摘要:

&NA;Quantitative information on soil water retention is in demand in hydrology, agrometeorology, agronomy, contaminant transport, and other soil‐related disciplines of earth and environmental sciences. Soil aggregate composition is an important characteristic of soil structure and, as such, is expected to affect soil water retention. Our objective was to determine whether and how aggregate size distributions affect soil water retention from saturation to the wilting point. Soil properties were studied for samples of Podzoluvisols, Planosols, Chernozems, Fluvisols, Calcisols, and Gleysols. Cumulative particle size and aggregate size distributions were used in regression trees to determine what fractions provide the least heterogeneous groups of samples as determined from values of water retention at matric potentials of −10, −33, and −1500 kPa as well as from van Genuchten parameters of the water retention curves, all on a gravimetric basis. Soil aggregate composition provided important grouping parameters for almost all water retention characteristics in this work. Contents of either small aggregates (<0.25 mm, <0.5 mm, <1 mm) or large aggregates (>7 mm, >10 mm) were the grouping variables in most cases for parameters of the van Genuchten equation. No aggregate size distribution parameters were included in the regression tree for the water content at −10 kPa, whereas contents of aggregates of medium sizes do affect water content at −33 and −1500 kPa. Aggregate size distribution parameters can be useful in estimating parameters of soil water retention from other soil properties. (Soil Science 2003;168:223‐233)

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

摘要:

&NA;The low‐molecular‐weight organic acids (LMWOAs) commonly present in root exudates may influence the mobility and bioavailability of Cd in soils through the formation of soluble Cd‐organic complexes in the soil rhizosphere. However, little is known about the dynamics of Cd released by LMWOAs from highly weathered tropical soils where large amounts of phosphate fertilizers are applied to correct phosphorus deficiency. Cadmium is a contaminant of phosphate fertilizers. The release of Cd from selected tropical soils treated with the Idaho monoammonium phosphate (MAP) fertilizer or the Cd perchlorate‐added MAP chemical reagent by LMWOAs (10−3Mand 10−2M) was investigated at 25°C and at an ionic strength of 0.1MNaNO3solution. The LMWOAs used in this study were acetic, citric, fumaric, malic, oxalic, and succinic acids. The surface soils used in this study were obtained from main agricultural areas in Kenya varying widely in physicochemical properties. The amounts of Cd released from the natural and the treated soils varied with the soils, the treatments, and the nature of the LMWOA. The amount of Cd released from the soils in the presence of LMWOAs also increased with the log stability constant values of the Cd‐LMWOA complexes, indicating that Cd was brought into solution by LMWOAs as Cd‐LMWOA complexes. A parabolic diffusion equation provided the best fit to the Cd released by LMWOAs during the short reaction period of 0.25 to 1 h. The overall diffusion coefficient values of the Cd released from the natural soils and the treated soils by LMWOAs varied with the soil type and the nature of the LMWOAs. The results showed that the LMWOAs enhanced the rate of Cd released from the soils, especially in the monoammonium phosphate‐treated soils. The continuous release of Cd from the soils by renewal of LMWOAs indicates the sustaining power of the soils to replenish the Cd labile pool of the soils. The results also indicate that LMWOAs commonly present in the root exudates play a vital role in the mobilization of Cd in tropical soils and, hence, may influence its uptake by the plants.

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

摘要:

&NA;The rate at which water percolates in soils may have a considerable effect on the kinetics of transport processes. Undisturbed samples of a Luvisol subsoil that differed in their hydraulic conductivities (k1: 4 to 6 cm d−1; k2: 20 to 21 cm d−1; k3: 42 to 59 cm d−1) were leached under saturated conditions with 4, 20, 102, and 205 mMBaCl2at a hydraulic gradient of 6.5, and the Darcy velocities were q1 (270 to 360 mm d−1), q2 (1280 to 1360 mm d−1) and q3 (2730 to 3830 mm d−1). Objectives were to investigate how different flux rates affect the amount of desorbed cations in an undisturbed soil when percolated with BaCl2solutions of different concentrations and whether ion concentrations in the percolates can be predicted. The model PHREEQC was used for the calculation of one‐dimensional transport, inorganic complexation, and multiple cation exchange, and the program UCODE was used for the parameter optimization. The best prediction performance of ion concentration and the sum of cations desorbed at q1 (73% satisfactory or good predictions) was obtained when the parameters were 0.6 cm (dispersivity &lgr;), 0.74 (fraction of mobile water f&thgr;), 3.4e‐5 s−1(mass transfer coefficient &agr;), and 0.91 (proportion of cation exchange capacity in contact with the mobile water fCEC‐m). At q2, the optimum performance (71% satisfactory or good predictions) was achieved with &agr; = 6.2e‐4 s−1and fCEC‐m= 0.58. At q3, the optimum parameters were &agr; = 6.8e‐4 s−1and fCEC‐m= 0.38, which resulted in 58% satisfactory or good predictions. This study suggests that &agr; increases with increasing pore water velocity and that fCEC‐mdecreases with increasing hydraulic conductivity. Application of UCODE and PHREEQC contributed to improved understanding of cation exchange reactions in undisturbed soil. (Soil Science 2003;168:253‐266)

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

摘要:

