ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

摘要:

A realistic, physically based simulation of water and solute movement in the unsaturated soil zone requires reasonable estimates of the water retention and unsaturated hydraulic conductivity functions. A variety of studies have revealed the importance of how these unsaturated soil parameters are assessed and subsequently distributed over the numerical mesh on modeling outcome. This study was initiated to acquire experimental data about the water flow characteristics of sandy soils to serve as a base for numerical analyses. Specific objectives were to clarify the effects of (i) the invoked procedure for estimating the soil hydraulic parameters and (ii) using increasingly refined spatial definitions of the hydraulic properties on simulated two dimensional water content and flow velocity distributions.Water flow in and drainage from a large sand tank (approximately 5 × 3 m2at the base, 6 × 5.6 m2at the top) was investigated using soil hydrologic and geophysical methods. Numerical analyses of variably saturated flow along a two-dimensional cross-section were carried out in attempts to describe the heterogeneous flow fields using the Richards equation-based HYDRUS-2D code. The unsaturated soil hydraulic properties were described using van Genuchten-Mualem type expressions. Information from bothin situand laboratory measurements was employed to obtain parameter estimates.The observed variability in discharge rate with time was reproduced best when an average water retention curve was used and the saturated water content was set equal to the porosity, whereas cumulative outflow was predicted best when all van Genuchten hydraulic parameters were fitted to the retention data. Using heterogeneously distributed hydraulic parameters (assuming a layered profile or a random distribution of the saturated hydraulic conductivity) improve neither predictions of the cumulative discharge rate nor the variability in the outflow rate when compared with the homogeneous case. Efforts to construct or numerically simulate heterogeneous flow experiments may, therefore, not always be justified when water flow in sandy substrates is studied.

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

摘要:

Accurate prediction and mapping of soil nutrient levels are essential for implementing variable rate technology. For the prediction of soil potassium (K), we evaluated the performance of the inverse distance weight of powers 1, 2, and 3, ordinary kriging, cokriging, multiple linear regression assuming independent error, and multiple linear regression with autocorrelated error structure. Two forms of ordinary kriging were evaluated: kriging the residuals from a trend surface regression (geographic locations only as predictors) and kriging the residuals from a regression of K on geographic location and other soil property predictors (soil pH and apparent electrical conductivity, ECa). The autocorrelated error model as implemented in the Statistical Analysis System (SAS) mixed linear model was employed to adjust for autocorrelated error structure in the regression models used for prediction. For cokriging, either soil ECa or soil pH was used as a secondary soil property to predict K. The root mean square error (RMSE) and mean error (ME) calculated from an independent validation data set (n= 68) were used as comparison criteria. The best result was obtained with the methods that incorporated geographic locations, other soil property predictors, and the correlated error structure. This investigation demonstrated the flexibility of the regression-based autocorrelated error model for spatial prediction compared with other methods. Further, the results of this study have important implications for screening economically acceptable soil and site characteristics that can be used to improve prediction of soil nutrients at unsampled locations within a field.

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

摘要:

With the availability of sensitive multielement analytical capability, it is no longer essential to use chemically aggressive soil tests to extract measurable levels of most trace elements. However, the relative abilities of mild and aggressive extractants to assess metal bioavailability in soils have rarely been compared. A greenhouse experiment was carried out to compare a mild soil extractant (hot 0.01MCaCl2) with an aggressive one (Mehlich 3) for predicting accumulation of trace metals by red clover (Trifolium pratenseL.). Clover was grown on large columns of nonacid fine-textured and acid coarse-textured soils that had been amended several years earlier by a heavy application of sewage sludge products, and pH was subsequently adjusted using CaCO3or H2SO4. The soil extractants (CaCl2and Mehlich 3) and clover tissue were analyzed for trace metals (As, Cd, Mo, Cu, Ni, Mn, Pb, and Zn) by axial-view ICP spectrophotometry. Linear regression analyses were performed to relate the concentration of each trace element in the red clover tissue to the concentration extracted from the soil. The results indicate that CaCl2extraction is more reliable than Mehlich 3 extraction when evaluating plant availability of trace elements in soils with a wide range of properties (especially pH). The strongly acidic nature of the Mehlich 3 extractant caused large quantities of metals such as Zn, Cd, Cu, and Ni to be extracted from metal-contaminated soils even when the plant availability of these metals was low because of near-neutral soil pH or high clay and organic matter content. Conversely, in coarse-textured and acid soils containing lower total concentrations of metals, plant-available metals were often relatively high, yet Mehlich 3 frequently extracted smaller quantities of metals from these soils than from the near-neutral soils. We conclude that dilute CaCl2is preferable to Mehlich 3 as a universal soil extractant for estimating short-term trace metal availability to crops.

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

摘要:

The toxicity of aluminum ions is the main cause of the unfavorable effect of acid soils on plants. The extent of this toxicity depends on a variety of factors, including organic acids. In the present paper, the effect of anions of four low molecular weight organic acids on the adsorption and desorption of Al by and from a kaolinite was investigated. The results showed that the presence of organic anions led to a decrease in the adsorption of Al. The ability of organic anions in hindering Al adsorption followed the order: Oxalate ≈ malonate > malate > lactate. The adsorption increased with the increase in pH in the lactate system and the control system, whereas an opposite trend occurred in the other three anion systems. The presence of organic anions also induced a decrease in the desorption of Al. The proportion of desorbed Al to adsorbed Al was higher than that in the control system. Both the adsorption and the desorption decreased with the increase in concentration of organic acid, with the difference between adsorption and desorption larger at low concentrations. The difference in effect among the four anion species was interpreted in terms of differences in their nature, with the dissociation constant of the anions and the stability constant of the Al-complexes as the parameters.

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

摘要:

A study using137Cs, a component of radioactive fallout, was carried out to estimate soil erosion and deposition on a small watershed in Piracicaba District, São Paulo State, Southeastern Brazil. Soil samples were collected along three transects employing different land uses and conservation practices on different slopes.137Cs concentration was measured with a high-resolution germanium semiconductor detector coupled to a multi-channel analyzer. The137Cs estimates of soil movement and redistribution were related to slope, land use, and conservation practices, allowing the identification of sites with soil loss and accumulation. This study shows that the application of the137Cs technique is viable for Brazil. A relationship between137Cs concentration in soil profiles and soil losses or accumulations for Brazilian conditions needs to be established to avoid the use of equations developed for the Northern hemisphere.

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

摘要:

Seal formation, runoff, and erosion are controlled by soil texture, rain kinetic energy (KE), and wetting rate (WR) of soils aggregates. The objective of this study was to determine the relative importance of WR and rain KE in seal formation, runoff production, and erosion, in soils varying in their clay and silt content, using a drip-type rain simulator. Four soils, ranging in clay content from 22.5 to 61.2%, were packed in 0.2×0.4-m trays, pre-wetted at WRs of 2, 8, or 64 mm h−1, and exposed to 60 mm of simulated distilled water (DW) rainfall with two KEs (8 and 15.9 kJ m−3). Runoff and erosion increased as rain KE and WR increased; however, the magnitude of change depended on clay content. In the loam (22.5% clay), the effect of rain KE on seal formation and runoff was significant, and the effect of WR was small. Conversely, in the clay soils (51.3 and 61.2% clay), the effect of WR on seal formation was significant and the effect of rain KE was negligible. In the sandy clay soil (38.1% clay), the effects of both WR and rain KE on seal formation and runoff were significant. The effect of rain energy on soil erosion was significant in all soils. Since water erosion is the product of sediment detachment and transport of the particles by overland flow, increase in the impact of rain drops increases both soil detachment and transport capacity and increases soil erosion in all soils. In clay soils, erosion also increased with the increase in WR. Disintegration of the aggregates by fast WR combined with detachment by rain impact increased erosion from clay soils. The conclusion reached is that for seal formation and runoff production, rain KE predominates in medium- and light-textured soils and WR predominates in heavy-textured soils. Conversely, for soil erosion from laboratory trays, detachment by rain KE is essential in all soils.

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

摘要:

Several studies of loess-paleosol sequences have been conducted with the objective of reconstructing the monsoonal climate changes in East Asia, but pedocomplexes have not been identified and studied. In this study, a pedocomplex was studied with the objective of evaluating paleoenvironmental changes. The data show that the paleosol S5of the Brunhes (780,000–0 yr B.P.), which developed over about 100 ka, is a pedocomplex and varies in thickness from 2.2 to 6.5 m. The paleosol is located at the Liujiapo near Xi’an, Renjiacun, near Baoji and Yaergou near Changwu and consists of four to five horizons. The first horizon (uppermost) is a well developed reddish brown clay horizon (Bts); the second horizon is a light reddish brown weathering-cracked loess (Ccs); the third is a dark yellowish brown weathering-cracked loess (Cs); the fourth is a light yellowish brown weathering-cracked loess (Cl); and the fifth horizon consists of CaCO3nodules (Ck). Weathering-cracked loess horizons have not previously been found in Chinese paleosol. The clay component is montmorillonite-illite with some kaolinite minerals. The pedocomplex suggests that a moist subtropical climate prevailed from 570,000 to 470,000 years B.P. in the central and southern parts of the Chinese Loess Plateau. Its profile is Bts-Ccs-Cs-Cl-Ck-Co in the Xi’an, Baoji areas and Bts-Cs-Cl-Ck-Co in the Changwu area. When the pedocomplex developed in the Xi’an and Baoji areas (southeastern Loess Plateau), the mean annual temperature and precipitation were about 16 °C and 1100 mm, respectively, about 4 °C higher and 500 mm more than at present. In the Changwu area (central Loess Plateau), the mean annual temperature and precipitation were 15 °C and 900 mm, about 6 °C higher and 300 mm more than at present. In the typical interglacial developing the S5, the Qinling Mountains lost their function as the boundary between the subtropical and temperate zones of China.

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

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID