摘要:

We examine plant water uptake from a uniformly rooted soil volume by numerically solving the nonlinear diffusion equation for a soil cylinder bounded externally by an insulating surface, located at the half-distance between adjacent roots and internally by the root surface. A sinusoidally varying uptake rate is employed to simulate the diurnal evaporative demand, while the actual water flux across the inner surface is controlled by the root surface matric potential through a stomatal adjustment function. The analysis is conducted for Chino clay, Pachappa sand, and Indio silt loam, using hydraulic parameters determined from piecewise continuous exponential functions of the dependent variable matching data for these soils. The analysis uses root radii of 0.003, 0.005, 0.010, and 0.020 cm and root densities of 0.080, 0.142, 0.318, and 0.650 cm root length per cubic centimeter of soil.Results show that the transient matric potential distribution in the soil depends strongly on the precise nature of the hydraulic prameters for each soil: as the soil loses water by transpiration, the value of the bulk soil matric potential decreases monotonically at a rate dependent upon the specific water capacity of the soil. For initial bulk soil potentials of −0.3 bar in the Chino and Indio, and −0.2 bar in the Pachappa, the initial rate of potential decrease with time is greatest in the Chino, followed by the Pachappa and Indio, respectively. The corresponding radial potential distribution around individual roots depends upon the instantaneous value of the soil water diffusivity, which decreases as the soil dries, creating steep potential gradients adjacent to the root, with the largest initial gradients occurring in the Chino clay, followed by equivalent gradients in the Pachappa sand and Indio silt loam. The relative rate of decrease in the bulk soil potential and the drawdown around a root for the three soils, however, changes with time as the soil dries. The drawdown around a root is reduced by increasing root densities, so that at low densities the root surface potential depends more closely upon the diffusivity, whereas at high densities the root surface potential depends more closely upon the specific water capacity. In addition, roots of smaller radii exhibit greater drawdown as the total root surface area decreases and the flux of water at the root surface increases. The simulation of root water uptake using piecewise continuous functional relationships for the diffusivity and specific capacity produces results that form a coherent illustration of plant survival in three diverse soil environments.

ISSN:0038-075X    

出版商:OVID    

年代:1984

数据来源: OVID

摘要:

We present a variance analysis for quantifying the different sources of errors induced on volumetric water content measurements and calculation of soil water storage with the use of a neutron moisture meter in one single access tube. For comparative purposes, we apply the analysis to field data obtained with two different probes. In each case the calibration curve is established by measuring simultaneously and independently neutron count rates and corresponding water contents. Two different approaches are considered, i.e. the unbiased treatment and the biased treatment. The unbiased treatment differs from the biased by its correction for measurement errors using the leastsquare technique. For the site under consideration, we show that the calibration component is the major contribution to the total variance associated with an individual water content estimation. The use of the unbiased statistical treatment notably decreases the total variance. In cases where the calibration curve is established very accurately, the instrument component can be reduced by increasing the number of count replications at each sampling point or the counting time or both. The loss of precision due to using neutron count rate ratios instead of simple count rates is negligible if several count replications are made in a standard medium or if the counting time is long enough. We show that the rule of integration of water content profiles in order to calculate water storage has a great importance: the use of Simpson's rule drastically decreases the associated variance as compared with the classical trapezoidal method.

ISSN:0038-075X    

出版商:OVID    

年代:1984

数据来源: OVID

摘要:

In acid peaty soils most of the copper in the soil solution is present as organic complexes. This paper describes methods for measuring total soil solution copper by flameless atomic absorption spectroscopy and estimating the proportion of this copper that is complexed using copper titration and a copper ion selective electrode. With these acidic peats of diverse origin, total copper in solution at natural pH values (3.4 to 3.5) ranged from 0.14 to 0.2 μM. Of this, 60 to 90% was complexed. Raising the pH of aqueous extracts to about 6.0 increased the degree of complexation of soluble copper in extracts of all three peats to >98%. We discuss the results in relation to current thinking about the nature of copper complexation by humic and fulvic acids.

ISSN:0038-075X    

出版商:OVID    

年代:1984

数据来源: OVID

摘要:

In laboratory studies on Cecil loamy sand and Webster sandy clay loam, I examined the influence of soil-water tension and soil temperature on14C-2,4-D degradation and the formation of nonextractable14C-residues, and I correlated the degradation rate with the growth rate of 2,4-D degrading microorganisms.14C-2,4-D rapidly mineralized in the two soils maintained at 0.1 and 0.33 bar of soil-water tension. Because the total amounts of metabolites in the solvent extracts never exceeded 5% of the total14C activity, the disappearance rate of extractable14C essentially represented the disappearance rate of extractable14C-2,4-D. Extractable14C in the14C-2,4-D treated soils maintained at 1 bar and below disappeared rapidly, and at the same time nonextractable14C rapidly formed, whereas extractable14C in soils maintained at 15 bars disappeared at much slower rates, and the formation of nonextractable14C was also slower. After 14 d of slow disappearance, however, extractable14C in the Cecil soil held at 15 bars started disappearing at a constant rate of 3.6% of applied14C per day. The disappearance rates in soils incubated at 35°C were generally smaller than those incubated at 25°C.Even though the initial most probable number (MPN) of 2,4-D degrading microorganisms in the Cecil soil was one fourth of that in the Webster soil, the organisms propagated more rapidly in the moist Cecil soil (0.33 bar) than in the moist Webster soil in response to the application of 10 μg 2,4-D/g of soil. As a result, 2,4-D in the Cecil soil degraded at a faster rate. The growth rates of 2,4-D degrading organisms in dry soils (15 bars) were small, in spite of the rapid degradation occurring in the Cecil soil after 14 d of incubation. In addition to forming nonextractable14C residues in humus components, as much as 4.2% of applied14C could be associated with microbial mass.

ISSN:0038-075X    

出版商:OVID    

年代:1984

数据来源: OVID

摘要:

We studied the adsorption and immobilization of zinc on six calcium- and sodiumsaturated soil samples that differed in their organic carbon content (OC), cation exchange capacity (CEC), and calcium carbonate (CaCO3). Homoionic (Ca- or Nasaturated) soils were equilibrated with chloride solutions of Zn2++ Ca2+and Zn2++ Na+at 10 meq L-1total electrolyte concentration. The data on adsorption equilibria were analyzed, using the thermodynamic approach, the Freundlich adsorption isotherm, and exchange selectivity coefficients.The values of standard free energy change of exchange reaction (ΔGr°) for the Ca2+- Zn2+system were negative and for the Na+- Zn2+system positive, anomalously suggesting a preference for Zn2+over Ca2+and a strong preference for Na+over Zn2+. A plot between logx(adsorbed Zn2+) versus logc(solution Zn2+) showed that these isotherms may be roughly divided into three parts: the first part showing a proportionately larger increase in logxper unit increase in logc, the second part with a relatively reduced slope, and the third part with a steep increase in logxfor a given increase in logc. The values of the Gapon selectivity coefficient (KG) for the Na+- Zn2+system were exceptionally high and gradually decreased with the increase in Zn2+concentration in the equilibrium solution. For the Ca2+- Zn2+system, the value of the exchange selectivity coefficient (K) did not follow any definite increasing or decreasing trend.Although the soil samples taken for this study differed considerably in their OC (0.1 to 1.9%), CaCO3(0 to 18.2%), and CEC (5.5 to 17.0 meq 100 g-1), the trend and magnitude of “apparently” adsorbed zinc in these soils differed only slightly. Possible conversion of a part of added Zn2+in soil to unidentified soluble or precipitated species, such as (Znx)+, Zn(OH)2, ZnCO3, Zn3(PO4)2, etc., seemed to mask the effects of OC, CaCO3, and CEC on adsorption and immobilization of zinc in these soil samples.

ISSN:0038-075X    

出版商:OVID    

年代:1984

数据来源: OVID

摘要:

Rapid degradation ofp-cresol in aerobic soil prohibited measurement of adsorption by the batch technique. Degradation ofp-cresol was not observed under anaerobic conditions over a period of 96 h. Adsorption ofp-cresol measured using the batch technique and soil incubated anaerobically demonstrated that adsorption equilibrium was attained within 24 h and that the FreundlichKvalues (K= 1.1) remained essentially constant for 96 h. Adsorption of 2,3-dichlorophenol and 2,4-dichlorophenol by aerobic, anaerobic, and autoclaved soil was compared. FreundlichKvalues describing adsorption of 2,3-dichlorophenol and 2–4 dichlorophenol by aerobic and anaerobic soil were similar. FreundlichKvalues obtained using autoclaved soil were significantly higher. These results indicated that an accurate measurement of adsorption of labile organic compounds can be achieved by using anerobic conditions to prohibit degradative losses.

ISSN:0038-075X    

出版商:OVID    

年代:1984

数据来源: OVID

摘要:

We used a simulation model based on the matric flux potential concept to describe two-dimensional infiltration from a line source. The model showed considerable stability with varying compartment sizes and trickle discharges. The simulated wetting patterns indicated closer agreement with published experimental data than did alternative models, especially under high rates of trickle discharge.

ISSN:0038-075X    

出版商:OVID    

年代:1984

数据来源: OVID

摘要:

We grew three successive crops of oats (Avena sativaL.) on Abitibi soils, either from individual horizons or from horizon mixtures. Soils were used as such or amended with 10 t of C/ha, added as peat or manure. Yield significantly decreased from the first to the third crop. In the third crop, yields of the manure-treated samples were significantly higher, at the 5% level, than those of the untreated samples. Adding an organic amendment caused a decrease of up to 9% of dry bulk density values. We also discuss the effects of cultivation on some chemical properties.

ISSN:0038-075X    

出版商:OVID    

年代:1984

数据来源: OVID