摘要:

Bioavailability of metals/metalloids is often rate limited by contact time (i.e., residence time) in soils and sediments, resulting in irreversible reactions. The fate and transport of the contaminants must be predicted/modeled not only on short-term (<48 h) adsorption/desorption studies but also on long-term (months-years) reactions. However, there is very little information on the long-term effects of metal/metalloid partitioning reactions in soils and soil components. In this study, residence time effects (3 days–1 yr) on As(V) adsorption/desorption reactions and on As(V) surface speciation at the aluminum oxide-water interface were investigated using batch adsorption/desorption experiments coupled with time-resolved Extended X-ray Absorption Fine Structure spectroscopy (EXAFS). Biphasic As(V) adsorption kinetics were observed at pH 4.5 and 7.8, and whereas the reaction at pH 4.5 was nearly completed after 3 days, slow adsorption continued at pH 7.8 after 1 year. The longer the residence time (3 days–1 yr), the greater the decrease in As(V) desorption at both pHs, suggesting nonsingular reactions. EXAFS analyses on aged As(V) reacted aluminum oxide at both pHs showed that As-Al interatomic distances were 3.11 − 3.14 Å (±0.13 Å) in all of the aged samples (3 days to 1 yr) at pH 4.5 and 7.8, suggesting that predominantly bidentate binuclear bonding environments were present. As a point of interest, X-ray Absorption Near Edge Structure spectroscopy (XANES) features suggested some changes in the local chemical structure of adsorbed As(V) with aging. The surface transformations such as (i) a rearrangement of surface complexes and/or (ii) a conversion of surface complexes into aluminum arsenate-like precipitates might be important chemical factors responsible for the decrease in As(V) reversibility with aging.

ISSN:0038-075X    

出版商:OVID    

年代:2002

数据来源: OVID

摘要:

The role of agricultural practices on soil carbon (C) dynamics is critical to improved soil management. Our main objective was to examine the C interactions resulting from crop changes under different tillage and residue treatments. Tillage choices were no-till (NT), moldboard plow (MB), and chisel plow (CH); residue options were residue harvested (h) and residue returned (r); and N fertilization rates were 0 and 200 kg N ha−1, applied annually only during the corn (Zea maysL.) (C4plant) years. Soil samples were taken from a long-term experiment representing 13 years of continuous corn followed by 4 years of soybean [Glycine max(L.) Merr.] (C3plant). Samples taken at 5-cm depth increments to either 45 cm (yr-13) or 30 cm (yr-17) and at 15-cm depth increments thereafter, down to 90 cm, were analyzed for C and natural abundance13C (δ13C). Specific objectives were to show: (i) differences in soil organic carbon (SOC) and δ13C caused by crop change; (ii) distributions of SOC and δ13C with depth at yr-13 and yr-17; and (iii) how much of the SOC, after 4 years of soybean, was current C (soybean-derived) and how much was relic C (after 13 yr of corn and earlier grass crops). There were three depth zones: 0 to 20, 20 to 40, and 40 to 90 cm. SOC decreased with depth, but in the 20- to 40-cm zone the decrease was relatively rapid. Values of δ13C increased with depth, except at depths greater than 40 cm when δ13C tended to decrease. Differences in SOC and δ13C caused by crop change generally occurred in the upper 20 cm. Decreases in SOC and δ13C under NT and CH as a result of crop change occurred at the very surface, whereas under MB, the decrease occurred down to 25 cm. The contribution of soybean to SOC ranged from −6 to 27%. When residues were returned, SOC in the 0- to 5-cm depth increased under NT (about 26%) and CH (about 19%). Under MB, the measured contribution of soybean at any 5-cm increment within the upper 25 cm was less than 8%; however, at the 25- to 30-cm depth with residue harvested, the contribution was greater than 14%. Except for the very surface of NTr and CHr and at the 25- to 30-cm depth under MBh, the SOC within the profile was predominantly relic C. A positive interaction between N application and corn residues returned showed that N could not sustain SOC without residue return. This interaction carried a residual through the soybean period, yet no N was applied to soybean. An overall relation between yr-13 and yr-17 suggested a 24% reduced SOC after soybean.

