摘要:

The soil temperature submodel of the daily version of the CENTURY ecosystem model, DAYCENT, has been improved for freezing/thawing conditions in soils. The simulation of the soil heat propagation in the model is based on the Fourier heat flow equation and approximated by use of the explicit finite difference scheme. In our study, we propose a tipping bucket algorithm to incorporate the discontinuous freezing/thawing process in the model by means of a calculated fusion energy pool. Daily average, maximum and minimum soil temperatures for each soil layer are estimated. The soil thermal diffusivity for each soil layer is simulated by the method of De Vries using upper and lower limits as well as a correction factor. A correction factor was optimized for the tipping bucket approximation of the freezing/thawing process. The model was tested and validated at three different sites with different climates and soils and showed good agreement with measured soil temperatures during winter periods. The mean deviation for maximum soil temperatures at 10 cm depth during the winter period was less than 1 °C for all locations; for the minimum temperatures the mean deviation was slightly higher. At a depth of 20 cm, mean deviations were higher but can nevertheless be seen as showing good results for the given approximations.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Seal formation at soil surfaces is significantly affected by raindrop kinetic energy (KE). Soil sealing enhances runoff and soil erosion. It has been hypothesized that in soils exposed to rain (i) runoff and interrill erosion levels in soils previously irrigated with effluents differ from those in soils previously irrigated with fresh water (FW), and (ii) that the difference in runoff and interrill erosion between the previously irrigated effluent and FW samples depends on rain KE. The effects of four droplet KE levels (3.6, 8.0, 12.4, and 15.9 kJ m−3) of distilled water (DW) on runoff and wash erosion of four smectitic soils that had been irrigated for many years with fresh water or effluents was studied with a drip-type rain simulator. The soils used were a loamy loess (Calcic Haploxeralf) from Be'er Sheva Valley, a dark brown sandy clay vertisol (Chromic Haploxerert) from the Pleshet Plains (vertisol HH), and two dark brown heavy clay vertisols (Chromic Haploxerert) from Zevulun Valley (vertisol Y) and Western Galilee (vertisol E), Israel. For all soils, total runoff and wash erosion values increased with the increase in rain KE. Total runoff levels were lower for the FW-irrigated samples than for the effluent-irrigated soils, except for the highest KE where no differences in runoff between the two types of soils was noted. Conversely, for each soil and each rain KE, wash erosion was higher in the effluent-irrigated samples than in the FW-irrigated ones. Total runoff was affected mainly by changes in rain KE at the lower to medium range of the KE studied. Thus, the effects of irrigation water quality (i.e., effluents vs. FW) on total runoff was pronounced mainly at the low-to-moderate rain KE range; at high rain KE, the effects of irrigation water quality on runoff were negligible. Wash erosion was affected mainly by changes in rain KE at the medium-to-high KE range studied. That wash erosion increased markedly at the high rain KE values, despite the negligible change in runoff level, suggested that for KE ≥ 12.4 kJ m−3, seal formation was completed, and the contribution of runoff in facilitating transport for the entrained material was secondary to the role of soil detachment in determining wash erosion. The dependence of wash erosion (through soil detachment) on rain KE was greater than its dependence on the quality of the irrigation water that the soil was exposed to in the past (FW vs. effluent).

