摘要:

Flow calorimetry can provide useful information about surface chemical reactions in soils that cannot be obtained readily by other methods. When flow calorimetry is conducted over a range of surface coverages, different sorption heats can be calculated to yield information about how binding energies vary with coverage, i.e., surface heterogeneity. The purpose of this study was to determine heats of exchange for K/Ca and K/Pb systems using flow calorimetry and to evaluate the degree of surface heterogeneity with respect to cation exchange. Surface horizon samples from a Typic Acrorthox and Typic Tropohumult from Puerto Rico were used. Lead was adsorbed specifically in both soils, but no adsorption heat was detected for this reaction in either soil. However, heats associated with reversible cation exchange between K and Pb were observed. Heats for K/Ca exchange were greater than those generated for K/Pb exchange in both soils. Heats of exchange were greater in the Ultisol than in the Oxisol. The differential heats of exchange were independent of exchange composition for both K/Pb and K/Ca exchange in the Oxisol, indicating that all cation exchange sites were similar energetically. In the Ultisol, the differential heats of exchange increased as exchangeable K decreased, indicating that the exchange sites were not similar energetically. These differences were attributed to the presence of smectite in the Ultisol, which was able, in part, to collapse when saturated with K.

ISSN:0038-075X    

出版商:OVID    

年代:2002

数据来源: OVID

摘要:

Flow calorimetry, which is ideally suited for measuring reactions occurring at the liquid/solid interface, has been used to study the surface chemistry of many types of solids, but little use of it has been made in the study of surface reactions of soils. The purpose of this study was to demonstrate the application of flow calorimetry to the study of two fundamental soil chemical processes, namely cation exchange and phosphate sorption. Surface horizon samples of a Typic Acrorthox and a Typic Tropohumult from Puerto Rico, a strong acid cation exchange resin (Dowex 50W-8), and an amorphous Al(OH)3were used. Heats for K/Ca exchange on the Dowex resin and the Oxisol, and K/Na exchange on the Ultisol, were consistent with literature values that were obtained using conventional batch calorimetry or derived from the temperature dependence of the exchange constant. Although peak areas associated with a given pair of exchange reactions were equal, peak shapes were generally not equivalent, indicating differences in the rate at which the two reactions occurred. For example, Ca displacing exchangeable K occurred more rapidly than the reverse reaction on the Dowex resin. The reaction of phosphate with the Ultisol and amorphous Al(OH)3was exothermic. Exposure of the soil to several cycles of phosphate was sufficient to saturate the sorption sites, as evidenced by the loss of a detectable heat signal. However, phosphate reactive sites were regenerated by flushing the column with a salt solution at pH 10. Precipitation of Al-phosphate was shown to be endothermic, confirming that precipitation was not the primary mechanism for phosphate sorption in this study. The results of this study show that flow calorimetry can provide valuable information about surface chemical reactions in soils that cannot be obtained readily by other methods.

ISSN:0038-075X    

出版商:OVID    

年代:2002

数据来源: OVID

摘要:

Addition of polysaccharides to soils or their formationin situthrough microbial activity increases the stability of soil aggregates. Like other components of soil organic matter, polysaccharides are expected to affect strongly the flocculation and dispersion behavior of clays, which are the most important soil component. The influence of polysaccharide concentration, molecule characteristics, and pH, on flocculation and rheological properties of Na-montmorillonite were studied. Flocculation values (FV) increased with increasing concentrations of the anionic polysaccharide polygalacturonic acid (PGA). The pattern was similar at pH 4 and 8: a relatively sharp increase in FV with small additions of PGA up to a concentration of 10 mg L−1(≈136.0 and 160 mmol L−1) and a slight to moderate increase thereafter. At pH 6 a different pattern was observed. The FV changes at the lower PGA concentrations (up to 10 mg L−1) were similar to those observed for pH 4. At higher concentrations, a further sharp increase in the FV was found. Addition of 1 mg L−1PGA at pH 10 resulted in a FV increase to 37.0 mmol L−1. For higher PGA concentrations, the FV remained basically unchanged.Results obtained by measurements of the rheological properties of suspensions to which PGA was added provided additional proof for the clay-PGA interaction and followed similar trends. The Na-montmorillonite suspension exhibited non-Newtonian flow properties. When PGA was added to the Na-montmorillonite suspension, the flow behavior gradually changed from non-Newtonian to Newtonian as the PGA concentration increased. The mechanisms proposed to explain the interactions between Na-montmorillonite and the anionic polysaccharide (PGA) are (i) edge charge reversal and (ii) mutual flocculation (heteroflocculation). These mechanisms are similar to those proposed previously for suspensions of Na-montmorillonite to which humic substances (HS) were added. The dissimilarities are, apparently, a result of the differences between structure and characteristics of the molecules of these organic materials.Additions of noncharged polysaccharides (dextrans) resulted in completely different responses. Additions of the low molecular weight dextran (T-40, MW 40000) did not result in significant changes in the FVs of Na-montmorillonite. In contrast, two neutral polysaccharides tested (T-500, MW 500,000 and T-2000, 2,000,000) enhanced clay flocculation and consequently caused a sharp decrease in the FV.

