摘要:

Salt diffusivity as a function of soil-water content and soil-water characteristic curves was measured on three sieved soils of different texture. The data were used together with ion diffusivity data for sieved soils from literature to test different diffusivity models. No significant differences between salt-, counter-, and self-diffusion data were observed, suggesting an insignificant effect of diffusion type in most cases. Two new models for predicting ion diffusivity, based on the Campbell soil-water retention model parameter b, were proposed: (i) a model with a b-dependent threshold water content &thetas;th(soil-water content where the ion diffusivity approaches zero) and (ii) a power function model where the power term η, representing the liquid phase tortuosity, is a linear function of b. In both models, the diffusivity at the soil-water content equal to the total porosity was estimated from a simple impedance factor model. Both b-dependent models gave improved predictions of ion diffusivity compared with existing models. The b-dependent power function model with η = 0.3b gave better predictions than both the &thetas;th-dependent model and a recent model for gas diffusivity in undisturbed soil (Part III of this series, Soil Sci. 161:366-375) where the tortuosity term η was best described as a function of b-1. For use of the new b-dependent ion diffusivity models in the absence of soil-water characteristic data, it is shown that b can be fairly accurately estimated from soil texture.

ISSN:0038-075X    

出版商:OVID    

年代:1996

数据来源: OVID

摘要:

Two chemical sorption models, a two-site equilibrium-kinetic sorption model (E-K) and an instantaneous equilibrium sorption model (I-E), were used to describe pesticide leaching and runoff. Concentrations of bromide, atrazine, and fenamiphos in leachate and runoff water from 1.0 m × 0.5 m × 0.1-m boxes of packed air-dry Tifton loamy sand (fine-loamy, siliceous, thermic Plinthic Kandiudults) were compared with simulated concentrations predicted by the Root Zone Water Quality Model (RZWQM). Atrazine and bromide (as KBr) were applied to the soil surface and fenamiphos was incorporated into the top 7 cm. Water (100 mm in 2 h) was applied using sprinkler rainfall simulator 24 h later. The E-K model and the I-E model, which are the two submodels for chemical sorption in RZWQM, were used for pesticide sorption simulations, and a nonuniform mixing model was used for pesticide runoff simulations. Model parameter values were obtained by measurements, from the literature, and by calibration with measured atrazine and fenamiphos breakthrough curves (BTCs) and a runoff chemograph from an independent experiment. Results showed that the E-K model simulated atrazine and fenamiphos leaching and atrazine runoff better than the I-E model. The I-E sorption model underestimated pesticide leaching and overestimated atrazine runoff. Compared with previous studies showing that the E-K model simulated pesticide sorption and leaching better than the I-E sorption model with initially saturated soils and under steady state water flow conditions, this study showed that the E-K model also worked better than the I-E sorption model for initially air-dry soil and under fast and transient water flow conditions for leaching as well as runoff of a pesticide.

ISSN:0038-075X    

出版商:OVID    

年代:1996

数据来源: OVID

摘要:

Heavy metal movement in soil profiles is a major environmental concern because even slow transport through the soil may eventually lead to deterioration of groundwater quality. In this study, three metal-contaminated soils (Fuquay, Dothan, and Clarendon) were selected from cropland where a high-metal flue dust had been applied annually for 6 years to raise soil pH, with application ending 4 years before sampling. One uncontaminated soil (Tifton) from the same physiographic area was also sampled as a control. Soil samples were collected in 15-cm increments from the surface to 105 cm in depth. Total contents of Zn, Cd, and Pb in the soil samples were determined. To better understand metal movement in relation to metal fractions in the soil profile, soil samples were also extracted sequentially for exchangeable (EXC), organic matter (OM), Mn oxide (MNO), amorphous Fe oxide (AFEO), crystalline Fe oxide (CFEO), and residual (RES) fractions. The Clarendon soil had more Zn, Cd, and Pb than the Dothan and Fuquay soils because of a higher application of flue dust. The metal concentrations in the contaminated topsoils were much higher than that in the Tifton (control) topsoil, whereas the differences in the subsoils were minimal except for the Zn in the Clarendon soil. High concentrations of Zn were found in the Clarendon subsoil (96 mg kg-1at 45-60 cm). A large portion of Zn in the subsoil was in the EXC fraction, which suggests that the Zn migrated in this soil profile. In the Fuquay and Dothan soils, movement of Zn was not evident. Except for slight movement of Cd in the Clarendon soil profile, there was no movement of Cd and Pb in these soil profiles. Cadmium and Pb had accumulated in the topsoils probably because of their affinity with the OM fraction. The application of high-Zn flue dust to coarse-textured soils low in organic matter may induce Zn movement in soil profiles.

