Soil Gas Diffusion Coefficient Research Articles

Effect of Soil Volumetric Water Content on the CO2 Diffusion Coefficient

Purpose: The soil air diffusion coefficient (Ds) is particularly important for the study of soil gas diffusion movement, and there are still many uncertainties in the widely used methods; as such, a method was designed to in situ measure the soil gas diffusion coefficient. Methods: Four different soil media were selected and studied by means of a designed in situ measurement of soil gas diffusion coefficients, and these were compared and analyzed with the predictions of several commonly used prediction models. In addition, they were combined with gas transport models to validate the results of the empirical models that were obtained by the in situ measurements. Results: The results of the data indicate that increasing the volumetric soil moisture content decreases the soil gas diffusion coefficient, with changes in the soil gas diffusion coefficient for the small-grained quartz sand medium being similar to those predicted by the Buckingham model. The soil gas diffusion coefficients for the large-grained quartz sand were similar to the Millington and Quirk model predictions at low humidity; for increased humidity, it was instead similar to the Buckingham model predictions. The soil gas diffusion coefficients of the two active media were closer to those of the SWLR model with high Cm. In addition, the R2 of the measured data was verified, by empirical modeling, to be greater than 0.54, and inversion experiments were conducted to verify that the results were consistent with those of the SWLR model. Conclusion: When measuring relative diffusion coefficients in the field, we recommend the in situ measurement method, which is more reflective of the actual situation in natural environments and provides more accurate data support for soil carbon flux studies.

Pore network modeling as a new tool for determining gas diffusivity in peat

Abstract. Peatlands are globally significant carbon stocks and may become major sources of the greenhouse gases (GHGs) carbon dioxide and methane in a changing climate and under anthropogenic management pressure. Diffusion is the dominant gas transport mechanism in peat; therefore, a proper knowledge of the soil gas diffusion coefficient is important for the estimation of GHG emissions from peatlands. Pore network modeling (PNM) is a potential tool for the determination of gas diffusivity in peat, as it explicitly connects the peat microstructure and the characteristics of the peat pore network to macroscopic gas transport properties. In the present work, we extracted macropore networks from three-dimensional X-ray micro-computed tomography (µCT) images of peat samples and simulated gas diffusion in these networks using PNM. These results were compared to the soil gas diffusion coefficients determined from the same samples in the laboratory using the diffusion chamber method. The measurements and simulations were conducted for peat samples from three depths. The soil gas diffusion coefficients were determined under varying water contents adjusted in a pressure plate apparatus. We also assessed the applicability of commonly used gas diffusivity models to peat. The laboratory measurements showed a decrease in gas diffusivity with depth due to a decrease in air-filled porosity and pore space connectivity. However, gas diffusivity was not extremely low close to saturation, which may indicate that the structure of the macropore network is such that it enables the presence of connected diffusion pathways through the peat matrix, even in wet conditions. The traditional gas diffusivity models were not very successful in predicting the soil gas diffusion coefficient. This may indicate that the microstructure of peat differs considerably from the structure of mineral soils and other kinds of porous materials for which these models have been constructed and calibrated. By contrast, the pore network simulations reproduced the laboratory-determined soil gas diffusion coefficients rather well. Thus, the combination of the µCT and PNM methods may offer a promising alternative to the traditional estimation of soil gas diffusivity through laboratory measurements.

A differential CO2 profile probe approach for field measurements of soil gas transport and soil respiration#