ISSN:0038-075X    

出版商:OVID    

年代:2002

数据来源: OVID

摘要:

The biodegradative capabilities of ten rhizobial species and four field isolates were evaluated in this study using growth studies, dense suspension cultures, and soil studies. After a preliminary screening, selected species were chosen for their degradative effect on aromatic, halogenated aromatic, and aliphatic substrates. The substrates utilized were: benzoate, phenol, 2-hydroxybenzoate, 3-hydroxybenzoate, 4-hydroxybenzoate, catechol, resorcinol, 2,3-dihydroxybenzoate, 2,4-dihydroxybenzoate, 2,6-dihydroxybenzoate, 3,4-dihydroxybenzoate, phenoxyacetate, 2,4-dichlorophenoxyacetate, cresols, mandelates, phthalate, 2-bromobenzoate, 2-chlorobenzoate, 4-chlorobenzoate, 2,4-dichlorobenzoate, 2-chlorophenoxyacetate, 4-chlorophenoxyacetate, 2-chlorophenol, 4-chlorophenol, 2,4-dichlorophenol, and hexadecane.Sinorhizobium xinjiangensis,Rhizobium leguminosarum,R. leguminosarumbiovartrifolii,Bradyrhizobium japonicum, and strain V2 (field isolate) were able to metabolize a variety of organic compounds such as hydroxy- and dihydroxybenzoates, halogenated aromatic compounds, and alkanes. Significant differences were observed among the strains showing interspecies variability in their response to the different compounds. Dense cell suspension experiments conducted evaluating substrate loss, 4-chlorophenol, 4-chlorobenzoate, 4-chlorophenoxyacetate, cresols, resorcinol, and mandelate, among others, were metabolized although they did not support growth. Soil studies were performed showing that the addition of rhizobial inocula increased degradation of hexadecane in a soil environment.

ISSN:0038-075X    

出版商:OVID    

年代:2002

数据来源: OVID

摘要:

Coal combustion by-products may offer significant benefits if used properly to neutralize soil acidity, but unintended release of trace components must be considered. A study was conducted with two objectives: (i) To compare the efficacy of two different preparations of fly ash with that of conventional ag lime for their ability to raise soil pH and reduce exchangeable Al; and (ii) to determine if the Al applied in fly ash produces detrimental changes in soil properties following subsequent acidification. Either fly ash in one of two forms, or conventional ag lime, was applied to three acid soils (Anselmo loam, Valentine sandy loam, and Holdrege sandy loam) in a pot study at rates equal to 0.5, 1.0, and 1.5 times the soils’ lime requirements. Soils were equilibrated in triplicate at approximately 33 kPa water potential in the greenhouse for 315 days (liming phase), during which pH and exchangeable aluminum (Al) were measured. The soils were then acidified under similar conditions for 439 days (acidification phase) by adding dilute acid solution to simulate management-induced acidification, and pH and exchangeable Al were then measured again. Both fly ashes and ag lime were effective in raising soil pH by up to 1.2 units and in reducing exchangeable Al by up to 5.6 mg kg−1. Two-way interactions involving soil, liming material, and rate of application produced different results for combinations of these factors. All amendments helped the soils resist subsequent acidification compared with zero-rate treatments but differed based on the 3-way combinations of soil, liming material, and rate of application. We concluded that overliming (as indicated by exceeding the target pH of 6.5) is a problem with all liming materials on the coarsest soils, suggesting that lime calibration should be re-examined. The fly ash materials seem to contribute to soil exchangeable Al after acidification, but this contribution is inconsequential if soil pH values are maintained at agronomic optima.

