摘要:

Air-pocket tensiometers with pressure transducer sensors are used widely in routine monitoring of soil water potential and in studies of soil water movement. Air temperature fluctuations in the exposed head space of the tensiometer can cause large air pressure changes leading to misinterpretation of the soil water potential. Treating the tensiometer head space as an ideal gas with water vapor in dynamic equilibrium, a set of coupled equations is derived that describes the relationship between head space air pressure, the rate of temperature change, the conductance of the tensiometer cup, and the head space volume. The interaction of these factors is illustrated through a series of simulations and then demonstrated with laboratory and field experiments. Air pressure deviations as large as ±80% were observed in tensiometers of low cup conductance and large head space when exposed to rapid temperature change. When the temperature change is slow relative to the cup conductance, changes in air-pocket pressure are corrected by water flow across the cup, and interpretation errors are small. Air pressure deviations from both rapid and slow temperature change may be estimated with the simulation equations using an approximation for the water pressure gradient across the tensiometer cup. The most practical outcome of this research is the theoretical and experimental confirmation that air temperature effects are reduced by maintaining high water levels in the tensiometer (that is, minimizing the air-pocket volume) and by maximizing the tensiometer cup conductance. In the field, insulating or shading tensiometers reduces temperature-related errors in general but may aggravate the problem if removal of the insulation (as might be necessary to read the tensiometer) causes a sudden temperature change in the air-pocket.

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID

摘要:

The forms of nitrogen (N) in fertilizers and soil properties influence gaseous loss of N by denitrification. A laboratory incubation study was conducted to evaluate the denitrification losses from liquid or dry granular forms of ammonium nitrate (at 200 mg N kg−1soil) as nitrous oxide in acetylene-inhibited systems. Myakka fine sand (Spodosol), Parkwood sand (Alfisol), and Tavares fine sand (Entisol) sampled at 0 to 15 cm depths were used with or without the addition of a readily available carbon (C) source (200 mg C kg−1, as glucose). The soils were incubated anaerobically for 96 h at field capacity moisture with 10% (vol/vol) acetylene atmosphere. Nitrous oxide emission was not detectable in the Parkwood sand throughout the study, despite its high organic C content and microbial activity, possibly because of incomplete inhibition of N2O reduction by acetylene in Parkwood soil as a result of its high clay content, which could have hindered the free diffusion of acetylene. In the Myakka fine sand and the Tavares fine sand, cumulative N2O production for 96 h was greater when ammonium nitrate was applied in the liquid form rather than the dry granular form. The difference was more pronounced in the Myakka sand (2.3-fold) than in the Tavares sand (1.9-fold). Denitrification was enhanced by the addition of readily available carbon in the form of glucose. The soil microbial population and soil organic carbon content seemed to play a major role in determining denitrification losses. Cumulative N2O emission was correlated significantly (r2= 0.92) with the microbial activity as measured by fluorescein diacetate hydrolytic activity.

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID

摘要:

Bacterial breakthrough curves from clean sand columns were used in two related studies of the effects of unsaturated flow and added surfactant on cell transport. In the first study, step-inputs of bacteria were used to evaluate the effects of water saturation (saturated and unsaturated condition), surfactant concentration (0 and 20 μmol/L sodium dodecyl-benzene sulfonate (DDBS)), and bacterial strain (aquifer isolates E3W7 and W31) on the kinetics of cell deposition and entrainment. These surfactant concentrations and bacterial strains did not have significant effects on the rate coefficients. Unsaturated columns, however, always had delayed cell breakthrough compared with saturated columns, and the deposition rate coefficient was significantly greater in unsaturated conditions compared with saturated conditions. After the outflow concentration of E3W7 from the saturated columns reached the inflow concentration, the columns were partially desaturated. This resulted in a rapid drop in outflow concentration-a decline of 29% with surfactant and 78% without surfactant. A second study was conducted to investigate the mechanism of surfactant-enhancement of bacterial transport by applying 100 μmol/L DDBS before a pulse of W31, concurrent with the pulse, following the pulse, at all times, or at no time. Only the presence of surfactant at all times and concurrently with the cell pulse resulted in significantly greater cell recovery compared with the no-surfactant control. This suggests that 100 μmol/L DDBS interacted with the cells to reduce their adsorption to gas-water interfaces in unsaturated sand.

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID

摘要:

The organic P (Po) status of wetland soils has a large influence on the internal P cycling, and therefore on the biological productivity and water quality, of wetland ecosystems. Chemical extraction and solution-state31P-nuclear magnetic resonance (NMR) spectroscopy were used to identify and quantify the distribution of the major forms of P in organic soils of three created wetland sites-Apopka Marsh, Eustis Muck Farm, and Sunny Hill Farm (SHF). Spectra were obtained on 0.25MNaOH-0.05MEDTA extracts of soils with and without previous chemical removal of labile soil P.31P-NMR spectral analysis of the NaOH-EDTA extracts revealed the presence of inorganic ortho P (Pi), ortho-P monoesters, and ortho-P diesters. Chemically determined proportions of total P (TP) as Pi in the NaOH-EDTA extracts were similar to those obtained by31P-NMR. P monoester was the predominant P form in Apopka Marsh (51% TP) and SHF (59% TP) soils. The Eustis soil contained a high proportion of Pi (65% TP), and the SHF soil contained significant (10% TP) P diester. Chemical removal of labile Pi, using 1MKCl and 0.5MNaHCO3, enhanced the quality of all NMR signatures and demonstrated that not all P diesters are removed in the NaHCO3extract. NMR spectra were obtained with acquisition times ranging from 0.5 to 1.2 h, which represents a significant reduction in data acquisition times reported in the previous31P NMR studies of soil extracts.

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID

摘要:

Rapid percolation enhances dissolved organic matter (DOM) transport in soils. One reason for this is nonequilibrium conditions, which make the time-dependence of the sorption/desorption an important parameter. We investigated the time-dependent DOM release and sorption of four soils (one topsoil, three subsoils) with different properties (organic carbon (OC): 2-56 g kg−1; dithionite-citrate-bicarbonate-extractable Fe: 2.4-21.9 g kg−1; clay: 30-450 g kg−1) in batch experiments during a 24-h period. Release of dissolved organic carbon (DOC) when a DOC-free solution was added occurred mainly within the first 15 min and then decreased drastically. After 2 to 4 h, the release almost stopped, but steady-state conditions were not reached within the experimental period. DOM fractionation with XAD-8 adsorber resin showed that the major portion of the released DOC exhibited hydrophilic properties. At an addition of 14 mmol DOC kg−1, the topsoil still released DOC. One subsoil, which was rich in Al and Fe oxides and low in OC, sorbed both hydrophilic and hydrophobic DOC strongly, with a preference for the hydrophobic fraction. The other two subsoil horizons showed a sorption of hydrophobic DOC that was accompanied by a steadily increasing release of hydrophilic DOC. This indicates competition between the two DOC fractions, with hydrophobic DOC having the stronger affinity to sorbent and displacing soil-bound hydrophilic substances. The reactions approached steady-state conditions after 2 to 4 h. The time-dependency of the release and sorption can be described adequately by the Elovich equation and the fractional-power model. In a few cases, the DOC release from soil was represented better by the parabolic-diffusion equation. The slope of the Elovich equation, which may be considered the apparent rate constant, was higher for the hydrophilic than for the hydrophobic DOC fraction. It is related to the ratio between OC and Al and Fe oxide content.

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID

摘要:

The severity of accelerated erosion is affected by slope gradient, slope shape, slope length, and tillage practices. Fly ash, the product of high temperature coal combustion, has been used previously as a sedimentation marker, but it has never been used on the upland as an indicator of soil loss from erosion. The objective of this study was to determine the extent of soil removal from a hillslope as a result of accelerated erosion and its subsequent deposition on a lower landscape position using the presence, depth distribution, and concentration of fly ash, organic carbon, and magnetic susceptibility to identify the remaining original soil as well as a sediment marker. The study was carried out by comparing the amount of fly ash contained in the soil profile at different landscape positions of a cultivated site with that in an uncultivated woodland site. Fly ash deposition began in 1855 with traffic on the Illinois Central railroad between Chicago and Cairo, Illinois. In 1889 and 1928, additional rail lines were located closer to both sites. Organic C, magnetic-mineral content, and magnetic susceptibility decreased regularly with depth at both sites. Magnetic susceptibility was generally higher on the uncultivated site compared with the cultivated site for all landscape positions except the lower footslope. Compared with the uncultivated site, there was 50% less fly ash on interfluve and shoulder landscape segments of the cultivated site, 35% less on the backslope, and 67% less on the depositional lower footslope position. When the entire transect site is considered, the cultivated site had 47% less fly ash in the upper 22.5-cm soil profile compared with the uncultivated site. These results indicated that 10.6 cm, or 47%, of the upper soil layer has been eroded from the hillslope since 1855 (142 years) as a result of accelerated erosion induced by cultivation. For the backslope, the presence of 65%, or 15 cm, of the original surface soil layers and 67% of the fly ash at the cultivated site places the soil in the moderately eroded phase of the Grantsburg soil.

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID

摘要:

The characteristics ofSoil Taxonomyare analyzed relative to various techniques for developing expert systems. Special emphasis is placed on computer program features that allow for more consistent application of classification systems and make them more user-friendly and understandable. We studied the functional logic and query processes employed bySoil Taxonomyto identify soil individuals and compared the methods with those used in other natural object classification systems. Numerical and classical identification methods and program features found in recent computer programs were evaluated for use withSoil Taxonomy.The keys inSoil Taxonomyare purely phenetic in nature and single-access in approach. In the absence of rule- and value confidence-weighting factors, the rules must be encoded without sequence modification to preserve the decision logic. Decisions inSoil Taxonomyquery a large, often incomplete, and sometimes faulty data set, requiring error-checking of data and the addition of expert rules to the encoded decisions to prevent indecision.Soil Taxonomyrules check within the soil individual for the presence or absence of spatial and nonspatial differentiae, specific property values, or other qualifications.Soil Taxonomyis suitable as the subject of an object-oriented expert system, and planning has begun on development of an automated prototype for the Histosol, Andisol, Spodosol, and Oxisol soil orders. Expert system features coupled with additional models and algorithms can be used to improve the use and user-friendliness ofSoil Taxonomy.

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID

摘要:

This study tested the feasibility of producing an automated expert system forSoil Taxonomyto identify soil order from stored data by building an expert system prototype.Soil Taxonomyrules for the Histosol, Spodosol, Andisol, and Oxisol orders were translated into decision tree format. Seventy independent properties were stored in tabular format for each pedon. Heuristic knowledge (expert rules) was added to the decision trees to query a minimum data set, with 13 field description properties required to contain data for each soil horizon, 20 default values, and three estimated values from lookup tables. The prototype expert system was developed using an object-oriented expert system shell. Twenty-seven subsections were named in the rules to identify the Histosol, Spodosol, Andisol, and Oxisol soil orders. Sixty-seven objects, 70 independent properties, and 135 calculated properties were needed to define these subsections and their properties. The tested prototype quickly and correctly identified the diagnostic horizons, nonspatial differentiae, and the soil order, proving the feasibility of developing an expert system forSoil Taxonomyusing existing computer programs and programming methods. We recommend improvements in policy and procedure for recording field description data and development of the expert rules to add dynamic links to outside models and software and incorporate fuzzy logic. The project should be continued to improve the prototype interface and data output features and to complete an expert system to add the remaining soil orders forSoil Taxonomy.

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID

ISSN:0038-075X    

出版商:OVID    

年代:1998

数据来源: OVID