Effect of reduced air pressure on soil thermal conductivity over a wide range of water content and temperature

To clarify the role of air molecules in coupled heat and mass transfer in soil, we measured the thermal conductivity of three kinds of soil (Ando soil, Red Yellow soil, and Toyoura sand) under reduced air pressure over a wide range of water content and temperature (10–75°C). The thermal conductivity increased sharply under reduced air pressure above a critical water content of the soil, becoming several times larger than that under normal pressure (101 kPa). The maximum thermal conductivity for each soil was obtained below 75°C and was similar to the thermal conductivity of some metals such as Mn, Hg and stainless steel. When the soil was drier than its critical water content, the thermal conductivity did not increase under reduced air pressure. The hydraulic diffusivity at the critical water content for each soil was of the order of 10^?8 m² s^?1. This suggests that the latent heat transfer is enhanced by the circulation of the condensed water. However, very little is known about the effect of circulating water on the latent heat transfer under reduced air pressure. To make this clear, the thermal conductivity would need to be measured in the steady state under reduced air pressure.     

Temperature dependence of thermal conductivity of soil over a wide range of temperature (5–75°C)

The coupled heat and mass transfer in soil can be analysed by examining the temperature dependence of thermal conductivity. We have measured the thermal conductivity of two kinds of soil (Ando soil and Red Yellow soil) as a function of both temperature (5–75°C) and water content by the twin heat probe method. From our results we concluded that the thermal conductivity resulting from the latent heat transfer can be separated from the apparent thermal conductivity by subtracting the thermal conductivity at a temperature near 0°C from that at a higher temperature. The relation between the phenomenological enhancement factor (β) and the volumetric air‐filled porosity was divided into two parts: β increases linearly as the volumetric air‐filled porosity increases from zero (that is, water saturation), to the point at which soil water potential corresponds to ?320 J kg^?1; from that point to oven‐dry condition, β decreased logistically with the volumetric air‐filled porosity. From these results, we could generalize the behaviour of β.     

Development of an apparatus for measuring one‐dimensional steady‐state heat flux of soil under reduced air pressure

One of the best ways to evaluate the coupled heat and mass transfer in soil is to measure the heat flux and water distribution simultaneously. For this purpose, we developed an apparatus for measuring the one‐dimensional steady‐state heat flux and water distribution in unsaturated soil under reduced air pressure. The system was tested using four samples with known thermal conductivity (0.6–8.0 W m^?1 K^?1). We confirmed that the system could measure the one‐dimensional steady‐state heat flux under a fixed temperature difference between ends of the samples over a wide range of thermal conductivity values. Time domain reflectometry was used to measure the water distribution with a repeatability of less than ± 1.0%. We used the apparatus to measure the soil heat flux and distribution of water content and temperature under steady‐state conditions with reduced air pressure. The initial volumetric water content, θ_ini, of the soil samples was set at 0.20 and 0.40 m³m^?3. For a θ_ini of 0.20, the heat flux was not significantly affected by air pressure, and the water content on the hot side decreased whilst that on the cold side increased, i.e. a pronounced water content gradient was formed. For a θ_ini of 0.40, the heat flux increased sharply with reduced air pressure, and the water content did not change, i.e. a homogeneous water distribution was observed. The increase in the heat flux with air pressure reduction is caused by the vapour transfer in soil pores. We found that a large vapour transfer took place in the soil with the homogeneous water distribution, and that the vapour transfer was less in the soil with the pronounced water content gradient. These experimental facts were entirely different from the traditional knowledge of vapour transfer in soil under temperature gradients. A lack of data on heat flux must have resulted in the previously incorrect conclusions. The new apparatus will serve to clarify the intricate phenomena of thermally induced vapour transfer in unsaturated soil in further experiments.     

Decrease in thermal conductivity with increasing temperature in nearly dry sandy soil

To clarify the role of the water bridges between soil particles on the transfer of heat we studied the dependence of thermal conductivity (λ) and electrical conductivity (E) on temperature between 278 and 338 K of sand and sand mixed with kaolin in the nearly dry state. The thermal conductivity decreased as temperature increased in the sand at volumetric water contents less than 0.07 m³ m^?3, but it increased in the sand–kaolin mixture over the measured range of water content. In the sand, the ratio of E in the soil solution to the electrical conductivity of pure water increased gradually with increasing water content at the water contents less than 0.05 m³ m^?3 and was almost constant at larger water contents. The ratio of E of the sand–kaolin mixture increased with increasing water content, particularly at the lower temperature. For both samples the ratio of E decreased as temperature increased, which suggested that the conduction of heat decreased through the decrease in the water bridges as temperature increased. Because the decrease in λwith increasing temperature could not be explained by the transfer of latent heat transfer, we considered that the temperature dependence of λwas due not only to the transfer of latent heat but also to the thermal bridge of water. We conclude that the condensation, conduction and evaporation in series involved in the latent heat transfer take place mainly through the water films. Our experimental results will help to understand the mechanism of the latent heat transfer in soil with the water films surrounding the soil particles.     

A method to estimate the water vapour enhancement factor in soil

Enhanced water vapour diffusion under temperature gradients has been proposed as a mechanism to explain the discrepancies between measured and predicted water fluxes in soils. Because of the difficulties in measuring soil vapour diffusion directly, modelling approaches have been used to estimate the vapour enhancement factor (η) by matching theory to measurements. In the method proposed by Hiraiwa & Kasubuchi (2000) , soil thermal conductivity associated with conduction heat transfer (λ_c) is assumed to be equal to the apparent soil thermal conductivity (λ) measured at a low temperature, and η is significantly under‐estimated. In the present study, an improved approach for estimating η is used, in which λ_c is taken as the apparent soil thermal conductivity associated with infinite atmospheric pressure. The λ at infinite atmospheric pressure is estimated by extrapolating λ measurements made at finite air pressures. By subtracting λ_c from measured λ values at a given atmospheric pressure, the contribution of thermal vapour diffusion to heat transfer (λ_v) is obtained and then used to estimate η. In the case of a lysimeter sand, λ_v accounts for 4–25, 8–29 and 13–35% of λ at 3.5, 22.5 and 32.5°C, respectively, at soil water contents greater than 0.02 m³ m⁻³. Thus, the latent heat transfer through vapour diffusion is important even at temperatures as low as 3.5°C. The agreement between predictions from the new method and selected literature values suggests that the improved approach is able to provide accurate estimate of η. The results from this study show that the magnitude of latent heat transfer resulting from thermal vapour diffusion is strongly soil texture‐dependent. Thus, it is important to estimate η on specific soils rather than assuming η from literature values.     

Alteration of soil thermal properties by structure formation

Temperatures were measured in a disturbed and a structured loess soil to study the influence of aggregation on thermal properties. The disturbance was done by mechanically destroying soil aggregates, and the structured soil was obtained by subjecting the disturbed soil to several irrigation and drying cycles. In all soils, five harmonics of a Fourier-series representation accounted for ≥99% of the variance of daily soil surface temperatures, and two to three harmonics did so below the soil surface. For soil water contents between 0.04 and 0.23 m³ m-³, the apparent thermal diffusivity, computed by the harmonic method, was higher in the structured soil than in the disturbed soil. The same was true for the apparent thermal conductivity, since the volumetric heat capacities of both the disturbed and structured soil were similar. The differences in the apparent thermal diffusivity and conductivity were attributed to increased heat conduction and water vapour transport in the structured soil.     

多针热脉冲技术测定土壤热导率误差分析

土壤热导率是研究土壤热传输、水热耦合运移的基本物理参数。为了探知多针热脉冲技术的误差,该研究以能够准确测定热导率的单针法作为参比,在4种质地土壤上,对多针热脉冲技术在不同体积质量、含水率和气压条件下测定的热导率进行了分析。结果表明,多针热脉冲技术的热导率结果与单针法总体符合较好,其热导率测定值的平均误差为0.074 W/(m·K)。干土热导率随气压增大呈现对数增长,这是由于气体分子平均自由程下降的原因。多针热脉冲技术的测定误差主要出现在中等含水率区域,关键问题是加热针的温度升高偏大,促进了水汽潜热传输。另外,土壤与探针之间的热接触阻力、探针导致的土壤体积质量改变、温度梯度引起的液水流也影响测定结果的准确性。该研究可为农业水土工程中的土壤热导率模拟提供依据。     

Gas diffusion through soil crumbs: the effects of compaction and wetting

Samples of 1–2 mm crumbs from a clay loam under permanent pasture were equilibrated at -5 kPa water potential then compacted to varying degrees. Gas diffusion coefficients D, (hydrogen through air), were measured immediately on compaction, again after re-equilibration at -5 kPa, then at other water contents between saturation and dryness. The relationship between diffusion coefficient and air content, was, as elsewhere, in two parts (dD/d small for drainage of pores within crumbs; large for pores between crumbs), but the transition from one part to the other occurred at smaller air contents with increased compaction. The air content at which D approached zero as the samples wetted was greatest in the loosest soil. Compaction from a bulk density of 0.86–1.29 g cm^?3 decreased the relative diffusion coefficient, D/D₀ (D₀ is the diffusion coefficient without impedance), from 0.35 to 0.22 (by 38%) at complete dryness, but from 0.19 to 0.035 (by 82%) in the soil initially at -5 kPa. On re-wetting and re-equilibrating at ?5 kPa, D/D₀ decreased further to 0.008 (total 97%) because of extra water held in the now smaller pores of the compacted soil. No single relationship between D/D₀ and fitted the results for even this one soil.     

Effects of NaCl concentration on the thermal conductivity of sand and glass beads with moisture contents at levels below field capacity

Abstract

Soil thermal conductivity is a key factor governing its thermal regime. In the present study, we measured the thermal conductivity of Toyoura sand and glass beads using a heat probe method to clarify the effects of gravimetric water content (w) and NaCl concentration (C) and to evaluate the estimation effectiveness of four models (Mochizuki, de Vries, Noborio and Kasubuchi). The de Vries and Kasubuchi models predict the effect of w on soil thermal conductivity, whereas the Noborio model describes the effects of solute concentration and the Mochizuki model describes both parameters. With or without NaCl, the thermal conductivity of both samples increased with increasing w, and the increase could be grouped into three ranges based on w. The upper and lower limits of each water content range were constant, even at varying NaCl concentrations, but the width of the range differed among the three ranges and between the sand and glass bead samples. Although soil thermal conductivity has previously been reported to generally decrease with increasing C, the thermal conductivities of some glass beads increased in the present study, particularly at moisture contents close to field capacity. The change in thermal conductivity as a function of C was linear in all cases. This trend was similar to that of a non-swelling clay in a previous study. The Mochizuki model, which regressed measured thermal conductivity on C and w, predicted the thermal conductivity of sand as well as previous models, but the calculations were easier and the method offers more flexibility for soils with different textures.     

温室墙体中覆铝箔封闭空气腔热工性能模拟分析

通过建立封闭空气腔二维稳态流动传热模型和温室墙体一维非稳态导热模型,模拟计算封闭腔内空气温度分布,研究了日光温室墙体中覆铝箔封闭空气腔的热工性能。结果表明:壁面覆铝箔可有效减少封闭空气腔的辐射换热量;封闭空气腔的热阻随封闭腔高度的增加而增大,高度达1.5 m后,热阻趋于不变;封闭空气腔的厚度小于0.03 m时,其热阻随厚度增加而增大,厚度超过0.03 m后,热阻逐渐减小;覆铝箔封闭空气腔高度为1.5 m、厚度为0.03 m、内外壁面温差为2~20 K时,热阻为0.70~0.55 K·m~2/W,保温隔热效果相当于0.81~0.64 m厚夯实黏土结构、0.55~0.43 m厚红砖砌体结构墙体或0.20~0.16 m厚煤渣、0.06~0.05 m厚珍珠岩、0.03~0.02 m厚聚苯板隔热材料。3组30 mm厚覆铝箔封闭空气腔加480 mm红砖复合墙体(360 mm红砖墙+3组30 mm封闭空气腔+120 mm红砖墙,240 mm红砖墙+3组30 mm封闭空气腔+240 mm红砖墙),其夜间向室内放热量较单一480 mm红砖墙体提高99.5%~104.2%,与相同结构聚苯板红砖复合墙体无明显差距。     

森林火灾对土壤热导率影响的实验和模型研究

An understanding of soil thermal conductivity after a wildfire or controlled burn is important to land management and post-fire recovery efforts. Although soil thermal conductivity has been well studied for non-fire heated soils, comprehensive data that evaluate the long-term effect of extreme heating from a fire on the soil thermal conductivity are limited. The purpose of this study was to evaluate the long-term impact of fire on the effective thermal conductivity of soils by directly comparing fire-heated and no-fire control soils through a series of laboratory studies. The thermal conductivity was measured for ten soil samples from two sites within the Manitou Experimental Forest, Colorado, USA, for a range of water contents from saturation to the residual degree of saturation. The thermal conductivity measured was compared with independent estimates made using three empirical models from literature, including the Campbell et al.(1994), Cté and Konrad(2005), and Massman et al.(2008) models. Results demonstrate that for the test soils studied, the thermal conductivity of the fire-heated soils was slightly lower than that of the control soils for all observed water contents.Modeling results show that the Campbell et al.(1994) model gave the best agreement over the full range of water contents when proper fitting parameters were employed. Further studies are needed to evaluate the significance of including the influence of fire burn on the thermal properties of soils in modeling studies.     

地表覆盖对土壤热参数变化的影响

覆盖条件下土壤热性质的研究对于包气带水热运移及覆盖技术的应用均有重要意义。使用11针热脉冲探头对沙黄土不同深度(6 mm、18 mm、30 mm)的土壤热扩散率、热容量和热导率三个热参数进行测定,并进行地表覆盖(石子覆盖、秸秆覆盖)处理,旨在探究覆盖条件下表层土壤热性质动态变化过程及土壤热参数与水分的内在联系。结果表明:(1)相对于裸土,石子和秸秆覆盖条件下土壤热参数增大,且覆盖对于靠近表层土壤热参数的影响更加明显;(2)随降雨的发生,土壤热参数均增大,在两次降雨期间,土壤热参数逐渐减小,覆盖与裸土热参数差异逐渐增大;(3)三个热参数随降雨的发生,其动态变化过程表现不同,热容量对降雨的响应最为敏感,热导率次之,热扩散率开始减小的时间较热导率和热容量滞后,三个深度滞后时间均在48 h以上,而且覆盖以后热扩散开始减小的时间较裸土推迟(48 h以上)。土壤容重不变的情况下,在频繁干湿交替的过程中土壤水分为土壤热参数变化的最主要影响因素。覆盖条件下土壤热参数与土壤含水量关系研究表明:石子和秸秆覆盖条件下土壤热参数与土壤含水量的变化关系与裸土条件下一致,热导率与含水量呈幂函数增加的趋势,热容量随含水量线性增加,热扩散率随含水量增加先增后减,本研究所用沙黄土热扩散率峰值对应的含水量在0.20 cm3cm-3左右。由以上结果可以发现覆盖对近表层土壤热参数的动态变化有显著的影响,覆盖的保水效应直接影响土壤热参数的变化。     

冻融期不同覆盖和气象因子对土壤导热率和热通量的影响

为了研究冻融期不同覆盖和气象因子对土壤导热率和土壤热通量的影响,在2015年11月-2016年4月期间,设置了裸地(BL)、自然积雪覆盖(SC)、6 000 kg/hm~2秸秆+积雪覆盖(SM1)、12 000 kg/hm~2秸秆+积雪覆盖(SM2)和18 000 kg/hm~2秸秆+积雪覆盖(SM3)5种不同的处理,测定了20、40、60和100 cm土壤含水率和温度,并计算出土壤导热率和土壤热通量。研究结果发现:在土壤冻结期,土壤导热率随着土壤的冻结而增大,直至完全冻结后基本保持不变,而在土壤融化期则逐渐减小。冻融阶段,积雪和秸秆覆盖会延缓土壤导热率的变化,减小土壤导热率的变化。冻结期,裸地处理的土壤导热率最大,平均为1.55 W/(m×K);融化期,裸地处理的土壤导热率最小,平均为0.79 W/(m×K)。在冻结期,土壤热量向上传递,传递量先增加后减小;在融化期,土壤热量向下传递,传递量逐渐增加。积雪和秸秆覆盖可以减小土壤热通量及其变化。积雪和秸秆覆盖条件下的土壤热通量比裸地少4.73~8.84 W/m~2。裸地处理的土壤导热率与水汽压的相关性最好,相关系数为-0.84,与风速的相关性最差,相关系数为-0.43。积雪和秸秆覆盖条件下的土壤导热率与环境温度的相关性最好,相关系数为-0.67~-0.73,与风速的相关性最差,相关系数为-0.18~-0.25。土壤热通量与太阳辐射的相关性最好,相关系数为-0.88~-0.91,与风速的相关性最差,相关系数为-0.44~-0.53。整体而言,积雪和秸秆覆盖会减小大气环境对土壤导热率和热通量的影响。     

日光温室热环境模拟模型的构建

该文建立了日光温室热环境模拟模型,定量描述了日光温室内的太阳辐射、对流换热、辐射换热、热传导、自然通风和水分相变带来的潜热对日光温室热环境的影响,根据质能平衡和传热学理论,得到一组关于覆盖物、室内空气、温室分层后墙、分层地面土壤、分层后坡和作物热平衡的微分方程组。利用MATLAB的强大计算能力与VB的良好用户界面建立模拟计算软件,可求得温室各组成部分的温度。通过试验验证,该模型能够比较准确预测日光温室环境温度。     

探针有限特性对热脉冲技术测定土壤热特性的影响

在利用热脉冲方法测定热特性时,通常对探针形状做理想化处理,即假设探针为线性热源,热导率无限大而热容量为零。在实际应用中,探针本身的有限特性(有限半径以及有限热容量)会导致热特性测定误差。为了研究探针有限特性对热脉冲技术测定土壤热特性的影响,该研究采用改进的热脉冲探针(直径2 mm、长度40 mm、间距8 mm)测定土壤热特性,并分别使用PILS(pulsed infinite line source,无限长线性脉冲热源)和ICPC(identical cylindrical perfect conductors,近似圆柱形完美导体)2种理论估计土壤热特性,比较分析了探针有限特性对热脉冲技术测定热特性结果的影响。结果表明:1)与PILS理论相比,利用ICPC理论拟合得到的温度升高曲线,可以有效减少探针有限半径和热容量对土壤热特性测定结果的影响。与ICPC理论相比,在0.03~0.25 m3/m3的含水率范围内,用PILS理论得到的砂土热扩散率和热导率分别偏低11.8%和5.2%;与模拟热容量相比,PILS和ICPC理论分别将热容量高估16.1%和7.9%;2)探针有限特性对土壤热特性的影响与含水率有关:在干土上最大;随着土壤含水率的增加,其影响逐渐降低。该研究对提高热脉冲技术测定土壤热特性的准确性具有指导意义。     

Partitioning of metals (Cd, Co, Cu, Ni, Pb, Zn) in soils: concepts, methodologies, prediction and applications – a review

Prediction of the fate of metals in soil requires knowledge of their solid–liquid partitioning. This paper reviews analytical methods and models for measuring or predicting the solid–liquid partitioning of metals in aerobic soils, and collates experimental data. The partitioning is often expressed with an empirical distribution coefficient or K_d, which gives the ratio of the concentration in the solid phase to that in the solution phase. The K_d value of a metal reflects the net effect of various reactions in the solid and liquid phases and varies by orders of magnitude among soils. The K_d value can be derived from the solid–liquid distribution of added metal or that of the soil‐borne metal. Only part of the solid‐phase metal is rapidly exchangeable with the solution phase. Various methods have been developed to quantify this ‘labile’ phase, and K_d values based on this phase often correlate better with soil properties than K_d values based on total concentration, and are more appropriate to express metal ion buffering in solute transport models. The in situ soil solution is the preferred solution phase for K_d determinations. Alternatively, water or dilute‐salt extracts can be used, but these may underestimate in situ concentrations of dissolved metals because of dilution of metal‐complexing ligands such as dissolved organic matter. Multi‐surface models and empirical models have been proposed to predict metal partitioning from soil properties. Though soil pH is the most important soil property determining the retention of the free metal ion, K_d values based on total dissolved metal in solution may show little pH dependence for metal ions that have strong affinity for dissolved organic matter. The K_d coefficient is used as an equilibrium constant in risk assessment models. However, slow dissociation of metal complexes in solution and slow exchange of metals between labile and non‐labile pools in the solid phase may invalidate this equilibrium assumption.     

A LABORATORY METHOD TO MEASURE GAS DIFFUSION AND FLOW IN SOIL AND OTHER POROUS MATERIALS

An apparatus was constructed to measure diffusivity of krypton-85 and gas permeability in an enclosed core of soil of field structure or in other porous material. Sample enclosure decreased water loss by evaporation, reduced mass flow caused by changes in ambient temperature and pressure during diffusion measurement, and allowed subsequent measurement of gas permeability without further sample disturbance. When a bundle of tubes was used as a test sample to calibrate the apparatus, the resistances to diffusion and viscous flow agreed approximately with those calculated from the tube size and number. Gas movement was measured in dry sieved soil and in undisturbed cores of silty loam soil to illustrate the practical value of the method. In the dry cores, diffusivity relative to free air (D_A/D_o) was greater in ploughed soil, 0.18, than in direct drilled soil, 0.14, nearly in proportion to the greater air porosity in the ploughed soil, but air permeability in ploughed soil was four times greater than in direct drilled soil and was about 1 000 times greater than in compacted sieved soil.     

A practical formula for quantifying the thermal conductivity of Tottori dune sand

Abstract

Sustainable agriculture needs appropriate management of water, chemicals and heat in soil. In this study, we focused on thermal conductivity, which is among the various soil physical properties that are crucial for the sustainable management of agricultural fields. To expand the Mochizuki model, which describes thermal conductivity as a function of water content and solution concentration, we considered the water content, solution concentration and temperature as independent variables. The thermal conductivity of Tottori dune sand was measured under conditions of various combinations of these three independent variables. We observed that the thermal conductivity increased linearly with increasing water content, 0.054–0.276 m³ m^?3, for fixed temperature and solution concentration, and varied linearly with solution concentration for fixed temperature and water content. These results are consistent with the Mochizuki model. Using the Mochizuki model, the experimental parameters, which are dependent variables of water content and solution concentration, are shown as functions of water content. From regression analyses of the relationships between the experimental parameters and temperature, we expanded the Mochizuki model into a new practical formula that quantifies the soil thermal conductivity as a function of water content, solution concentration and temperature.     

Temperature variations in unfrozen soils with variable hydrothermal properties

A comprehensive understanding of the hydrothermal properties of soil is required to model heat distribution in unsaturated soils. In this study, we aim to model heat distribution throughout the profile of unfrozen soil while its thermal diffusivity varies with time and depth. The proposed model is based on the fundamental solution of the one‐dimensional transient heat conduction equation using the decomposition method. We calibrate our model using experimental data from soils of different textures in the literature. The new model can estimate soil thermal diffusivity at different depths and times and uses easily accessible characteristics such as the degree of saturation and the texture of the soil. In this study, the performance of the new model is compared to the performance of the simplified model in which constant thermal diffusivity is considered throughout the profile. Moreover, the model is validated by comparing it with in‐situ temperature measurements within depth of soil profiles with different textures. The results show a very good agreement between the predicted and the measured temperature throughout the soil profiles. Such a validation shows that with increasing degree of soil saturation, depletions in temperature for fine‐textured soils are more significant than those for coarse‐textured soils. Finally, the new model is applied to a double‐layer soil in the Alsace region to define temperature variation in the profile of soil with different characteristics in each layer. For a double‐layer profile, the continuity of the temperature as well as the heat flux is verified at the interface between the two layers.     

Gas diffusivity in soils compared to ideal isotropic porous media

Evidence of anisotropy is reported for advective air and water permeabilities in soils. Thus, anisotropy is likely to exist also for diffusive gas fluxes. Information about direction‐dependent soil gas diffusivity is scarce and most modeling approaches assume isotropy. At hundreds of closely lying positions in a compacted and adjacent undisturbed forest soil, gas diffusivity (D_s/D₀) was measured either in vertical or horizontal direction. The volume‐independent diffusion efficiency (i.e., diffusivity divided by air‐filled porosity) was fitted by a generalized additive model (GAM). Significant regressors were air‐filled porosity (?), soil depth, and the discrete diffusion direction. The model yields in all cases higher vertical diffusion efficiencies. The compaction factor did not yield a significant regressor of its own, i.e., the reduction of diffusivity in the compacted soil was the same as in low‐porosity samples of the undisturbed profile. To elucidate the role of sharing vertically and horizontally orientated pore space and a potential competition between diffusivity in different spatial directions, simple geometric models consisting of 3‐dimensionally crossed pores have been parameterized. These models provided a good explanation of the typical nonlinear D_s/D₀(?) relationship. By simple one‐parameter correction (linear or power function), this mechanistic model could be fitted to the data. The one‐parameter correction of the geometric model could be a straightforward approach to consider direction dependence of measured diffusivities. However, by applying this approach to the observations the anisotropy effect was not clearly evident, which could be attributed to a changing D_s/D₀(?) relationship with depth. As a reason for the preference of the vertical gas diffusion the dominance of vertical stresses and the activity of anecic earthworms are discussed. Direction dependency of gas diffusivity seems to be a basic feature of natural pore systems and has to be considered for modeling gas fluxes in soils. Generally, a preferential vertical diffusion direction reduces horizontal balancing and increases the heterogeneity of gas concentrations in the soil air.