ISSN:0038-075X    

出版商:OVID    

年代:2002

数据来源: OVID

摘要:

Plant species and plant communities can alter element cycling promoting divergent pathways of soil development. We hypothesized that the exotic invasive crucifer,Lepidium latifolium, altered biogeochemical cycling relative to pre-existing vegetation such that sodium-affected subsoils are ameliorated. We tested this hypothesis at the Fleming unit within the Honey Lake Wildlife Refuge in northeastern California by comparing the distribution and cycling of various elements in paired L. latifolium invaded and noninvaded sites occupied byElytrigia elongata.Soils are fine, smectitic, calcareous, mesic Fluvaquentic Endoaquolls. In 1998, per unit area, live biomass ofL. latifoliumwas more than 3 times greater than that ofE. elongata.Content of C, Ca, Mg, K, and S in aboveground tissue ofL. latifoliumwas significantly (P ≤ 0.05) greater than content inE. elongata.Biogeochemical fluxes of C, N, P, Ca, Mg, and S were from 3 to 11 times greater inL. latifolium-occupied areas than inE. elongata-occupied areas.L. latifoliuminvasion fostered elevated soil solution levels of Mg+2and Ca+2, thereby reducing sodium adsorption ratios that could lead to sodic soil amelioration. Invasion byL. latifoliumhas the potential to alter soil properties and processes, thereby altering the trajectory of soil evolution.

ISSN:0038-075X    

出版商:OVID    

年代:2002

数据来源: OVID

摘要:

In several states in the Northeastern U.S., the Morgan or Modified Morgan soil tests are used as the basis for both fertilizer recommendations and P runoff risk indices. However, private laboratories servicing these states typically use the Mehlich-III solution as their basic soil test extractant. To meet Natural Resources Conservation Service standards for nutrient management and to use land grant university research databases to derive recommendations, accurate conversions from Mehlich-III and Modified Morgan to Morgan are needed. A study was conducted in New York to: (i) develop models that convert Mehlich-III P, K, Ca, and Mg soil tests to Morgan equivalents; and (ii) evaluate the impact of the use of these prediction models on agronomic recommendations for corn and the New York P index. Soils from 235 locations (27 different New York soil types) were analyzed for pH, modified Morgan P, and Morgan-and Mehlich-III extractable P, K, Ca, Mg, Fe and Al. Multiple regression analysis was used to derive conversion models that fit the data. The models were validated, and the impact of their use on fertilizer recommendations was assessed using a set of >10,000 independently collected New York soil samples. Morgan and Mehlich-III extractable K, Ca, and Mg were correlated linearly with slopes approaching 1. For P, the best model fit was obtained using pH, Mehlich-III P, Ca, and Al as independent variables (r2= 0.88). A slightly less reliable prediction was obtained without Al (r2= 0.82). The use of Morgan equivalents did not alter the percentage of low, medium, high, and very high soils in the New York data set, nor did it affect the P index classification. Recommendations for corn derived from a database with independently collected Mehlich-III soil test data (without Al) were identical to those obtained using measured Morgan values for 57% of all samples. An additional 32% showed <10 kg P2O5ha−1difference. Eight percent predicted P applications that were 15–20 kg P2O5ha−1larger or smaller, whereas 3% showed deviations >20 P2O5ha−1. Inclusion of Mehlich-III Al data is expected to improve the accuracy of the recommendations and assessment of the P index. We conclude that conversion equations can be used to derive accurate Morgan equivalents based on pH, Mehlich-III P, Ca, and Al, but the results need to be verified based on land use history.

ISSN:0038-075X    

出版商:OVID    

年代:2002

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:2002

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:2002

数据来源: OVID