&NA;The objective of this research was to study the rhizosphere of tree species of acid forest soils derived from amphibolite (AMP) and granite (GR) rocks, so that the influence of the type of pedogenesis (generated by these materials) on its chemistry could be established. Umbric A1horizons were sampled in three AMP soils (Lithic Hapludands, Andic Dystrudept), under oak, pine, and eucalyptus stands, and in three nearby GR soils (Typic Dystrudepts) under identical stands. The A1GR horizons had a lower pH (4.2 vs 4.6), and greater aqueous soluble Al (0.40 vs 0.06 mmol kg−1) and NH4Cl‐extractable Al contents (6.8 vs 2.6 cmolckg−1) than the A1AMP horizons. Mean values of sodium pyrophosphate extractable‐Al (Alp) and ammonium oxalate extractable‐Al (Alo) in the A1AMP horizons were 12.0 and 16.2 g kg−1, respectively, and the molar ratio between Alpand sodium pyrophosphate extractable‐C (Cp) ranged between 0.1 and 0.2. Mean values of Alp, Alo, and Alp/Cpin the A1GR horizons were significantly (P< 0.05) smaller (5.2 g kg−1, 5.3 g kg−1, and 0.06, respectively). In the A1GR horizons, we observed a lower pH (significant atP< 0.05) in the rhizosphere than in the bulk soils, a pattern which was not observed in the A1AMP horizons. The balance of nutritive cations and anions between rhizosphere and bulk soils did not explain the patterns of pH change. Based on the relationships found among organic acids, soil pH, and Alp/Cp, we propose that, in the A1GR horizons, where organic matter is Al‐undersaturated, the increased organic acidity produced in the rhizosphere cannot be completely buffered by Al alkalinity, whereas in the AMP soils, where mineral amorphous Al coexists with Al‐humus complexes, organic compounds become stabilized with reactive Al surfaces, maintaining organic acidity at low levels, even in the rhizosphere (whenever the stabilization capacity of these surfaces is not exceeded). Our results imply that both variations in organic matter content and the degree of metal saturation of the organic matter need to be considered when investigating the causes behind changes in soil pH in the rhizosphere of acid forest soils.

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

摘要:

&NA;We determined the concentrations of 25 elements in soils typical of the Southeast Atlantic Coastal Plain, Piedmont, and Blue Ridge at 50 sites in South Carolina. Our purpose was to gather baseline data on the elemental content of soils for use in dealing with land application of wastes and other environmental issues involving metal content in soils. Concentrations of more than one‐half of the elements were higher in the Blue Ridge and Piedmont soils than in those in the Carolina Sandhills, Upper Coastal Plain, or Lower Coastal Plain. Concentrations of Al, Ba, Be, Cd, Co, Cr, Fe, K, Li, Mn, Na, Ni, Rb, S, Sb, Sc, V, and Zn were correlated positively with clay content. Selenium and Y were correlated negatively with clay content. Several of these elements (Al, Ba, Fe, K, Mg, Mn, Ni, S, Se, V, and Zn) were correlated positively with soil pH, but correlation coefficients were generally less than for correlations with clay content. Land use, according to cropping history, was not a significant factor affecting elemental concentrations. Sites that had been cropped to cotton or pastures for long periods of time had slightly higher concentrations of As and Cu, respectively, but differences were of little consequence. Soil Ni concentrations were higher at forest sites compared with all other sites, which may have been due to more effective biogeochemical recycling at forest sites resulting in higher soil retention. We found no evidence that soil at the 0‐15‐cm depth was enriched with any elements. Concentrations within the 0‐15‐cm depth were no different than those within the 15‐30‐cm depth, indicating that any enrichment that may have occurred because of anthropogenic activities had become distributed throughout the 30‐cm depth as a result of tillage and natural processes. The maximum value and geometric mean of Cd concentrations were considerably greater than values reported in two other surveys of U.S. soils. Some sites in all of the land resource and land use categories had concentrations of As and Cd that translated into soil loads exceeding 30% of the cumulative pollutant load limits cited in the USEPA 503 regulations.

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID

摘要:

&NA;There is no consistent theory that explains the patchy distribution of chernozemic soils in Central Germany. Mounting evidence suggests that prehistoric human activities may have interfered with natural pedogenesis on a large scale. Therefore, we undertook to identify parameters that would enable researchers to infer a human interference with pedogenesis. We selected a soil with a known history of prehistoric influence (Seeben), later referred to as ancient soil, and a reference soil (Zöberitz) with properties representative of the chernozemic soils in the vicinity. We did a survey of parameters that are commonly used to characterize pedogenetic dynamics and combined this with an investigation of organic matter properties.We found that prehistoric land use has predominantly modified the properties of the subsoil, rendering it unlikely that such modifications are the result of either (i) contemporary agricultural management or (ii) natural input dynamics via root systems or bioturbation. Soil color was darker, bulk density higher, and C/N ratio wider in the ancient soil. The mineral matrix was uniform down to a depth of 95 cm in the ancient soil, whereas a B‐horizon with a corresponding change in clay mineral assemblage was developed in the reference soil. Compared with the reference soil, the carbon stock was elevated by a factor of 2.5 in the ancient soil, with more carbon associated with the clay fraction. We identified large contributions of charred organic matter in the ancient soil by two independent methods (Benzene polycarboxylic acid BPCA; Transmission electron microscopy, TEM), and confirmed by means of solid‐state crosspolarization/magic angle spinning/nuclear magnetic resonance (13C‐CPMAS‐NMR) spectroscopy that organic matter aromaticity was higher in the ancient soil. We suggest that any simultaneous occurrence of several of the observations listed above should be regarded as a strong indication for human interference with soil genesis. (Soil Science 2003;168:292‐306)

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

ISSN:0038-075X    

出版商:OVID    

年代:2003

数据来源: OVID