Function Of Air-filled Porosity Research Articles

Spatial and temporal variability of soil gas diffusivity, its scaling and relevance for soil respiration under different tillage

Soils are an important source and sink for carbon. Soil management such as reduced or no-tillage management has been reported to increase soil organic matter budgets, probably due to a hampered microbial mineralization of organic components. While soil respiration is mainly controlled by temperature and soil moisture, it can be also limited by the soil pore system facilitating diffusive gas fluxes between the soil and the atmosphere. However, soil gas diffusivity as a controlling factor for soil respiration has not been assessed under different soil management. Moreover, no adequate methods have been developed yet that facilitate the description of spatial or temporal variations of the highly non-linear soil gas diffusivity functions. Therefore, the objectives of this study were to deduce and apply a scaling rule for gas diffusivity, and to observe and analyze spatio-temporal variations of soil respiration and gas diffusivity under conventional tillage (CT) and no-tillage (NT). We measured soil respiration rates and gas diffusivities along a transect on an arable field in Hollabrunn (Lower Austria) within the 2014 vegetation period. We also determined the soil hydraulic properties and gas diffusivities as a function of air-filled porosity. By adopting the similar media approach of Miller and Miller we facilitated scaling of spatially variable gas diffusivity model functions. The scaling performed well to derive representative mean parameters while preserving the spatial variability in the scaling factors. The comparison of scaling factors for soil water retention, hydraulic conductivity, and gas diffusivity revealed that flow pathways were not the same for water and gases. This finding was explained by the continuity of pores that are accessible for water or gas movement. Compared to NT, the CT plot was characterized by greater soil respiration rates, gas diffusivities, total porosities, and unsaturated hydraulic conductivities, while soil water retention, observed volumetric water contents, and the spatial variability of these properties were smaller. Soil respiration rates were mainly changing with time as a result of soil temperature and soil water content. However, we also found that the diffusive soil properties slightly influenced CO2 efflux rates.

Predicting Air Permeability in Undisturbed, Subsurface Sandy Soils from Air-Filled Porosity

A model for predicting air permeability ( ka ) as function of air-filled porosity (e) in undisturbed subsurface sandy soils, relevant for vapor extraction system design and operation, was developed using data from eight undisturbed soils (approximately 240 samples). The model requires only one measurement of corresponding values of ka and e as input to estimate ka at any desired value of e . The soils used represent both urban, agricultural and forest locations. The model is based on the fact that the relationships between log( ka ) and log(e) in sandy soils are approximately linear and on average pass through a common interception point, although with very different slopes. An expression for predicting ka at −100 cm H2 O soil water potential ( ka100 ) from e at the same potential ( e100 ) for sandy soils was also developed. Using this expression together with the new ka (e) predictive model enables prediction of the entire ka (e) relation without any ka measurements using only a measurement of e100 . Thi...

GAS TRANSPORT PARAMETERS ALONG FIELD TRANSECTS OF A VOLCANIC ASH SOIL

Variations in gas transport parameters at the field scale govern the transport, fate, and emission of greenhouse gases and volatile organic chemicals in soil. In this study, we evaluated predictive models for soil-gas diffusivity (D p /D o ) and air permeability (k a ) based on measurements along a 117-m transect and a parallel 33-m transect of a humic volcanic ash soil (Andisol) in Nishi-Tokyo, Japan. Measurements were done on 100-cm 3 undisturbed soil samples, with 3-m spacing between sampling points, and included water retention, soil-gas diffusion coefficient (D p ), k a at .different soil-water matric potentials, and saturated hydraulic conductivity. Traditionally used predictive gas diffusivity models underestimated D p /D o in wet soil and largely overestimated D p /D o under dry conditions because of soil aggregation effects. A linear model for D p /D o as a function of air-filled porosity (e), taking into account inactive/ remote air-filled pore space, accurately described D P (e)/D o from wet to oven-dry conditions and well captured the spatial variations in D P /D o along the transects. The k a exhibited a nonlinear relation with e, and k a (e) was best predicted from a recently presented power-law model, with measured k a at -100 cm H 2 O of soil-water matric potential (k a,100 ) as a reference point. Trends of decreasing soil-water retention and increasing e along transects were observed. Similar trends in k a and saturated hydraulic conductivity were not observed because the convective fluid transport parameters were mainly governed by soil structure and not by fluid phase contents. Autocorrelograms suggested a spatial correlation range of 10 to 20 m for gas transport parameters (D p /D o and k a ). Measurements of e and k a at conditions close to -100 cm H 2 O of soil-water matric potential are suggested for rapid assessment of the magnitude and spatial variations in gas transport properties at the field scale.

Gas Diffusion Measurement and Modeling in Coarse-Textured Porous Media

Conventional gas diffusion measurements in coarse-textured and aggregated porous media are severely limited due to hydrostatically induced variations in water content and air-filled porosity. Motivated by the need to measure gas diffusion in coarse-textured plant growth media designed for use in microgravity (e.g., aboard the International Space Station), our objectives were (i) to develop and test an automated diffusion measurement system on earth with water content adjustment capability and that minimizes hydrostatic effects, and (ii) to model characteristics of gas diffusion in partially saturated aggregated porous media. The horizontally oriented O2 diffusion cell design for reducing the gravitational effect was based on a thin profile rectangular cell. Continuous measurement of O2 in sealed dual-chamber diffusion cells provided concentration data for fitting diffusion coefficients where water content was controlled volumetrically using a porous membrane with an imposed pressure for incremental addition or removal of water. Gas diffusion was modeled as a function of air-filled porosity in millimeter-sized aggregated particles exhibiting a substantial difference between internal and external aggregate pore sizes. For this case, the internal aggregate porosity contribution to diffusion compared with external aggregate pore space was minor as described by a dual-porosity diffusion model. The measurement approach described can be used in other coarse-textured and structured porous media.

Measurement of effective air diffusion coefficients for trichloroethene in undisturbed soil cores

In this study, we measure effective diffusion coefficients for trichloroethene in undisturbed soil samples taken from Picatinny Arsenal, New Jersey. The measured effective diffusion coefficients ranged from 0.0053 to 0.0609 cm 2/s over a range of air-filled porosity of 0.23–0.49. The experimental data were compared to several previously published relations that predict diffusion coefficients as a function of air-filled porosity and porosity. A multiple linear regression analysis was developed to determine if a modification of the exponents in Millington's [Science 130 (1959) 100] relation would better fit the experimental data. The literature relations appeared to generally underpredict the effective diffusion coefficient for the soil cores studied in this work. Inclusion of a particle-size distribution parameter, d 10, did not significantly improve the fit of the linear regression equation. The effective diffusion coefficient and porosity data were used to recalculate estimates of diffusive flux through the subsurface made in a previous study performed at the field site. It was determined that the method of calculation used in the previous study resulted in an underprediction of diffusive flux from the subsurface. We conclude that although Millington's [Science 130 (1959) 100] relation works well to predict effective diffusion coefficients in homogeneous soils with relatively uniform particle-size distributions, it may be inaccurate for many natural soils with heterogeneous structure and/or non-uniform particle-size distributions.

A New Perspective on Soil Thermal Properties

The soil thermal properties—heat capacity (C), thermal diffusivity (α), and thermal conductivity (λ)—are important in many agricultural, engineering, and meteorological applications. Soil thermal properties are largely dependent on the volume fraction of water (θ), volume fraction of solids (vs), and volume fraction of air (na) in the soil. In many natural settings θ, vs, and na vary greatly over time and space, but data showing the effects of these variations on thermal properties are not readily available. We used a heat‐pulse method to measure the thermal properties of 59 packed columns of four medium‐textured soils covering large ranges of θ, vs, and na The measured data reveal the commonly overlooked but dominant influence of na on soil thermal properties. Notably, the measurements show that the λ of these soils at 20°C can be accurately described as a decreasing linear function of na Good agreement exists between the measured data and common models for λ and C

Predicting the Gas Diffusion Coefficient in Undisturbed Soil from Soil Water Characteristics

The gas diffusion coefficient in soil (DP), and its dependency on soil physical characteristics, governs the diffusive transport of oxygen, greenhouse gases, fumigants, and volatile organic pollutants in agricultural, forest, and urban soils. Accurate models for predicting DP as a function of air‐filled porosity (ϵ) in natural, undisturbed soil are needed for realistic gas transport and fate simulations. Using data from 126 undisturbed soil layers, we obtained a high correlation (r2 = 0.97) for a simple, nonlinear expression describing DP at −100 cm H2O of soil water potential (DP,100) as a function of the corresponding air‐filled porosity (ϵ100), equal to the volume of soil pores with an equivalent pore diameter >30 μm. A new DP(ϵ) model was developed by combining the DP,100(ϵ100) expression with the Burdine relative hydraulic conductivity model, the latter modified to predict relative gas diffusivity in unsaturated soil. The DP,100 and Burdine terms in the DP(ϵ) model are both related to the soil water characteristic (SWC) curve and, thus, the actual pore‐size distribution within the water content range considered. The DP(ϵ) model requires knowledge of the soil's air‐filled and total porosities and a minimum of two points on the SWC curve, including a measurement at −100 cm H2O. When tested against independent gas diffusivity data for 21 differently textured and undisturbed soils, the SWC‐dependent DP(ϵ) model accurately predicted measured data and gave a reduction in root mean square error of prediction between 58 and 83% compared to the classical, soil type‐independent Penman and Millington‐Quirk models. To further test the new DP(ϵ) model, gas diffusivity and SWC measurements on undisturbed soil cores from three 0.4‐m soil horizons (sandy clay loam, sandy loam, and loamy sand) within the 4 to 7 m depth below an industrially polluted soil site were carried out. For these deep subsurface soils the SWC‐dependent model best predicted the measured gas diffusivities.

Modeling the Reduction of Vapor Phase Emissions from Surface Soils Due to Soil Matrix Effects: Porosity/Tortuosity Concepts

ABSTRACT A major route for transport of volatile organic compounds within porous media is vapor phase diffusion. The diffusion rate through a porous medium is less than that through free-air due to the decreased cross-sectional area available for gas movement and the increased path length due to pore tortuosity. Numerous empirical expressions have been published that relate the diffusion coefficient in porous media to the diffusion coefficient in free-air (unobstructed gas phase). Published measurements of relative diffusivity and air-filled porosity were combined into a database. Empirical expressions available in the literature, including the popular Millington-Quirk equation, were evaluated along with a fourth-degree polynomial expression developed by the authors to determine the best type of equation to predict relative diffusivity as a function of air-filled porosity over the domain of values for porosity ranging from 0.071 to 1 for different types of materials. Mean square deviations were used as the statistical test to compare equations. The polynomial expression developed in this project produced a significantly different effective diffusion coefficient (1.3 x 10-6 m2/sec) compared to values of 9.2 x 10-6 m2/sec and 3.1 x 10-6 m2/ sec predicted by forms of the Millington-Quirk equation for a specific case.