ISSN:0038-075X    

出版商:OVID    

年代:2002

数据来源: OVID

摘要:

The nature of ammonium released from 2:1 layer silicates with varying degrees of tetrahedral/octahedral isomorphous substitution has not yet been defined. This study investigated the influence of 2:1 clay charge caused by octahedral or tetrahedral sheet substitution on the amount of ammonium released. Twelve surface soil (0–30 cm) clays in Taiwan were examined. Deferrated clays with added ammonium fixed had −0.8 volts (V) current applied for 30 min to release ammonium from soil clay suspensions. The mean value of total ammonium released ranged from 0.72 to 3.66 cmol kg−1. The percent of total ammonium released with respect to total ammonium fixed ranged from 5.81 to 83.22%. The added ammonium fixed plus the tetrahedral sheet contribution to ammonium fixed showed a correlation coefficient (r) of 0.797 with the amount of ammonium released.

ISSN:0038-075X    

出版商:OVID    

年代:2002

数据来源: OVID

摘要:

Using an XQC-300 electronic temperature recorder, a continuous automatic record of thein situdiurnal changes in soil temperature under different weather conditions over several years was collected in a valley of the Sichuan Basin in China. A vast amount of experiment data was obtained. Analysis of the measurements of diurnal fluctuation in soil temperature showed that the fluctuation could be described by a damping wave equation. A comparison was made between the damping wave equation and the sine wave equation to describe the diurnal fluctuation under different weather conditions and in different seasons. The results showed that the damping wave equation was more accurate. Using given boundary conditions, a damping wave equation describing the diurnal fluctuation at different depths of the soil body was derived from the heat flow equation. The calculated result and the measured data at different depths in an upland and a paddy soil were found to be in good agreement. These results show that the damping wave equation is much better than the sine wave equation in describing the diurnal changes of soil temperature and that the sine wave equation is a special case of the damping wave equation when the damping coefficient equals zero.

ISSN:0038-075X    

出版商:OVID    

年代:2002

数据来源: OVID

摘要:

Herbicide mobility has been studied extensively in nonvolcanic soils but not in the volcanic soils that are important to the agricultural systems of many regions, including Pacific-Rim countries. Volcanic soils have unique properties that include high contents of organic carbon and soil water at 1500 kPa tension, low bulk density and saturation of bases, and the presence of amorphous clay. We hypothesized that the chemical and physical characteristics of these soils would affect the mobility of selected herbicides. Therefore, thin-layer chromatography was used to measure the relative mobility of atrazine, diethylamine salt of 2,4-D, and dicamba in volcanic soils of south-central Chile, including Arrayan silt loam (Typic Haploxerands), Santa Barbara silt loam (Typic Haploxerands), Arenales loamy sand (Dystric Xeropsamments), and weathered Collipulli clay loam (Xeric Palehumults) series. Dicamba was the most mobile herbicide in all of the soils; atrazine was the least mobile herbicide in the weathered Collipulli soil and in the surface horizons of the Arrayan and Arenales soils, but 2,4-D was the least mobile in the Santa Barbara soil. Herbicide mobility in the allophanic soils (Arrayan, Santa Barbara, and Arenales) could be estimated using the contents of soil organic matter and 1500 kPa water, and also soil pH. The relative mobility of atrazine was greater than expected in the Arrayan and Santa Barbara soils and was probably caused by the protonation and subsequent repulsion of atrazine by positively charged soil surfaces. The relative mobilities of 2,4-D and dicamba were smaller than expected in the Santa Barbara soil and were attributed to the retention of dissociated 2,4-D and dicamba by positively charged soil surfaces.

ISSN:0038-075X    

出版商:OVID    

年代:2002

数据来源: OVID

摘要:

Cultivation strips of different widths were compared in the southwestern area of the Argentinean semiarid region to evaluate their efficiency in controlling wind erosion and soil water storage. The strips ranged in width from L to 4L, with L being the length of the field free of barriers, as calculated by the wind erosion equation (WEQ). The movement of soil particles and the moisture level in the soil profile were determined during three erosive events for a high susceptibility soil condition (fallow: flat and bare surface). Weeping lovegrass (Eragrostis curvula) was used as the barrier crop.Strips of L width showed the least soil movement and the highest soil water content. The amount of soil blown away increased and the level of soil moisture diminished with increasing strip width. Between L and 4L, the amount of soil captured can be explained as a function of the downwind distance from the shelter barrier (r2= 0.88 to 0.95, p < 0.001). A similar relationship was observed between soil water content and strip width (r2= 0.34 to 0.78, p < 0.05).

ISSN:0038-075X    

出版商:OVID    

年代:2002

数据来源: OVID