Soil Air-filled Porosity Research Articles

Mixture of winter cover crops improves soil physical properties under no-tillage system in a subtropical environment

The monocrop of winter cover crops is a common practice under no-tillage system (NTS) in subtropical environment but increase diversity of cover crop species can be an alternative to improve soil physical quality, with positive effects on crop production. This study evaluated whether using a greater diversity of cover crop species affects soil physical properties, as well as grain yield of soybean (Glycine max L.) and common bean (Phaseolus vulgaris L.) under NTS in three growing seasons. The experimental design was in randomized blocks with four replications. The treatments were monocrop of black oat (Avena strigosa Schreb.); intercropping of black oat and turnip (Brassica napus L.); and a mixture of black oat, turnip, white oat (Avena sativa L.), rye (Secale cereale L.), white lupine (Lupinus albus L.), and common vetch (Vicia sativa L.). Cover crop biomass yield, soil physical properties, and grain yield of summer crops were evaluated. Using a mixture of cover crops improved soil physical properties by decreasing soil bulk density up to 5 %, increasing soil air-filled porosity at –100 hPa (0.0–0.2 m) up to 27 % and saturated hydraulic conductivity (0.0–0.2 m) up to 300 %. Common bean grain yield and 1000-grain weight in succession to the mixture of cover crops increased by 15 % and 4 %, respectively. Soybean yield components were not affected by treatments. The increased diversity of winter cover crops improved soil physical properties in relation to monocrop in NTS, as well as improved crop yield of common bean, which is a shallower root growth specie, more sensitive to soil constraints.

AIR-FILLED POROSITY AND YIELD OF IRRIGATED COFFEE IN AN OXISOL

Soil air-filled porosity (β) is directly related to the plants development and yield. Thus, the aim of this study was to evaluate how hourly aeration porosity (β) affects the productivity of irrigated coffee cultivated in an Oxisol in the region of Lavras, southeastern Brazil, in order to determine its ideal range. The experiment was conducted with the Rubi MG-1192 cultivar, spaced at 3.5 x 0.8 meters, in randomized blocks with six treatments and three replicates. The treatments consisted of applying percentages of the gross irrigation depth required (Li), being: non-irrigated, 60, 80, 100, 120 and 140% of Li. The soil matric potential (ym) in each treatment was measured with sensors, performing hourly readings at three depths (0.25, 0.50 and 0.75 m). The increase in gross irrigation depths provides a linear reduction of the average air-filled porosity in the soil cultivated with coffee. The T100 treatment showed a significant productivity increase, and from this value the gross irrigation depth showed a tendency to reduce coffee productivity and soil air-filled porosity. The ideal range of β between 0.152 and 0.163 m3 m-3 provided adequate aeration conditions and higher yield for coffee.
 Keywords: irrigated agriculture, productivity, soil aeration.

Physical recovery of an oxisol subjected to four intensities of dairy cattle grazing

The expansion of dairy farming in southern Brazil, mainly under a rotational grazing system, has generated soil physical changes with loss of soil quality, which in turn must be evaluated to determine how long they persist and how the soil recovers from these changes. The objective of this study was to evaluate the natural recovery of the soil physical attributes of a clayey Oxisol (Latossolo Vermelho distrófico típico, with clayey texture), which had been managed under different intensities of dairy cattle grazing: T0 = no grazing, T30 = consumption of 30 % of available forage, T50= consumption of 50 % of available forage and T80= consumption of 80 % of available forage. The rotational grazing system consisted of a grazing period (an average of 4 h) with a variable stoking rate to achieve forage consumption. For T0, zero animals, T30 three animals, T50 six animals and T80 nine animals per 100 m2 plot were used, followed by animal exclusion until the forage reached an average height of 35 cm. The grazing system was conducted for approximately 7 months, after which the animals were permanently removed from the area. The recovery of soil physical properties was monitored by soil sampling every three months for one year. The evaluated soil physical properties were porosity, bulk density and air permeability. The increase in grazing intensity negatively affected the soil physical quality. At all grazed areas, T30, T50 and T80, there was a reduction of the total porosity, the macroporosity and the soil air permeability, as well as an increase of the soil bulk density compared with the no grazed area, T0. However, 90 days after exclusion, increases in total porosity and macroporosity were observed. The angular coefficient between soil air-filled porosity and air permeability decreased as the grazing intensity increased and increased with time after grazing exclusion. Rotational grazing areas, when managed properly, can be an important tool for sustainable production. The use of grazing exclusion periods alleviates compaction, reducing the risk for forage production for the following grazing season.