ISSN:0038-075X    

出版商:OVID    

年代:1996

数据来源: OVID

摘要:

Oxidative degradation of plant tissue leads to the formation of natural dissolved organic carbon (DOC) and humus. Infrared (IR) and13C nuclear magnetic resonance (NMR) spectrometry have been used to elucidate the chemical reactions of the early stages of degradation that give rise to DOC derived from litter and compost. The results of this study indicate that oxidation of the lignin components of plant tissue follows the sequence of O-demethylation, and hydroxylation followed by ring-fission, chain-shortening, and oxidative removal of substituents. Oxidative ring-fission leads to the formation of carboxylic acid groups on the cleaved ends of the rings and, in the process, transforms phenolic groups into aliphatic alcoholic groups. The carbohydrate components are broken down into aliphatic hydroxy acids and aliphatic alcohols.

ISSN:0038-075X    

出版商:OVID    

年代:1996

数据来源: OVID

摘要:

During the stabilization of plant residues into soil humus, organic matter is transformed continuously to different chemical compounds. To obtain a better understanding of these changes, we used13C nuclear magnetic resonance (13C NMR) and pyrolysis-field ionization mass spectrometry (Py-FIMS) to characterize plant tissue, isolated fractions, and whole surface soils and subsoils from a forest system and a maize (Zea maysL.) system. Both methods indicated that chemical components of the light fraction (LF) were similar to those in the plant material from which the LF was derived, but a lesser amount of carbohydrates and a greater amount of sterols in the LF signalled the early stages of decomposition of organic matter in soil. Accumulation of alkyl C in the maize LF was attributed to microbial structures or metabolites. Larger differences in the abundance and range of organic components were observed between the LF and sand-size fraction (SSF) of the soil under maize. The mass spectra showed that fewer lignin monomers and dimers, lipids, and alky-aromatic compounds were present in the SSF compared with the LF. Carbon-13 NMR data indicated that the SSF contained relatively lesser amounts of carbohydrates and aliphatic compounds and had a higher degree of aromaticity than the LF.Differences between the organic matter in the soils under forest and maize reflected the effects of deforestation, cultivation, and cropping to maize on soil organic matter. Carbon-13 NMR results indicated that the surface soil under maize had less O-alkyl and alkyl C but more aromatic and carboxyl C than the forest soil. In addition, Py-FIMS results indicated that lipids and sterols, which are derived from plant material, were reduced in the soil under maize. Microbial degradation of these high-molecular-weight compounds probably resulted in their transformation into polysaccharides in the soil humus. The presence of numerous N-compounds in the soils under maize was attributed to N from fertilizers that had been stabilized in heterocyclic forms.