AbstractBackgroundGas exchange between soil and atmosphere is of great importance for greenhouse gas cycles. As gas transport in soil is generally dominated by diffusion, the soil gas diffusion coefficient (DS) is crucial to understand fluxes between soil and atmosphere. Estimating DS is still a great source of uncertainty when calculating soil gas fluxes such as soil respiration from soil gas profiles. In situ measurement of the effective exchange coefficient (Deff) not only reduces this uncertainty, but also allows to quantify non‐diffusive transport processes in addition to the purely diffusive exchange (DS), which cannot be investigated by laboratory measurements or the application of soil gas diffusivity models. Even though several methods for in situ Deff measurement exist, they often lack in the temporal resolution to identify short‐term effects on Deff or require laborious set‐ups, which makes them unsuitable for a fast and mobile application.AimsOur objective was to test an improved profile probe for model‐based soil gas flux analyses that allows in situ monitoring of (1) soil CO2 profiles with high temporal resolution, (2) soil gas transport coefficients, including non‐diffusive transport processes, (3) soil–atmosphere CO2 flux, and (4) soil respiration profiles.MethodsWe developed a CO2 profile probe with build‐in sensors that can easily be installed in soil to gain continuous CO2 concentration profiles. The probe includes the option to inject CO2 as a tracer gas to estimate Deff. To account for changes in natural CO2 concentrations in the soil, we tested two approaches: firstly, a differential approach using two probes, an injection probe and a reference probe, and secondly, a stand‐alone approach in which changes in natural CO2 concentrations are estimated by a statistical model using its main environmental drivers. The resulting tracer gas profiles were used to fit a finite element gas diffusion model to derive Deff. Using the derived Deff values and the CO2 profiles allowed calculating CO2 fluxes. The approach was tested with controlled laboratory experiments using different mineral substrates to compare the diffusivity estimates of the in situ method with laboratory measurements on soil cores. Additional laboratory experiments included artificial CO2 sources to simulate soil respiration in order to evaluate the gradient‐based estimation of soil respiration profiles. In a second step, both approaches were tested under natural conditions in the field.ResultsThe derived Deff values agreed well with laboratory measurements of DS and DS‐transfer functions. The artificial CO2 sources could accurately be estimated with the gradient method. In the field under calm conditions, both approaches for the estimation of the changing natural CO2 profile gave comparable results. Under windy conditions, the stand‐alone approach gave unrealistically high values, whereas the differential approach using a reference profile still gave reliable results. We observed a clear relationship between Deff and wind‐induced pressure pumping in the topsoil. The estimated surface CO2 efflux agreed well with chamber measurements.ConclusionsThis new set‐up enables monitoring of diffusive and non‐diffusive soil gas exchange and soil respiration in situ with high resolution and relatively low effort.

Plants control soil gas exchanges possibly via mucilage

AbstractBackground: Gaseous matter exchanges in soil are determined by the connectivity of the pore system which is easily clogged by fresh root exudates. However, it remains unclear how a hydrogel (e.g., mucilage) affects soil pore tortuosity and gas diffusion properties when drying.Aims: The aim of this viewpoint study is to extend the understanding of gas exchange processes in the rhizosphere by (a) relating it to the patterns formed by drying mucilage within pore space and (b) to give a concept of the effect of drying mucilage on soil gas diffusivity using the combination of experimental evidence and simulations.Methods: To describe the effect of mucilage on soil gas exchanges, we performed gas diffusion experiments on dry soil–mucilage samples and took images of glass beads mixed with mucilage to visualize the formation of mucilage after drying, using Environmental Scanning Electron Microscopy. Finally, we set up simulations to characterize the geometric distribution of mucilage within soil during the drying process.Results: Experiments of gas diffusion show that mucilage decreases gas diffusion coefficient in dry soil without significantly altering bulk density and porosity. Electron microscopy indicates that during drying mucilage forms filaments and interconnected structures throughout the pore space reducing gas phase connectivity. The evolution of these geometric structures is explained via pore scale modelling based on identifying the elastic strength of rhizodeposition during soil drying.Conclusion: Our results suggest that releasing mucilage may be a plant adaption strategy to actively alter gas diffusion in soil.

Gas diffusion characteristics of agricultural soils from South Greenland

AbstractThe Arctic is warming at twice the global average, which may impact agricultural production in Greenland. Therefore knowledge of the functional properties of Greenlandic soil resources is necessary. The relative soil gas diffusivity [the soil gas diffusion coefficient (Dp)–free‐air diffusion coefficient (Do) ratio] is the chief parameter controlling gas transport in soils. Predictions of Dp/Do are needed to estimate root zone aeration and terrestrial greenhouse gas fluxes. We used existing models to analyze the Dp/Do of soils from Greenlandic fields and a pore connectivity index (Cip) to infer their degree of structural development and identify the main parameters controlling Dp/Do. In total, 201 × 3 intact 100‐cm3 soil samples were sampled across six fields with clay and organic C contents of 0.016 to 0.089 and 0.016 to 0.105 kg kg−1, respectively. The Dp/Do was measured with the one‐chamber nonsteady‐state method at soil water potentials between –10 and –1,000 cm H2O. Accurate determination of total porosity (Φ) was ensured by calibrating a particle density model on 129 samples. The soils exhibited a less developed structure and highly tortuous pore networks, resulting in low Dp/Do as a function of air‐filled porosity (ε). Density‐corrected models with air saturation (ε/Φ) reduced the RMSE. Furthermore, Cip at –1,000 cm H2O soil water potential increased linearly with dry bulk density (ρb), suggesting that ρb is a key controller of Dp/Do, which is important for planning cultivation practices for Southern Greenlandic soils. Lastly, we found that an air saturation >35% is required for adequate soil aeration.