Biochar increases plant growth and alters microbial communities via regulating the moisture and temperature of green roof substrates

Green roofs have increasingly been designed and applied to relieve environmental problems, such as water loss, air pollution as well as heat island effect. Substrate and vegetation are important components of green roofs providing ecosystem services and benefiting the urban development. Biochar made from sewage sludge could be potentially used as the substrate amendment for green roofs, however, the effects of biochar on substrate quality and plant performance in green roofs are still unclear. We evaluated the effects of adding sludge biochar (0, 5, 10, 15 and 20%, v/v) to natural soil planted with three types of plant species (ryegrass, Sedum lineare and cucumber) on soil properties, plant growth and microbial communities in both green roof and ground ecosystems. Our results showed that sludge biochar addition significantly increased substrate moisture, adjusted substrate temperature, altered microbial community structure and increased plant growth. The application rate of 10–15% sludge biochar on the green roof exerted the most significant effects on both microbial and plant biomass by 63.9–89.6% and 54.0–54.2% respectively. Path analysis showed that biochar addition had a strong effect on microbial biomass via changing the soil air-filled porosity, soil moisture and temperature, and promoted plant growth through the positive effects on microbial biomass. These results suggest that the applications of biochar at an appropriate rate can significantly alter plant growth and microbial community structure, and increase the ecological benefits of green roofs via exerting effects on the moisture, temperature and nutrients of roof substrates.

Reply to A.V. Smagin: IV. Surface diffusion or random noise?

Анализируется эксперимент проф. А.В. Смагина, из которого он получает численное значение коэффициента поверхностной диффузии газов в почве. Нами предпринята попытка оценить погрешность этого коэффициента. Обосновывается пессимистическая точка зрения: поскольку погрешность оказалась во много раз больше самого коэффициента диффузии, то не имеет смысла говорить о поверхностной диффузии – из рассмотренного эксперимента мы ничего не можем заключить о ее интенсивности, ибо статистически значимо она не отличается от нуля.

Soil methane oxidation in both dry and wet temperate eucalypt forests shows a near-identical relationship with soil air-filled porosity

Abstract. Well-drained, aerated soils are important sinks for atmospheric methane (CH4) via the process of CH4 oxidation by methane-oxidising bacteria (MOB). This terrestrial CH4 sink may contribute towards climate change mitigation, but the impact of changing soil moisture and temperature regimes on CH4 uptake is not well understood in all ecosystems. Soils in temperate forest ecosystems are the greatest terrestrial CH4 sink globally. Under predicted climate change scenarios, temperate eucalypt forests in south-eastern Australia are predicted to experience rapid and extreme changes in rainfall patterns, temperatures and wild fires. To investigate the influence of environmental drivers on seasonal and inter-annual variation of soil–atmosphere CH4 exchange, we measured soil–atmosphere CH4 exchange at high-temporal resolution (< 2 h) in a dry temperate eucalypt forest in Victoria (Wombat State Forest, precipitation 870 mm yr−1) and in a wet temperature eucalypt forest in Tasmania (Warra Long-Term Ecological Research site, 1700 mm yr−1). Both forest soil systems were continuous CH4 sinks of −1.79 kg CH4 ha−1 yr−1 in Victoria and −3.83 kg CH4 ha−1 yr−1 in Tasmania. Soil CH4 uptake showed substantial temporal variation and was strongly controlled by soil moisture at both forest sites. Soil CH4 uptake increased when soil moisture decreased and this relationship explained up to 90 % of the temporal variability. Furthermore, the relationship between soil moisture and soil CH4 flux was near-identical at both forest sites when soil moisture was expressed as soil air-filled porosity (AFP). Soil temperature only had a minor influence on soil CH4 uptake. Soil nitrogen concentrations were generally low and fluctuations in nitrogen availability did not influence soil CH4 uptake at either forest site. Our data suggest that soil MOB activity in the two forests was similar and that differences in soil CH4 exchange between the two forests were related to differences in soil moisture and thereby soil gas diffusivity. The differences between forest sites and the variation in soil CH4 exchange over time could be explained by soil AFP as an indicator of soil moisture status.