ISSN:0038-075X    

出版商:OVID    

年代:1996

数据来源: OVID

摘要:

Dissolved organic carbon (DOC) leached from the forest floor is an important source of acidity in forest soils. The study of DOC and its acidic properties has been particularly difficult because of uncertainties about the use of isolated fractions of DOC. To evaluate the acidic properties of soluble organic substances in the forest floor we collected and examined leachates from a spruce-fir Spodosol in east-central Maine. Filtered, lyophilized, bulk leachates and DOC fractions were analyzed by potentiometric titration in constant ionic media (0.01MKCl) to determine the charge density, exchange acidity, and apparent acidity constants (pKa) of each organic material. Based on the titration data, hydrophilic acids had the greatest content of acidic functional groups (e.g., carboxylic acid groups). Bulk leachates, hydrophobic acids, and hydrophilic acids had mean (± SE) exchange acidities of 0.131 ± 0.002, 0.127 ± 0.002, and 0.198 ± 0.003 µeq µmol-1C, respectively, and pKas of 3.71 ± 0.05, 4.10 ± 0.03, and 3.70 ± 0.03. From the titration of bulk leachates, we calculated organic anions contributions of 267 to 858 µeq L-1. An organic charge contribution model was also used to evaluate the titration data, along with DOC fractionation percentages and total DOC in the forest floor leachates. The acidic properties calculated from this model and those of the bulk leachate samples agreed closely (r2= 0.93), suggesting that acidic properties of DOC were not altered by isolation procedures. Forest floor organic anion contribution to zero-tension lysimeter solutions was also estimated by three methods (a charge contribution model, anion deficit calculations, and chemical equilibrium model simulations) that yielded similar results. For the best estimation of organic anions in soil solutions, empirically derived constants from large samples of bulk leachates (isolation not needed) and use of a chemical equilibrium model are recommended.

ISSN:0038-075X    

出版商:OVID    

年代:1996

数据来源: OVID

摘要:

Capillary electrophoresis (CE) is suitable for the analysis of cations and anions, including organic acids, in soil solution. In the concentration range typical for soil solutions, the standard deviation of the linear calibration function is, with the exception of NH+4and Na+(≅ 12%), better than 6%. Using drift correction procedures, CE is applicable for routine analysis. The advantage of this analytical technique is the minimal sample consumption of about 5 to 10 nL per analytical run, which enables new approaches for soil solution collection and analysis on a microscale level. As an example, a podzol profile was covered by a vertical matrix of 6 × 10 microlysimeters (sampling distance 2 cm). High resolution maps of soil solution chemistry showed, that, although some solutes had concentrations related to soil horizons (UV as measure for DOC, SO2-4, Al3+), in general there was little correlation between the clearly visible soil horizon borders and concentration isolines. For all solutes analyzed even within soil horizons marked concentration gradients could be observed.

ISSN:0038-075X    

出版商:OVID    

年代:1996

数据来源: OVID

摘要:

Binary systems, such as K - Ca or NH4- Ca pairs, are used commonly in traditional Quantity/Intensity studies, and it is assumed that the results obtained can be extrapolated to ternary systems. It is also believed that K and NH4behave analogously with respect to micaceous clay minerals. The existing data on the subject of ternary systems, however, and on the interaction between K and NH4in soils do not provide enough evidence that ternary exchange reactions, K - Ca - NH4, in soils can be predicted from binary exchange data. In this study, the Q/I relationships of K-Ca in the presence and absence of NH4and of NH4-Ca in the presence and absence of K were investigated in two soils of micaceous clay mineralogy: an acidic Alfisol (Ultic Haploxeralf) and a calcareous Entisol (Typic Xerorthent). In both soils, as expected, the addition of NH4to the K-Ca system decreased the potential buffering capacity for potassium (PBCK), and the addition of K to the NH4-Ca system decreased the PBCNH4. The data also revealed that in these two solid, K and NH4at low exchangeable loads did not behave as the true exchange analogs one commonly expects. Ammonium was more capable of displacing K from the solid surfaces than K was of displacing NH4. This indicates that K-NH4interactions are complex. In certain soils, addition of NH4fertilizers might increase potassium availability, whereas in others, the opposite can be true.

ISSN:0038-075X    

出版商:OVID    

年代:1996

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1996

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1996

数据来源: OVID