Approach for using measured soil gas diffusion coefficients in Hydrus 1D with examples from forest soils

AbstractThe use of Hydrus‐1D for modeling soil gas fluxes can be improved by introducing a parameter f obtained by fitting the Millington–Quirk (MQ) tortuosity model to measured gas diffusion coefficients. The approach was tested for data from soil horizons of two sandy forest sites located in Northeast‐Germany. f‐values ranged between 0.41 and 0.54, indicating a more tortuous pore systems than predicted by MQ model (f = 1). The parameter optimization procedure can be carried out by using the statistical software R and the enclosed R‐script.

CHARACTERIZING PORE-STRUCTURAL PARAMETERS OF VOLCANIC ASH SOIL: COMPARISON BETWEEN NON-DESTRUCTIVE AND INDIRECT METHODS

Mass transport within porous media is governed by their pore networks, which is highlyinfluenced by pore structure parameters such as pore size distribution, porosity, pore tortuosity and porecoordination number. Micro-focus X-ray computed tomography (MFXCT) has emerged as a powerful nondestructive tool for the direct visualization and better understand soil pore geometry. In this study, soilmacropore networks (typically, pore diameter ≥ 30 µm) were visualized and gas transport parameters weremeasured for volcanic ash soils taken from Nishi Tokyo City, Japan. Especially, the study aimed to identifythe effect of moisture content on pore structural parameters based on MFXCT analysis and compare theMFXCT derived parameters to indirectly-estimated parameters from soil gas diffusion coefficient and airpermeability such as pore tortuosity-connectivity factor (XG) and equivalent pore diameter (dG) for gas flow.Both undisturbed and repacked samples were used for characterizing soil pore networks. In MFXCT analysis,the pore structural parameters such as effective pore diameter (dMFXCT), pore tortuosity (TMFXCT), andcoordination number (CMFXCT) were measured. Results showed the moisture content affected clearly porestructural parameters, TMFXCT and CMFXCT decreased with increasing soil air filled porosity, however, measureddMFXCT were independent on the moisture content. For both undisturbed and disturbed samples, the measuredTMFXCT and XG became close and plotted in a narrow range between 1:1.5 and 1.5:1. On the other hand, highvariations were observed between dMFXCT and dG, indicating that the indirectly estimated equivalent porediameter from soil gas transport parameters does not correspond to the mean diameter of soil macropore.

Relationship between greenhouse gas emissions and changes in soil gas diffusivity in a field experiment with biochar and lime

AbstractReducing greenhouse gas emissions from arable soil while maintaining productivity is a major challenge for agriculture. Biochar is known to reduce N2O emissions from soil, but the underlying mechanisms are unclear. This study examined the impact of green waste biochar (20 Mg ha−1) and lime (CaCO3; 2 Mg ha−1) application on soil gas transport properties and related changes in these to soil N2O and CO2 emissions measured using automated chambers in a field experiment cropped with maize. In situ soil water content monitoring was combined with laboratory measurements of relative soil gas diffusion coefficient (Dp/D0) at different matric potentials, to determine changes in Dp/D0 over time. Cumulative N2O emissions were similar in the control and lime treatment, but much lower in the biochar treatment. Cumulative CO2 emissions decreased in the order: lime treatment > biochar treatment > control soil. When N2O emissions were not driven by excess N supply shortly after fertilisation, they were associated with Dp/D0 changes, whereby decreases in Dp/D0 corresponded to N2O emissions peaks. No distinct pattern was observed between CO2 emissions and Dp/D0. Cumulative N2O emissions were positively related to number of days with Dp/D0 < 0.02, a critical limit for soil aeration. These results indicate that improved soil gas diffusivity, and hence improved soil aeration, may explain the effect of biochar in reducing N2O emissions. They also suggest that knowledge of Dp/D0 changes may be key to explaining N2O emissions.

Linking pore network structure derived by microfocus X-ray CT to mass transport parameters in differently compacted loamy soils

Mass transport in soil occurs through the soil pore network, which is highly influenced by pore structural parameters such as pore-size distribution, porosity, pore tortuosity, and coordination number. In this study, we visualised the networks of meso- and macro-pores (typical pore radius r ≥ 10 μm) using microfocus X-ray computed tomography (MFXCT) and evaluated pore structural parameters of two loamy soils from Japan and New Zealand packed at different degrees of compaction. The effect of compaction on pore structural parameters and relationships between pore structural parameters and measured mass transport parameters were examined. Results showed a clear influence of compaction on pore structural parameters, with the MFXCT-derived mean pore radii and pore tortuosities decreasing and the mean pore coordination number increasing with increasing dry bulk density. Especially, pores with r > 80 µm became finer or were not well formed due to compaction. The MFXCT-derived pore structural parameters were not well correlated with the equivalent pore radii from measured water retention curves. However, volumetric surface areas and pore-network connectivity-tortuosity factors derived from MFXCT allowed a fair prediction of several important mass transport parameters such as saturated hydraulic conductivities, soil-gas diffusion coefficients, and soil-air permeabilities. Further studies are needed to link micro-pores with radii smaller than the X-ray CT resolution to meso- and macro-pores visualised by X-ray CT to improve the prediction of mass transport parameters in soil.

A comparison of measured and predicted diffusion coefficients applied to sand and silt sized acid mine drainage materials

Determining the rate at which Acid Mine Drainage (AMD) sulfide oxidation occurs in mining waste products is a central requirement for safe and sustainable long term design of storages, including tails storage facilities, ore stockpiles and waste rock dumps. Inappropriate design can result in AMD acidification, mobilisation of heavy metals and pollution of ground and surface waters. The use of soil gas (oxygen) transport modelling to model AMD based sulfide oxidation and potential acidity loads is widespread, but diffusion coefficients used for modelling are based on existing diffusion coefficient models derived for natural and agricultural soils. Mining wastes are often well sorted due to mineral processing, and differences in soil structure and porosity can impact on diffusion coefficient behaviour compared to natural soils. This study compares a variety of approaches to estimate the diffusion coefficient and compare these results to measured values for comparison and analysis. A diffusion column apparatus is used to compare diffusion coefficient models from the literature with test results from the laboratory in several mining derived AMD materials. The results of the comparison indicate that laboratory testing of diffusion provides more accurate estimation of soil gas diffusion coefficients in mine materials. The use of diffusion coefficient safety factor approaches are explored as possible alternatives, if measurement of mine material matrix soil gas diffusion coefficient is not possible.

Effect of Soil Texture on Estimates of Soil‐Column Carbon Dioxide Flux Comparing Chamber and Gradient Methods

Core Ideas Carbon dioxide efflux from soil of different textures was compared. The dominant mechanism of CO2 efflux changed from coarse to fine soil. The gradient method should be used very carefully in coarse soils. A key component of the global C balance includes soil CO2 emissions. Both chamber‐ and gradient‐based methods have been effectively used for determination of soil CO2 efflux. The gradient‐based method, however, is installed belowground and requires assessment of the soil gas diffusion coefficient, which varies with soil properties such as soil texture. We hypothesized that the gradient and chamber methods would yield similar results when soil texture was appropriately accounted for. Our objective was to evaluate soil gas efflux by applying both methods in three different soil textures (loam, silty loam, and silty clay loam) packed in laboratory soil columns. Water was incrementally applied to soil surfaces while volumetric soil water contents (VWC) and CO2 concentrations were continually monitored at target locations in parallel with surface chamber measurements. The Soil Air Phase Individual Resistances (SAPHIR) model was used to determine the relative gas diffusion coefficients (DP/D0). The gradient‐based CO2 effluxes were compared with estimates derived from the CO2–efflux chamber. Soil CO2 effluxes estimated using the gradient method approximated those using the chamber for all three soil textures. The particle shape factor (p) in the SAPHIR model varied with silt content of the three soils. The SAPHIR model's water blockage factor (w) also varied with soil texture to a lesser extent. Under the repacked soil condition, these results confirm that the gradient method can provide consistent CO2 measurements in comparison with chamber‐based results from soils of varied texture when the diffusion coefficient takes into account the effects of soil texture. The gradient method should be used cautiously in coarse‐textured soils.

An in situ method for real‐time measurement of gas transport in soil

SummarySoil aeration is central to the biogeochemistry of soil. Gas transport in soil is governed mainly by molecular diffusion and depends on the soil gas diffusion coefficient (DS). Several methods exist to determine this property based on field and laboratory measurements. These methods, however, are unsuitable for continuous monitoring of DS over time. Moreover, non‐diffusive processes can affect gas transport in soil during certain situations that last several hours only and discontinuous measurements might fail to identify such processes. We developed a novel in situ method for the real‐time monitoring of gas transport in soil. Helium (He) was injected continuously into the soil and the resulting steady‐state gas profile was monitored. We used a gas sampling probe with very permeable membranes, which enables passive sampling of the soil gas at different depths. The DS profile was modelled by inverse finite‐element modelling (FEM) of an exact geometrical model of the sampler and soil. Molecular diffusion was assumed to be the only process of gas transport. The method was tested in the laboratory with different granular materials and in two field studies. The DS values obtained with this new method agreed well with reference measurements from soil cores and diffusion models. Soil gas diffusivity was monitored over a few days that included a major rain event. During this period the effect of increasing soil moisture on gas transport in soil could be observed in real time. Our novel method is suitable for monitoring gas transport in soil over several days. This enables the monitoring and investigation of situations including non‐diffusive transport processes that result from barometric pressure changes or strong wind events.Highlights We present a novel method for in situ monitoring of gas transport in soil. Helium was continuously injected into the soil and its distribution in the soil modelled. Soil gas diffusivity profiles determined in situ agreed well with laboratory measurements of DS. Our novel method is ideal for the investigation of specific situations in gas transport in soil.

Evaluation of gas diffusivity models for no-tilled and tilled volcanic ash soils

ABSTRACTAccurate quantification of soil gas diffusion is essential to understand the gas transport mechanism in soils, especially for soil greenhouse gas emissions. To date, the performance of soil gas diffusivity (Dp/D0, where Dp is the soil gas diffusion coefficient and D0 is the diffusion coefficient in free air) models has seldom been evaluated for no-tilled and tilled volcanic ash soils. In the present study, six commonly used models were evaluated for volcanic ash soils under two treatments by comparing the predicted and measured soil gas diffusivities at water potentials of pF 1.3–3. The Buckingham-Burdine-Campbell (BBC), soil-water-characteristic-dependent (SWC-dependent), and two-region extended Archie’s Law (2EAL) models showed better performance for both no-tilled and tilled volcanic ash soils, which is likely because porosity and pore size parameters of bimodal soils were taken into consideration in these models. Since the BBC model showed better accuracy than the SWC-dependent and 2EAL models and required fewer, more easily measurable parameters, this study recommends the BBC model for predicting soil gas diffusivity of volcanic ash soil under different tillage managements. In future studies, the BBC model should be further tested at water potentials of pF > 3, and may be improved by including the parameters of pore continuity and saturation.

Gas diffusivity in chinampas soils in Mexico City .

In this laboratory experiment we measured soil gas diffusion coefficients (<em>D</em>) on undisturbed cores of anthropogenic <em>chinampas</em> soils and tested the validity of some classical gas diffusivity models for predicting the ratio of <em>D</em> to the gas diffusion coefficient in free air (<em>D</em><sub><em>0</em></sub>) as a function of the soil air-filled porosity (<em>ε</em>). The A1 horizon (0-7 cm) of <em>chinampas </em>soils had the highest gas diffusivity and a linear relationship between <em>D/D</em><sub><em>0</em></sub> and <em>ε</em>, and thus, the Penman model gave an adequate prediction for this sub-horizon. The Millington-Quirk model was similar to the <em>D/D</em><sub><em>0</em></sub> at all values of <em>ε</em> for the A2 sub-horizon (7-18 cm) and at <em>ε</em> < 0.5 cm<sup>3</sup> cm<sup>-3</sup> for the A3 (18-30 cm) and A4 (30-50 cm) sub-horizons. Gas diffusivities in <em>chinampas</em> soils were lower than in mineral soils, as predicted by <em>D/D</em><sub><em>0</em></sub>(<em>ε</em>) models, likely due to the high content of soil organic carbon. The predictive models could be used for the evaluation of greenhouse gases emission from <em>chinampas</em> soil.

Soil Surface Wetting Effects on Gradient‐Based Estimates of Soil Carbon Dioxide Efflux

The gradient method is widely used in conjunction with Fick's Law to estimate emissions of soil CO2. This requires accurate estimation of the soil gas diffusion coefficient (DP) and the CO2 concentration gradient, typically from measured water content and CO2 concentration. Shallow application of water via precipitation or irrigation causes a temporary reduction in the near‐surface soil gas diffusion coefficient, which compromises gradient‐based flux estimates when undetected by a water content sensor. Our objectives were to analyze the effects of soil surface wetting and temperature on CO2 concentration and efflux using laboratory and field measurements and to compare four widely used models for estimating DP. A laboratory test was conducted to determine the effects of water application on the soil CO2 concentration and the CO2 efflux calculated with the gradient method. The DP parameter was very sensitive to the soil water content and was most reliably calculated using a power function model in a Millville silt loam field soil, while both power function and SAPHIR models were similarly reliable in a Kidman fine sandy loam laboratory test. A single CO2 sensor at a depth of 5 cm with water content monitored at 2.5 cm provided reasonable estimates of the soil CO2 efflux validated with an automated chamber. We found that under most conditions, the CO2 concentration gradient in the soil profile is a reasonable estimator of CO2 flux when measurements of the soil water content and known porosity values are used to estimate the gas diffusion coefficient. However, shallow wetting events require improved monitoring of spatial and temporal changes near the surface or appropriate modeling of hydrodynamics there.

Tillage and drainage management effect on soil gas diffusivity

Subsurface drainage influences the gaseous exchange in soils and improves crop productivity. Thus, gas diffusivity were monitored on a long-term drainage/tillage experiment established in 1994 at the Waterman Farm of The Ohio State University, Columbus, OH, USA. Specific objectives of the present study were to compare the gas diffusion and physical properties (bulk density and water retention) of soils managed under no-till (NT) and chisel-tillage (CT) systems with subsurface drainage management. Soils of the experimental site are classified as Crosby silt loam (Fine, mixed, mesic, Aeric Ochraqualf; fine, mixed, mesic, Typic Argiaquall). Treatments included: NT with tile drainage (NT-D), NT with no-drainage (NT-ND), CT with drainage (CT-D), and CT with no-drainage (CT-ND). The research site has been under continuous corn (Zea mays L.) cropping system since the start of the experiment. Intact core samples (n=36) from 0–10, 10–20, and 20–30cm depths were collected during November 2011 in three replicated plots of NT and CT systems under D and ND treatments. Results from this study showed that drainage treatments significantly influencing the relative gas diffusion (Dp/D0), is defined as the ratio of the soil gas diffusion coefficient to that in free air. The Dp/D0 for NT soils (23.1×10−3) were 26% higher than those for CT (18.3×10−3). Similarly, the ratio was 22% higher for soils under D (25.0×10−3) compared with those under ND (20.5×10−3). The tillage by drainage interaction was also significant for the Dp/D0 at the 0–10cm depth. Corn yield was positively correlated with relative gas diffusion (R2=0.36). It can be concluded from this study that NT system under drainage management can improve the gas diffusivity, enhance the soil structure and increase crop yield.

Poly-Use Multi-Level Sampling System for Soil-Gas Transport Analysis in the Vadose Zone

Soil-gas turnover is important in the global cycling of greenhouse gases. The analysis of soil-gas profiles provides quantitative information on below-ground turnover and fluxes. We developed a poly-use multi-level sampling system (PMLS) for soil-gas sampling, water-content and temperature measurement with high depth resolution and minimal soil disturbance. It is based on perforated access tubes (ATs) permanently installed in the soil. A multi-level sampler allows extraction of soil-gas samples from 20 locations within 1 m depth, while a capacitance probe is used to measure volumetric water contents. During idle times, the ATs are sealed and can be equipped with temperature sensors. Proof-of-concept experiments in a field lysimeter showed good agreement of soil-gas samples and water-content measurements compared with conventional techniques, while a successfully performed gas-tracer test demonstrated the feasibility of the PMLS to determine soil-gas diffusion coefficients in situ. A field application of the PMLS to quantify oxidation of atmospheric CH4 in a field lysimeter and in the forefield of a receding glacier yielded activity coefficients and soil-atmosphere fluxes well in agreement with previous studies. With numerous options for customization, the presented tool extends the methodological choices to investigate soil-gas transport in the vadose zone.

A Simple Tool for the Inverse Estimation of Soil Gas Diffusion Coefficients

For the calculation of gas fluxes through soils, the soil gas diffusion coefficient Ds needs to be known. Ds can be derived from diffusion chamber experiments or from field experiments measuring diffusive gas fluxes across larger spatial scales. Within these methods, the gas concentration depletion or enrichment in an accumulation chamber is used to derive Ds based on Fick's law. However, since this approach does not account for retardation of soluble gases in the soil water phase, the applicability is restricted to rather inert gases. The method also does not account for convective fluxes due to sampling. To overcome these limitations and to enable the applicability of an earlier presented two‐side in situ method, Ds needs to be estimated inversely from the transient diffusion experiments. In this paper, a convenient computer tool is presented that allows the estimation of Ds from diffusion chamber experiments and two in situ methods.

Multifractal properties of porosity as calculated from computed tomography (CT) images of a sandy soil, in relation to soil gas diffusion and linked soil physical properties

Relationships between soil porosity and diffusive gas flux are poorly understood, partly because of a difference in measurement scales between the two. The complexity of soil pore systems can be described by multifractal analysis at the microscopic scale, whereas relative soil gas diffusion coefficients (Ds/Do) are usually evaluated at the core scale. The objectives of this study were to (i) define a quantitative ‘pseudo‐macroporositygas’ from high‐resolution X‐ray computed tomography (CT) scanning images and characterize it for 10 intact soil cores, (ii) analyse the frequency distribution of pseudo‐macroporesgas in the columns with a multifractal approach and (iii) assess relationships between Ds/Do measured at the core scale and multifractal parameters describing the pore system heterogeneity within a core. The shape and symmetry of the singularity spectra and the degree of curvilinearity of the Rényi spectra show that the multifractal behaviour of the pseudo‐macroporositygas distribution for a given CT image thresholding varied among soil columns. Correlations found between Ds/Do and some parameter estimates of the singularity spectrum suggest that the distribution of pseudo‐macroporesgas, depending on the CT image thresholding, influenced Ds/Do. In particular, a strong correlation between Ds/Do and the entropy dimension (Dq = 1) indicates that Ds/Do was influenced by the degree of spatial heterogeneity of the pseudo‐macroporositygas distribution. The correlation dimension (Dq = 2) was also linked to Ds/Do, suggesting that a second‐order power law might describe the scaling relationship between pseudo‐macroporositygas distribution and Ds/Do. In conclusion, the multifractal description of soil porosity as calculated from CT images may be regarded as a way to improve our understanding of gas movement in soils at the core scale.

Vadose Zone Biodegradation of Benzene Vapors in Repacked and Undisturbed Soil Cores

Bench testing of petroleum biodegradation rates in vadose zone soils is typically associated with errors because transport conditions in laboratory systems are different from those found in the vadose zone. This work addressed the effect of soil structure and gas transport properties on hydrocarbon biodegradation in the unsaturated zone and we present a novel method for measuring biodegradation rates in intact and undisturbed soil columns of 100 cm3. To determine whether soil structure and gas diffusivity, defined as the ratio of the gas diffusion coefficient in soil to that in free air (Dp/D0), influence the outcome of aerobic benzene biodegradation experiments, measurements using identical sandy soils were performed on (i) undisturbed 100‐cm3 core samples; (ii) sieved (2‐mm) and repacked 100‐cm3 core samples; and (iii) soil samples (10 g) prepared as slurry microcosms. While slurry reactor experiments changed the first‐order rate constant (kw,1) significantly compared with undisturbed core samples, this was not the case for soil cores that had been sieved and repacked. This suggests that soil structure on a millimeter scale does not affect aerobic biodegradation in relatively unstructured sandy soils. Within differently textured soil cores, the biodegradation rate was found to increase with gas diffusivity when Dp/D0 < 0.02. This establishes gaseous O2 and petroleum vapor diffusion and distribution in soil profiles as a controlling factor for natural biodegradation of petroleum vapors.