention function.

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID

摘要:

The application and validation of complex atmosphere-soil water transport models demands knowledge of the parameters that describe hydraulic properties over extensive areas. Such information is rarely available, but Pedo Transfer Functions (PTFs) provide a means of predicting these parameters from soil survey data. However, most PTFs have been derived and validated using information from soils of temperate regions and have not been tested for the soils of tropical areas, for which chemical, physical, and pedogenetic processes are different. The equations of Rawls and colleagues, for example, overestimate water content when applied to the soils of Brazilian Amazonia. In this paper, we have developed a PTF to predict Brooks-Corey parameters from texture using data from soils of Amazonia. Multiple linear regressions were fitted to estimate, from soil texture (% sand, silt, and clay), the bulk density and porosity and the water content at a range of matric potentials. Brooks-Corey parameters were then derived and correlated independently with soil texture, providing a straightforward method for deriving soil retention parameters from the percentage of clay and silt. The method was validated using an independent data set for which textural and water release data were available. The agreement between the observed and measured values was very significant, but the results showed that the differences between predictions and measurements also depended on bulk density. However, the Amazonian soil survey data, which may be used to extrapolate these results spatially, do not generally include bulk density, and for this reason they were not included in the regressions.

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID

摘要:

Information in regard to nondestructive and repetitive measurements of changes in soil porosity (ϵ) that may occur during wetting and drying is limited, particularly information regarding changes on a very small scale. The objectives of this study were (i) to determine whether the changes in ϵ that may occur during wetting and drying at scales as small as 2 × 2 mm can be discriminated by computer-assisted tomography and (ii) to use a published theoretical equation to estimate surface fractal dimension (D) from ϵ and to determine whether D is sensitive to wetting and drying. Computer-assisted tomography was applied to gamma-ray attenuation to measure dry bulk density (ρ), before and after wetting, at 2 × 2-mm resolution of water-stable soil aggregates (WSA) 2 to 4, 0.71 to 1.40, and 0.25 to 0.71 mm in size and packed separately in acrylic cylinders. Columns with similar particle size were also prepared for unstable soil aggregates (USA). Before wetting, ϵ computed from ρ in WSA, ranged from 0.621 to 0.740; after wetting the range was 0.604 to 0.709. In USA, ϵ ranged from 0.489 to 0.562 before wetting and from 0.457 to 0.516 after wetting. The lack of a 1:1 relationship between the before and after wetting data for ϵ indicated there were significant differences between the two. Initial aggregate size (x), wetting, (w), and the interaction w x accounted for 74% of the variability in ϵ of USA compared with only 47% of the variability for WSA. The estimates of D, obtained using the theoretical equation and ϵ, ranged from 2.154 to 2.236 for WSA and from 2.055 to 2.12 for USA. Wetting, x, and w x accounted for 47% of the variability in D of USA compared with 69% for WSA. Pore continuity (PC), estimated using a theoretical relation involving PC, ϵ, and D, decreased from 0.45 to 0.30 after wetting in USA and from 0.60 to 0.55 in WSA.

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID

摘要:

The statistical description of the spatial variability of soil properties is a preliminary step to the application of other quantitative techniques, for example, optimal spatial interpolation by kriging. The spatial correlation between the experimental data is characterized by the covariance function that is usually inferred from the experimental measurements. An appealing method for performing this inference is maximum likelihood estimation.This paper describes the results of the application of maximum likelihood estimation to the inference of the spatial covariance of infiltration measurements for a small catchment. Because there are a large number of experimental data, the approximate maximum likelihood method has been used to overcome the computational burden imposed by the calculation of the complete likelihood. As will be shown, this method, used in conjunction with classical graphical variogram analysis (method of moments), provides more objective inference of covariance parameters.