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Molecular weight (MW) of humic materials is a key factor controlling proton and metal binding and organic pollutant partitioning. Several studies have suggested preferential adsorption of higher MW, more aromatic moieties to mineral surfaces; quantification of such processes is fundamental to development of predictive models. We used high pressure size exclusion chromatography (HPSEC) to quantify MW changes upon adsorption of a muck fulvic acid (MFA) extracted from a peat deposit to kaolinite and goethite, at pH 3.7, 6, and 8 at 22 °C, I = 0.01 (NaCl), 24-h reaction time. MFA adsorption affinity was greater for goethite than for kaolinite. At concentrations less than the adsorption maximum (Amax) for both adsorbents, the weight-average MW (Mw) of MFA remaining in solution decreased by as much as several hundred Daltons relative to control samples, indicating preferential adsorption of the higher MW components. At concentrations more than Amax, Mwof MFA in solution did not change appreciably. Although total adsorption decreased significantly as pH increased, fractionation as measured by change in Mwremained similar, perhaps indicating greater selectivity for higher MW components at higher pH.Absorptivities at λ = 280 nm normalized to mg C L−1(ϵ) suggested preferential adsorption of more aromatic moieties to kaolinite. ϵ could not be used for goethite-reacted samples because high Fe concentrations in the aqueous phase brought about by goethite dissolution interfered with the spectroscopic analysis. Preliminary kinetic experiments suggested that smaller molecules adsorbed first and were replaced by larger molecules whose adsorption was thermodynamically favored.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Most studies about lime requirement and the pH buffering capacity of soil have either calibrated buffer solutions to soil-CaCO3reactions or have correlated pH buffering with soil properties such as clay and organic matter content. A different approach is to address two fundamental questions: where do OH−ions react when they are added to acid soils, and can we quantify these reactions? The experimental hypothesis was that alkali added to a soil (cmol/kg) could be accounted for quantitatively by summing the increase in effective cation exchange capacity (ECEC) (cmol/kg), the decrease in exchangeable Al (cmol/kg), and, possibly, the decrease in exchangeable Mn (cmol/kg). It was presumed that this model would begin to fail as pH approached neutrality, where the precipitation of CaCO3would start to exert an influence on pH buffering capacity. Hence, this model was intended to account for the reaction of applied OH−in the approximate range 4 < pH(CaCl2) < 7.Twelve soils were titrated with Ca(OH)2at near constant ionic strength and soil:solution ratio so as to minimize changes in (H+) caused by redistribution between solution and adsorbed/exchangeable phases. After 16 h of reaction, the exchangeable cations were measured in each soil at each concentration of Ca(OH)2addition. The data indicated that exchangeable Mn was not a sink for OH−under the experimental conditions. Precipitation of Ca2+occurred during the reaction period, causing an apparent loss of the applied alkali. However, this reaction could be expected to reverse over a longer period of time and thus was considered an experimental artifact rather than a component of buffering capacity. The experimental artifact could be overcome by expressing pH buffering in terms of the measured slope of pH change versus the sum of measured changes in ECEC plus exchangeable Al. It was concluded that in the pH(CaCl2) range 4 to 7, the soil sink for added alkali can be accounted for quantitatively by the increase in ECEC plus the decrease in exchangeable Al. Therefore, soil pHBCin the alkaline direction can be estimated from changes in ECEC and exchangeable Al.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Because acidic soils limit the establishment, growth, and persistence of forage legumes, new information about symbiotic development in these soils, particularly during the early stages, will likely contribute to the solution of this worldwide problem. A previously described laboratory model system (Staley, T. E. and D. R. Morris. 1998. Soil Sci. 163: 230-240) using an acidic (pHw4.62), fresh Ultisol showed the ability to discern enhancement of growth and nodulation of white clover (Trifolium repensL.) as early as 3 weeks after planting, even when limed to only pHw4.83. We now report on the reproducibility of soil chemical alterations and plant and nodule responses using the identical system with soil collected from the same field plot about 1 year later. The effects of low-level liming rates (pHw4.54-4.94) and rhizobial (Rhizobium leguminosarumbv.trifolii) inoculation levels (50-5000 CFU g−1dry soil), of both fresh and re-wet soil on earlier (2 weeks) and later (4 weeks) growth and nodulation of Huia white clover were determined. Plant parameters (shoot mass, shoot N uptake, and root mass) were generally unaffected by liming at any of the inoculation levels at Cut 1 (2 weeks) in both soils, whereas most of these parameters were positively affected by liming at all inoculation levels at Cut 2 (4 weeks), especially in the fresh soil. Positive lime rate effects on nodulation were found at both cuts in both soils, but almost exclusively at the highest inoculation level. Thus, the interactive effects of liming and rhizobial population on nodulation were demonstrated, at 2 weeks, before significant N2-fixation. Growth rates of asymbiotically grown roots, determined by short-term (hours) assays in soil-over-agar, paralleled the root mass responses to liming in the soil model system. Rhizobial viability, assessed in soil solutions extracted from the limed soils, decreased from about 107to 106CFU mL−1in 21 h. These results suggest that although seedling root growth and likely rhizobial viability are compromised within hours after planting and inoculation by soil acidity-related chemical factors, maintenance of a threshold level of viable rhizobia can lead to improved nodulation of white clover even at very low soil pH values.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

We evaluated soil reserve potassium mobilized by plants in a greenhouse K exhaustion experiment from 15 smectite-dominant soils in relation to soil K status determined by chemical estimates and electro-ultrafiltration (EUF) fractions of K and soil mineralogy. Cumulative K uptake from the soils over a period of 420 days of cropping varied from 160 ± 11 to 823 ± 78 mg kg−1, and the plant mobilization rate of soil reserve K varied from 0.151 ± 0.009 mg kg−1day−1to 1.880 ± 0.164 mg kg−1day−1. Soils with higher K estimates by chemical and EUF methods showed greater K uptake and a higher plant mobilization rate of soil reserve K. The plant was used as an extractant in place of the usual chemical extractants for studying release kinetics. Kinetic studies indicated that mobilization of soil K by plants could be explained best by the parabolic diffusion equation (r= 0.985-0.998; SE=0.016-0.059) among the different mathematical models used, suggesting that plant mobilization of interlayer K is a diffusion-controlled exchange process. Release rate constants varied among the soils from 45.2 h−1/2× 10−3to 62.1 h−1/2× 10−3. The relationship of the mobilization rate of reserve K with soil mineralogy indicated that mica in silt, as well as in the clay fraction of soils, was a major source of K for K uptake by plants.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Quantitative identification of lithologic discontinuities (LDs) is essential for the study of pedogenesis. This study examines three representative soils on three landscape positions-summit, upper backslope, and lower backslope-along a toposequence in southern Taiwan. The objectives were to evaluate whether LDs in the residuum of parent material occurred in these soils which formed in a subtropical region of Taiwan and to test the utility of various parameters for the determination of LDs. The depth of LDs was identified by examination of the depth functions of clay-free particle sizes, sand/silt ratios, uniformity value indices, and Ti, Zr, and Ti/Zr ratios. The soils, derived from sandstone and shale, are Typic Paleudults. The results of this study indicated that the recognition of LDs in these soils was difficult in the absence of different particle size distribution in field observations and distinct Ti and Zr elements analysis in laboratory analysis. Pedogenic processes have blurred or destroyed the evidence of such discontinuities. Results indicated that clay-free particle size distributions (Cf-PSDs), sand/silt ratios, and uniformity value (UV) indices are the most useful parameters in identifying the LDs in this toposequence. The depth functions of the contents of Ti and Zr, and the Ti/Zr ratios in the clay-free sand fraction, were unable to identify the discontinuities of parent materials in this site.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID