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  • Soils
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  • 1
    In: Soil Science Society of America Journal, January 2012, Vol.76(1), pp.51-60
    Description: Solute diffusivity in soil plays a major role in many important processes with relation to plant growth and environmental issues. Soil solute diffusivity is affected by the volumetric water content as well as the morphological characteristics of water‐filled pores. The solute diffusivity in intact soil samples from two different tillage treatments (soil from below the depth of a harrow treatment and soil from within a moldboard plowed plow layer) was estimated based on concentration profiles using a newly developed method. The method makes use of multiple tracers (two sets of counterdiffusing tracers) for a better determination of the diffusivity. The diffusivity was higher in the below‐till soil than the plowed soil at the same soil water matric potential due to higher water content but also due to higher continuity and lower tortuosity of the soil pores. We measured identical solute diffusivities independent of the tracer set used. We analyzed the whole data set using Archie's law and found a linear relation between Archie's exponent and the logarithm of the soil water matric suction in centimeters of water (pF). An analysis of seven data sets from the literature showed that this was a general trend for soils with moderate to low clay contents.
    Keywords: Clay ; Data Collection ; Soil Water Content ; Soil Pore System ; Soil Treatment ; Solutes ; Soil Sampling ; Soil Water ; Plowing ; Tracer Techniques ; Plant Growth ; Water Content ; Harrows ; Diffusivity;
    ISSN: 0361-5995
    E-ISSN: 1435-0661
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  • 2
    Language: English
    In: Journal of Hydrology, February 2015, Vol.521, pp.498-507
    Description: The present study proposes a new two-step approach to prediction of the continuous soil water characteristic (SWC) from saturation to oven-dryness from a limited number of measured textural data, organic matter content and dry bulk density. The approach combines dry- and wet-region functions to obtain the entire SWC by means of parameterizing a previously developed continuous equation. The dry region function relates gravimetric soil fractions to adsorptive forces and the corresponding water adsorbed to soil particles. The wet region function converts the volumetric particle size fractions to pore size fractions and utilizes the capillary rise equation to predict water content and matric potential pairs. Twenty-one Arizona source soils with clay and organic carbon contents ranging from 0.01 to 0.52 kg kg and 0 to 0.07 kg kg , respectively, were used for the model development. The SWCs were measured with Tempe cells, a WP4-T Dewpoint Potentiameter, and a water vapor sorption analyzer (VSA). The model was subsequently tested for eight soils from various agricultural fields in Denmark with clay contents ranging from 0.05 to 0.41 kg kg . Test results clearly revealed that the proposed model can adequately predict the SWC based on limited soil data. The advantage of the new model is that it considers both capillary and adsorptive contributions to obtain the SWC from saturation to oven-dryness.
    Keywords: Capillarity ; Adsorption ; Unsaturated Soil ; Water Retention ; Soil Moisture ; Geography
    ISSN: 0022-1694
    E-ISSN: 1879-2707
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  • 3
    Language: English
    In: Journal of Hydrology, 06 May 2014, Vol.512, pp.388-396
    Description: The saturated hydraulic conductivity ( ) is an essential effective parameter for the development of improved distributed hydrological models and area-differentiated risk assessment of chemical leaching. Basic soil properties such as the particle size distribution or, more recently, air permeability are commonly used to estimate . Conversely, links to soil gas diffusivity ( / ) have not been fully explored even though gas diffusivity is intimately linked to the connectivity and tortuosity of the soil pore network. Based on measurements for a coarse sandy soil, potential relationships between and / were investigated. A total of 84 undisturbed soil cores were extracted from the topsoil of a field site, and / and were measured in the laboratory. Water-induced and solids-induced tortuosity factors were obtained by applying a two-parameter / model to measured data, and subsequently linked to the cementation exponent of the well-established Revil and Cathles predictive model for saturated hydraulic conductivity. Furthermore, a two-parameter model, analogue to the Kozeny–Carman equation, was developed for the − / relationship. All analyses implied strong and fundamental relationships between and / .
    Keywords: Soil Gas Diffusivity ; Tortuosity ; Saturated Hydraulic Conductivity ; Porosity ; Particle Size Distribution ; Geography
    ISSN: 0022-1694
    E-ISSN: 1879-2707
    Source: ScienceDirect Journals (Elsevier)
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  • 4
    Language: English
    In: Soil Science Society of America journal, 2012, Vol.76(5), pp.1509-1517
    Description: The gas diffusion coefficient (Ds,g) and solute diffusion coefficient (D(s,l)) and their dependencies on fluid content (κ) (equal to soil–air content θ for D(s,g) and soil–water content ɛ for D(s,l)) are controlling factors for gas and solute transport in variably saturated soils. In this study, we propose unified, predictive models for D(s,g)(ɛ) and D(s,l)(θ) based on modifying and extending the classical Maxwell model at fluid saturation with a fluid-induced reduction term including a percolation threshold (ɛ(th) for D(s,g) and θ(th) for D(s,l)). Different percolation threshold terms adopted from recent studies for gas (D(s,g)) and solute (D(s,l)) diffusion were applied. For gas diffusion, ɛth was a function of bulk density (total porosity), while for solute diffusion θ(th) was best described by volumetric content of finer soil particles (clay and organic matter), FINES(vol). The resulting LIquid and GAs diffusivity and tortuosity (LIGA) models were tested against D(s,g) and D(s,l) data for differently-textured soils and performed well against the measured data across soil types. A sensitivity analysis using the new Maxwell’s Law based LIGA models implied that the liquid phase but not the gaseous-phase tortuosity was controlled by soil type. The analyses also suggested very different pathways and fluid-phase connectivity for gas and solute diffusion in unsaturated soil. In conclusion, the commonly applied strategy of using the same, soil-type-independent model for gas and solute diffusivity in analytical and numerical models for chemical transport and fate in variably-saturated soils appears invalid, except for highly sandy soils. The unified LIGA model with differing percolation thresholds for diffusion in the liquid and gaseous phases solves this problem. ; p. 1509-1517.
    Keywords: Clay ; Sandy Soils ; Bulk Density ; Solutes ; Mathematical Models ; Organic Matter ; Porosity ; Diffusivity
    ISSN: 0361-5995
    E-ISSN: 14350661
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  • 5
    Language: English
    In: Soil Science Society of America Journal, Nov-Dec, 2013, Vol.77(6), p.1965(12)
    Description: Soil structure maintains prime importance in determining the ability of soils to carry out essential ecosystem functions and services. This study quantified the newly formed structure of 22-mo field-incubated physically disturbed (2-mm sieved) samples of varying clay mineralogy (illite, kaolinite, and smectite) amended with organic material (7.5 Mg ha-1). The newly formed structure was compared with that of sieved, repacked (SR) and natural intact samples described in previous studies. Assessment and comparison of structural complexity and organization was done using water retention (pore size distribution), soil gas diffusivity, air permeability, and derived pore network complexity parameters. Significant decreases in bulk density and increases in pores 〉100 ?m were observed for incubated samples compared with SR samples. For the soils studied, the proportion of pores 〉100 ?m increased in the order: smectite 〈 illite 〈 kaolinite, with no effect of organic amendment. Soil structural complexity, quantified by soil gas diffusivity, air permeability, and derived pore network indices, was greater for incubated than SR samples. For illitic soils, incubated samples had lower water content and higher air-filled porosity and air permeability than natural intact samples at a matric potential of -10 kPa. Despite this, soil pore organization was similar for both natural and incubated soils, but pore network complexity increased in the order: SR 〈 incubated 〈 natural soils. Finally, the air permeability percolation threshold corresponding to the physically based diffusion threshold increased with structural complexity (SR = 0.02 ?m2; incubated = 0.20 ?m2; natural = 0.70 ?m2). Thus, critical reexamination is needed of the often-used 1.0-?m2 percolation threshold for convective air transport when analyzing pore network complexity. Lack of a clear effect of organic amendment for incubated samples suggests using higher application rates in future studies.
    Keywords: Porosity -- Analysis ; Soil Permeability -- Analysis ; Soil Research
    ISSN: 0361-5995
    E-ISSN: 14350661
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  • 6
    Language: English
    In: Soil Science Society of America Journal, March-April, 2014, Vol.78(2), p.377(10)
    Description: Quantitative characterization of aggregate pore structure can reveal the evolution of aggregates under different land use and management practices and their effects on soil processes and functions. Advances in X-ray computed tomography (CT) provide powerful means to conduct such characterization. This study examined aggregate pore structure of three differently managed same textured Danish soils (mixed forage cropping, MFC; mixed cash cropping, MCC; cereal cash cropping, CCC) for (i) natural aggregates, and (ii) aggregates regenerated after 20 mo of incubation. In total, 27 aggregates (8-16 mm) were sampled from nine different treatments; three natural soils and three repacked lysimeters without and three with organic matter (OM; ground rape) amendment. Three dimensional X-ray CT images, tensile strength, and organic carbon (OC) were obtained for each aggregate. Aggregate-associated OC differed significantly between the three soils as 2.1, 1.4, and 1.0% for MFC, MCC, and CCC, respectively. Aggregate porosity and pore connectivity were significantly higher for CCC aggregates than for MFC and MCC aggregates. The CCC aggregates had an average pore diameter of 300 ?m, whereas MFC and MCC had an average pore diameter of 200 and 170 ?m, respectively. Pore shape analysis indicated that CCC and MFC aggregates had an abundance of rounded and elongated pores, respectively, and those of MCC were in-between CCC and MFC. Aggregate pore structure development in the lysimeters was nearly similar irrespective of the soil type and organic matter amendment, and was vastly different from the state of natural aggregates. Aggregate porosity (〉30 ?m) was observed to be a good predictor for the mechanical properties of aggregates. In general natural aggregates were stronger than lysimeter aggregates.
    Keywords: Cat Scans -- Usage ; Porosity -- Research ; Soil Research
    ISSN: 0361-5995
    E-ISSN: 14350661
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  • 7
    Language: English
    In: Soil Science Society of America Journal, March-April, 2014, Vol.78(2), p.422(12)
    Description: Changes in soil water content are known to affect soil reflectance. Even though it was suggested some time ago that the phenomenon of increased forward scattering due to the presence of water in the soil is related to water film thickness and matric potential, there has been no detailed investigation of this in any studies. The effects of moisture conditions on visible near-infrared (vis-NIR) spectra of four representative soils in Denmark have been assessed as a function of both water film thickness (expressed as the number of molecular layers) and matric potential. Complete water retention curves, from wet (pF 0.3, pF = log(|j|), where ? is the matric potential in cm) to hyper dry end (oven-dried and freeze-dried soil), were obtained by initial wetting followed by successive draining and drying of soil samples, performing NIR measurements at each step. Soil reflectance was found to decrease systematically, yet not proportionally, with decreasing matric potential and increasing molecular layers. The changes in molecular layers were best captured by the soil reflectance of clay-rich soils. Here the largest increase in reflectance occurred between pF 3 and 4, caused by the shift from capillary to adsorptive surface forces. In support of this, the smallest changes in reflectance were seen in the sandiest soil. Freeze drying the soil highest in organic C increased reflectance, possibly due to an alteration in organic matter during freezing. The different reflectance behavior of soil with a higher organic C content may be linked to differences in the amount, but also the quality (higher hydrophobicity) of the organic matter. However, this needs to be confirmed in further studies.
    Keywords: Reflectance -- Research ; Molecules -- Research ; Soil Research
    ISSN: 0361-5995
    E-ISSN: 14350661
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  • 8
    Language: English
    In: Soil Science Society of America journal, 2011, Vol.75(4), pp.1315-1329
    Description: Accurate predictions of the soil-gas diffusivity (Dp/Do, where Dp is the soil-gas diffusion coefficient and Do is the diffusion coefficient in free air) from easily measureable parameters like air-filled porosity (ε) and soil total porosity (ϕ) are valuable when predicting soil aeration and the emission of greenhouse gases and gaseous-phase contaminants from soils. Soil type (texture) and soil density (compaction) are two key factors controlling gas diffusivity in soils. We extended a recently presented density-corrected Dp(ε)/Do model by letting both model parameters (α and β) be interdependent and also functions of ϕ. The extension was based on literature measurements on Dutch and Danish soils ranging from sand to peat. The parameter α showed a promising linear relation to total porosity, while β also varied with α following a weak linear relation. The thus generalized density-corrected (GDC) model gave improved predictions of diffusivity across a wide range of soil types and density levels when tested against two independent data sets (total of 280 undisturbed soils or soil layers) representing Danish soil profile data (0–8 m below the ground surface) and performed better than existing models. The GDC model was further extended to describe two-region (bimodal) soils and could describe and predict Dp/Do well for both different soil aggregate size fractions and variably compacted volcanic ash soils. A possible use of the new GDC model is engineering applications such as the design of highly compacted landfill site caps. ; p. 1315-1329.
    Keywords: Data Collection ; Sand ; Soil Profiles ; Texture ; Landfills ; Prediction ; Engineering ; Soil Density ; Porosity ; Models ; Peat ; Greenhouse Gas Emissions ; Aeration ; Air ; Volcanic Ash Soils ; Diffusivity
    ISSN: 0361-5995
    E-ISSN: 14350661
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  • 9
    Language: English
    In: Soil Science Society of America journal, 2011, Vol.75(3), pp.795-806
    Description: The air permeability (ka) and soil gas diffusion coefficients (Dp) are controlling factors for gas transport and fate in variably saturated soils. We developed a unified model for ka and Dp based on the classical Archie's law, extended by: (i) allowing for two-region gas transport behavior for structured soils, with the natural field moisture condition (set at −100 cm H2O matric potential [pF 2]) as the reference (spliced) point between the large-pore (drained pore diameter ≥30 μm at pF ≤ 2) and the small-pore (subsequently drained pores 2) regions, and (ii) including a percolation threshold, set as 10% of the total porosity for structureless porous media or 10% of the porosity in the large-pore region for structured soils. The resulting extended Archie's law with reference point (EXAR) models for ka and Dp were fitted to the measured data. For both structureless and structured porous media, Archie's saturation exponent (n) was higher for Dp than for ka, indicating higher water blockage effects on gas diffusion. For structured soils, the saturation exponent for the large-pore region (n1) was lower than for the small-pore region (n2). Generally, n1 values of∼1 for ka and 2 for Dp and n2 values of 4/3 for ka and 7/3 for Dp described the data well. Two reference-point expressions for ka at pF 2 were also developed and tested together with existing models for Dp at pF 2 against independent data across soil types. The best-performing reference-point models were a ka model based on the classical Kozeny equation and the Moldrup Dp model. ; p. 795-806.
    Keywords: Models ; Water ; Soil Air ; Soil Types ; Porous Media ; Air ; Equations ; Permeability ; Porosity ; Diffusivity
    ISSN: 0361-5995
    E-ISSN: 14350661
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  • 10
    Language: English
    In: Waste Management, 2011, Vol.31(12), pp.2464-2472
    Description: ► The effects of soil physical properties on gas transport parameters were investigated. ► Higher values of and exhibited in the ‘+gravel’ than the ‘−gravel’ fraction at same soil–air content ( ). ► Recent power law models for (WLR) and (RPL) were modified. ► Model parameters were linearly related to easily measurable dry bulk density ( ). Landfill sites are emerging in climate change scenarios as a significant source of greenhouse gases. The compacted final soil cover at landfill sites plays a vital role for the emission, fate and transport of landfill gases. This study investigated the effects of dry bulk density, , and particle size fraction on the main soil–gas transport parameters – soil–gas diffusivity ( / , ratio of gas diffusion coefficients in soil and free air) and air permeability ( ) – under variably-saturated moisture conditions. Soil samples were prepared by three different compaction methods (Standard and Modified Proctor compaction, and hand compaction) with resulting values ranging from 1.40 to 2.10 g cm . Results showed that and values for the ‘+gravel’ fraction (〈35 mm) became larger than for the ‘−gravel’ fraction (〈2 mm) under variably-saturated conditions for a given soil–air content ( ), likely due to enhanced gas diffusion and advection through less tortuous, large-pore networks. The effect of dry bulk density on and was most pronounced for the ‘+gravel’ fraction. Normalized ratios were introduced for all soil–gas parameters: (i) for gas diffusivity / , the ratio of measured to in total porosity ( ), (ii) for air permeability / , the ratio of measured to at 1235 kPa matric potential (=pF 4.1), and (iii) for soil–air content, the ratio of soil–air content ( ) to total porosity ( ) (air saturation). Based on the normalized parameters, predictive power-law models for ( / ) and ( / ) models were developed based on a single parameter (water blockage factor for and for ). The water blockage factors, and , were found to be linearly correlated to values, and the effects of dry bulk density on and for both ‘+gravel’ and ‘−gravel’ fractions were well accounted for by the new models.
    Keywords: Air Permeability ; Gas Diffusivity ; Dry Bulk Density ; Landfill Final Cover ; Engineering ; Chemistry
    ISSN: 0956-053X
    E-ISSN: 1879-2456
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