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID

摘要:

Characterizing the amino sugar signature in particle-size fractions allows elucidation of the fate of microbially derived compounds during the alteration and turnover of soil organic matter (SOM) in soils of different climate regimes. The purpose of this study was to evaluate amino sugar pools and the effect of climate on such pools in particle-size fractions of 18 surface (0-10 cm) soil samples along a climosequence in the native North American prairie. Soils were fractionated into clay (<2 μm), silt (2-20 μm), fine sand (20-250 μm), and coarse sand (250-2000 μm). Soil organic carbon, glucosamine, mannosamine, galactosamine, and muramic acid were then determined in the fractions.The major proportion of the three hexosamines (69%) and muramic acid (79%) was attached to clay. The proportions of clay-associated amino sugars were positively related to mean annual temperature (MAT), and those of silt were negatively related to MAT. This indicated a shift of amino sugars from silt to clay as MAT increased. The total concentration of the four amino sugars in SOM increased markedly from coarse sand (30 g kg−1SOM) to clay (93 g kg−1SOM), indicating a progressive accumulation of microbially derived components in SOM with decreasing particle-size. The enrichment factors of the three hexosamines in the SOM of clay were correlated positively with MAT (r= 0.79***), whereas those of muramic acid related positively to mean annual precipitation (MAP) (r= 0.60**). The result suggested that the dynamics of hexosamine were different from those of muramic acid. The amino sugar ratios were controlled by MAT and MAP, but the effect was different in different size fractions. Nevertheless, the two climatic elements are the key site variables that determine the fate of microbially derived compounds in the native grassland soils.

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID

摘要:

Establishment and maintenance of forage legumes on acidic soils continues to be a worldwide problem. In an attempt to develop a more rational approach to the selection of plants and bacteria, a laboratory model system was constructed using an acidic, fresh Ultisol from an old-field. The objective of the study was to evaluate this system by determining the effect of low-level liming of the soil to pHw4.62, 4.76, 4.83, and 4.95, and rhizobial inoculation, on the establishment and functioning of theTrifolium repens,L./Rhizobium leguminosarumbv.trifoliisymbiosis. The soil, as taken from the field, was low in pHw(4.65) and base saturation (45%), relatively high in Al (2.4 cmol kg1), and essentially devoid of naturalized rhizobia. For the symbiotically-grown (inoculated) plants, beneficial effects of liming to only pHw4.83 on shoot mass, shoot N uptake, nodule number, and ARA, but not root mass and length, were detected as early as 3 weeks. Nearlinear responses to soil pH increases were found for shoot mass, shoot N uptake, nodule number, and acetylene reduction activity at 5 and, with the exception of ARA, 7 weeks. Root mass, but not length, also responded positively to liming at these later times. Soil pH and basic cation- and aluminum-associated properties were significantly (P≤ 0.05) correlated with shoot mass and shoot N uptake. Although correlations (r) improved with time, they never exceeded 0.80, even at 7 weeks. Asymbiotically grown (NH4NO3-treated) plants gave similar shoot responses, but at different times, indicating dissimilar growth rate responses compared with the symbiotically grown plants. Rhizobial inoculum levels altered the outcome of the symbiosis significantly, indicative of a rhizobial survival limitation. Taken together, these results suggest that both partners of the symbiosis, plants and rhizobia, were affected adversely by the stressful chemical composition of the soil. More importantly, these results demonstrate the utility of the model system for studying the early (3 weeks or less) effects of low-level liming on the development of the white clover symbiosis in a near-natural, acidic Ultisol.

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID

摘要:

Intensively cropped dryland systems in the central Great Plains require adequate N fertilization for optimum residue and grain production. However, this N fertilization could be slowly changing the chemistry of the surface soil because of a decrease in soil pH and an increase in soil organic matter (SOM) and basic cations, even in previously well buffered calcareous soil systems. We investigated the effects of five increasing ammonium-N fertilizer rates in a Platner loam, on physical and chemical changes at the 0 to 5, and 0 to 15-cm depths after three cycles of no-till wheat (Triticum aestivumL.)-corn (Zea maysL.)-fallow rotation. We measured soil pH, texture, bulk density, cation exchange capacity (CEC), total P, soluble and total soil organic carbon (SOC), nitrate-N to a depth of 60 cm, and grain yields. No significant changes were found with soil texture, bulk densities, CEC, and total P. The data showed a significant reduction in surface (0-5 cm) soil pH (6.5 to 5.1) with the highest N rate (112 kg/ha), but this was accompanied by a 40% increase in SOC. Although there were significant increases in Al and Mn and decreases in Ca concentrations in the surface 0 to 5 cm at the highest N rate, no reduction in grain yields occurred relative to lower N levels with near neutral pHs. Because only a shallow depth of the soil was affected, residue, SOM, and rapid root growth could be compensating for surface acidity. Over the longer term, we need to monitor the effects of ammoniacal-N on downward soil acidity and yield trends under these new intensive cropping systems.

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID