ISSN:0038-075X    

出版商:OVID    

年代:1997

数据来源: OVID

摘要:

More than a half century ago,timewas popularized as one of the state factors of soil formation. The accompanying concept of the “chronosequence” significantly impacted research efforts to understand the role of time in soil genesis. Two difficulties have plagued chronosequence soils studies: (i) small data sets (limited by what is available in nature), and (ii) variability within the data sets (introduced by geographical variations in other soil-forming factors). This paper describes a theoretical approach to developing more accurate chronofunctions in transgressive estuarine pedogenetic systems by minimizing these obstacles; it is here called thechrono-continuum. The encroachment of the sea on upland soils during the most recent coastal transgression caused a dramatic shift in pedogenetic processes, which brought about a number of significant changes in physical, chemical, mineralogical, and biological soil properties. The slowly rising sea level (a few mm/yr) has permitted the establishment of tidal marsh vegetation. The subsequent vertical accretionary growth of marsh vegetation generally has kept pace with sea-level rise. The rate of sea-level rise during an interval of marsh soil formation can be determined by various dating techniques, which can be related to the elevation of the submerged land surface through a topographic survey. In this way it is possible to recreate the relationship between sea level and the land surface at any point in the history of the marsh. Thus, the continuum in space (along the topographic surface) can be translated into a continuum in time. This permits one to sample a large number of points along the landscape that represent different points in time (addressing the first difficulty), and this can be done within a relatively small geographic distance (addressing the second difficulty).

ISSN:0038-075X    

出版商:OVID    

年代:1997

数据来源: OVID

摘要:

An important step in research involving the solid-liquid interface is obtaining contaminant-free samples because accurate information concerning reaction paths and mechanisms can only be deduced from clean surfaces. It is necessary to consider the carbon dioxide species as potential sample and surface contaminants as they are pervasive in nature. This study evaluates the efficiency of pure air in removing adsorbed CO2from a titanium dioxide suspension. The purging process is monitored using a three-chamber apparatus. The first chamber of deionized water humidifies the pure air as it enters the apparatus. The pure air flow causes a shift in equilibria in the second chamber which holds the sample. This shift leads to CO2desorption from the liquid suspension, and, ultimately, from the oxide surface. The third chamber, consisting of an alkaline solution maintained at a pH of 9.000, functions as a trap for the desorbed CO2. The system can be calibrated by purging known quantities of NaHCO3. The time dependence of the desorption process is described by first order kinetics. The calculated apparent desorption rate constant kDes 9value was (4.41 ± 1.24) × 10-3min-1.

ISSN:0038-075X    

出版商:OVID    

年代:1997

数据来源: OVID

摘要:

Previous studies have examined the effects of soil osmotic potential (Ψs) on net rates of mineralization and nitrification. Because net rates represent the difference between gross production and consumption processes, it is unclear which process is being affected. We used an15N isotopic dilution method to evaluate the effects of Ψson gross rates of nitrification, ammonification, NH+4assimilation, and NO-3assimilation, and net rates of nitrous oxide production in a Penoyer sandy loam at field capacity. To avoid creating specific ion toxicities that normally do not occur in this soil, we used a chemical equilibrium model to predict how solute concentrations in the soil solution change during evapo-concentration; then we used solutions containing these mixtures of solutes to create individual Ψstreatments. A nitrification potential assay was also performed to determine the effect of Ψson nitrification rates at high substrate concentrations. In soil slurries with elevated NH+4concentration (1110 μM), nitrification rates declined exponentially with reduced Ψs(increased salt concentration); however, in soil samples incubated at field capacity without added NH+4(9.7 μM, or 2 mg N kg-1), the gross nitrification rate was independent of Ψs. The differential response between slurries and soil at field capacity was attributed to differences in NH+4concentrations, and indicated that the effects of Ψswere secondary to NH+4concentrations in controlling nitrification rates. Nitrification rates in slurries declined more when a single salt (K2SO4) was used than when the mixture of salts that more closely approximated the solute composition predicted to occur in the field was used to lower Ψs. This suggests that nitrifying bacteria are capable of adapting to specific ion toxicities. Gross rates of ammonification declined exponentially with decreased Ψsbetween 0 and -500 kPa but were independent of Ψsat potentials of -500 to -1750 kPa. Rates of microbial assimilation of NO-3exceeded NH+4assimilation by a factor of 4, indicating that under NH+4limited conditions substantial NO-3assimilation can occur. Microbial assimilation of both NH+4and NO-3declined exponentially with decreased Ψs, and were insignificant at <-1500 kPa Ψs. Because NO-3assimilation declined more rapidly than gross nitrification, net nitrification rates actually increased with declining Ψs. Rates of nitrous oxide (N2O) production were also inversely correlated with Ψs. Our results indicate that in previous studies, measurement of net rates, use of inappropriate salts, and addition of substrate may have resulted in over-estimation of the adverse effects of low Ψson rates of N-transformations.

ISSN:0038-075X    

出版商:OVID    

年代:1997

数据来源: OVID

摘要:

Complexation of HA (and FA) by goethite, hematite, gibbsite, and boehmite was studied in pre-dried systems. Hematite showed the highest fixation of HA at various oxide: HA ratios and at all ph ≥ 7.0. A gradual reduction in HA/FA fixation from 2.0 to 10.0 was observed for all minerals except gibbsite, which showed a very sharp decrease at pH > 7.0 and a maximum at pH 5.0. Exchangeable cations have a remarkably dissimilar influence on HA complexation by the four minerals. Thus, various cationic forms of gibbsite showed a drastic loss of HA fixation capacity compared with the original (pronated) surface, whereas in boehmite, the reverse behavior was observed. Most of the metal ion-substituted hematites showed excellent HA fixation, but goethites revealed a mixed trend. It is inferred that two major modes of HA bonding are operative in hematite and goethite, viz., cation bridges forming oxide-M-HA links and direct bonding of HA to coordination centers at the oxide surface; forces of such bonding are strongest in hematite. In boehmite, cation bridging is the major interactive mode, whereas in gibbsite, HA fixation occurs primarily to coordination centers at the surface.

ISSN:0038-075X    

出版商:OVID    

年代:1997

数据来源: OVID

摘要:

The processes of adsorption and desorption of Hg(II) by 15 soils were investigated. Within an initial Hg solution concentration range of 1.0 × 10-7to 1.1 × 10-4M, all soils with a large organic matter content (> 11.0 g C kg-1) had an S-type isotherm, whereas all soils with a low organic matter content (< 7.5 g C kg-1) had an L-type isotherm. Changing the concentration of NaNO3background electrolyte had no effect on adsorption, suggesting that adsorption was not dominated by electrostatic interaction. Mercury(II) speciation calculations indicated that <4% of the total Hg could be a adsorbed via electrostatic interaction. A model that incorporated the interactions of aqueous Hg with both dissolved organic matter and soil solids was developed to describe the isotherms and to evaluate the adsorption capacities of soils. The adsorption capacities correlated with the soil organic carbon content (R2=0.92). The adsorption and desorption isotherms did not follow the same track. Soil organic matter was the most important component responsible for the observed hysteresis. The restricted diffusion of Hg through organic matter was probably the major factor that resulted in the hysteresis.

ISSN:0038-075X    

出版商:OVID    

年代:1997

数据来源: OVID

摘要:

Anthropogenic Pb is the world's largest and most widespread heavy metal contamination. Inspired by recent evidence suggesting a faster redistribution of Pb through the mineral soil profile than was previously expected, we investigated the effects of rodent activity on Pb redistribution. Total Pb was analyzed at the 0-1, 1-4, and 4-7-cm depths in a rodent-affected soil and in an undisturbed soil, in the same proximity and with the same parent material, in the Box Springs Mountains near Riverside, California. Six replicate sites of each condition were sampled. Lead was recovered by a digest in 4MHNO3and measured using a graphite furnace atomic absorption spectrophotometer. Anthropogenic Pb content to a 7cm depth averaged 19 mg kg-1in undisturbed soils and 10 mg kg-1in rodent-affected soils. In both soils, the highest concentrations of Pb were located in the top 4 cm of the profile. After accounting for an estimated native Pb (≈3.3 mg kg-1), we determined that 20 to 38 kg ha-1Pb has been deposited on these soils through air pollution. Our findings suggest rodents significantly modify the distribution of anthropogenic Pb in the rodent-affected soils of the Box Springs Mountains primarily in two ways: (i) by reducing Pb concentration in surface soils, thereby decreasing the potential for erosional redistribution of Pb, and (ii) by decreasing Pb transport time through the soil profile as a result of physical mixing. This redistribution mechanism is likely applicable to other surface deposited anthropogenic contaminants that have similarly low soil mobility.

ISSN:0038-075X    

出版商:OVID    

年代:1997

数据来源: OVID

摘要:

The applicability of an equation that suggests the effective diffusion coefficient of ions through a charged porous medium is linearly related to the square root of the concentration of ions is evaluated. Data from experiments on diffusion of NaCl and LiCl solutions through Na and Li bentonites were analyzed using the proposed equation. Plots of effective diffusion coefficient versus square root of concentration are consistent with the formulation that the effective diffusion coefficient declines linearly with the square root of concentration. The analysis also indicates that effective diffusion coefficient of an ion is dependent on the counter ion in solution as well as that at the clay exchangeable site. For solutions up to 0.07 meq/cc, the results further indicate that it is feasible to combine all the porous medium characteristics into a single factor referred to as the apparent tortuosity factor.

ISSN:0038-075X    

出版商:OVID    

年代:1997

数据来源: OVID

摘要:

Quantity-intensity (Q/I) relationships for phosphate (P) in long-term cropping systems were studied for soils obtained from two research sites; Clarion-Webster Research Center (CWRC) at Kanawha and Galva-Primghar Research Center (GPRC) at Sutherland, in Iowa. Soil samples were collected from three cropping systems: continuous corn (CCCC), corn-soybean-corn-soybean (CSCS), and corn-oats-meadow-meadow (COMM), with or without N fertilizer treatment. In these studies, a 1-g sample of <60-mesh (250 μm) soil was equilibrated with 25 mL of standard KH2PO4solution containing 0 to 10 mg P L-1at 25°C for 24 h. Inorganic P in the filtrate was determined. Quantity-Intensity diagrams were constructed by plotting the amount of P adsorbed by or desorbed (ΔP) from the soil against the solution P concentration. Statistical analysis (ANOVA) revealed significant differences in P adsorption/desorption among replicates of the experimental field plots as the result of spatial variability of phosphate in these fields. Results showed that Q/I parameters (intercept labile P,a; equilibrium buffering capacity, EBC; and equilibrium P concentration, EPC) varied significantly between and within sites for the cropping systems studied. Results also showed that N application, in general, increased the equilibrium buffering capacity of the soils, leaving less P in the labile and solution phases. At the CWRC site, application of N fertilizer significantly increased the EBC for soils under various cropping systems in the order: COMM > CSCS > CCCC. At the GPRC site, however, the order was CCCC > COMM > CSCS. Theavalues were correlated significantly with the results of seven P tests evaluated, and thervalues ranged from 0.637**to 0.846***. Theavalues were significantly, but negatively, correlated (r= -0.743***) with the ECB values, and positively correlated (r= 0.893***) with the EPC values. Useful relationships were developed between EBC and organic C, CEC, clay content, and exchangeable Ca.

ISSN:0038-075X    

出版商:OVID    

年代:1997

数据来源: OVID

摘要:

Low phosphorus availability is a critical impediment to agricultural use of Vertisols. Several studies have explored the P sorption characteristics of Vertisols, but few have investigated the relationship between soil physicochemical and the P sorption parameters calculated from uniform-surface and two-surface Langmuir equations. Phosphate sorption was investigated in surface-samples of 19 Vertisols in southwestern Spain. The phosphate sorption maxima (PAM) deduced from the uniform-surface Langmuirequationranged from 150 to 2566 mg P kg-1, with a mean value of 1115 mg P kg-1. The calculated sorption maxima (TPAM) for the two-surface Langmuir model ranged from 410 to 15406 mg P kg-1, and the mean value (4296 mg P kg-1) increased about 3 times compared with the basic Langmuirequation.The two-surface Langmuir model fit the sorption data better than the uniform layer model. Therefore, an underestimation of the P-sorption was observed when the traditional Langmuirequationwas used. Total surface area was highly correlated with sorption maxima:n= 0.708***for PAM and 0.900***for TPAM. For the two-surface equation, correlation analysis suggested that the “active” CaCO3(ACCE) was the most active sorbent of P on the high energy sites, whereas citrate-dithionite-bicarbonate extractable Fe (Fed) contributed to P sorption on the low energy sites. The prediction of the PAM and TPAM was improved by combining all or several of following soil properties: Fed, ACCE, clay, and total surface area in a multiple-regression analysis. The equations obtained could offer a rapid estimation of P-sorption in Vertisols of southwestern Spain. Moreover, the results obtained from a greenhouse experiment withAgrostis tennuisand two levels of P (0 and 218 mg P kg-1) showed that the amount of available soil P was a positive function of Fedbut a negative function for ACCE.

ISSN:0038-075X    

出版商:OVID    

年代:1997

数据来源: OVID