Soil Methane Uptake Increases under Continuous Throughfall Reduction in a Temperate Evergreen, Broadleaved Eucalypt Forest

Soils in temperate forests ecosystems are the greatest terrestrial CH4 sink globally. Global and regional circulation models predict decreased average rainfall, increased extreme rainfall events and increased temperatures for many temperate ecosystems. However, most studies of soil CH4 uptake have only considered extended periods of drought rather than an overall decrease in rainfall amount. We measured soil CH4 uptake from March 2010 to March 2012 after installing passive rainfall reduction systems to intercept approximately 40% of throughfall in a temperate broadleaf evergreen eucalypt forest in south-eastern Australia. Throughfall reduction caused an average reduction of 15.1 ± 6.4% (SE) in soil volumetric water content, a reduction of 19.8 ± 6.9% in soil water-filled pore space (%WFPS) and a 20.1 ± 6.8% increase in soil air-filled porosity. In response to these changes, soil CH4 uptake increased by 54.7 ± 19.3%. The increase in soil CH4 uptake could be explained by increased diffusivity in drier soils, whilst the activity of methanotrophs remained relatively unchanged. It is likely that soil CH4 uptake will increase if rainfall reduces in temperate broadleaf evergreen forests of Australia as a consequence of climate change.

English

The least limiting water range (LLWR) and degree of compactness (DC) can be useful indicators of soil physical quality and crop yield. This study focused on assessing of LLWR, DC and evaluation of critical values to crop growth of an Alfisol and Oxisol under no-till management. Undisturbed soil cores were taken from the layer 0.00 - 0.20 m depth. Soil water retention curve, soil penetration resistance curve, air-filled porosity and bulk density (Bd) were measured. The range of LLWR variation was limited by volumetric water content at field capacity and penetration resistance. Values of LLWR varied from 0.00 - 0.14 m 3 m -3 to Alfisol and 0.00 - 0.04 m 3 m -3 to Oxisol. The critical values of the Bd and DC for crop development were 1.79 Mg m -3 and 1.35 Mg m -3 and 96% and 74% to Alfisol and Oxisol, respectively. Further researches relating LLWR, DC and crop response are still required in soils with different conditions and management.

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.

Impact of tillage on physical characteristics in a Mollisol of Northeast China

Soil management is aimed at the maintenance of optimal soil physical quality for crop production. In order to explore the effects of tillage practices on soil physical properties, a study was conducted to compare the effects of no tillage (NT), moldboard plow (MP) and ridge tillage (RT) on soil bulk density (BD), soil penetration resistance (SPR), soil water content (SWC), soil macroporosity (MAC) and soil air-filled porosity (AFP) in Northeast China. Results showed that both NT and RT led to significant BD increment than MP at 0–20 cm (P < 0.05). Compared with MP, NT and RT increased SPR at the depths of 2.5–17.5 cm (P < 0.05). SWC of 0–10 cm layer was significantly higher in NT and RT than MP soils (P < 0.05). NT showed a significantly lower MAC than MP and RT at 0–20 cm soil depths (P < 0.05). All AFP values were above the limit of 0.10 cm<sup>3</sup>/cm<sup>3</sup> under all tillage treatments. RT improved the soil physical quality as evidenced by decreased BD and SPR, and increased SWC, MAC and AFP relative to NT.

Accounting for Time-Variable Soil Porosity Improves the Accuracy of the Gradient Method for Estimating Soil Carbon Dioxide Production

Soil porosity is usually taken as a constant over time for a given field, although in reality it decreases with time after tillage. For the gradient method, estimating soil CO2 production with a fixed porosity may lead to large errors when soil porosity varies over time. In this study, we compared soil air-filled porosity, gas diffusivity, and CO2 production based on a temporally variable soil porosity (ϕV) with those based on a constant porosity, either initial porosity just after soil tillage (ϕi) or final porosity at harvest after a tilled soil has settled (ϕf). Soil porosity was measured seven times during a maize (Zea mays L.) growing season, and an exponential relationship of soil porosity with time was developed to describe ϕV for the 0- to 5-cm soil layer. Soil CO2 production was estimated from the gradient method and the mass conservation law. Soil-surface CO2 efflux was measured with a dynamic chamber throughout the growing season. The ϕi value was 0.49 m3 m−3 and the ϕf value was 0.43 m3 m−3. Compared with results obtained from ϕV, soil air-filled porosity, gas diffusivity, and CO2 production values obtained from ϕf were 6, 11, and 22% lower, whereas values obtained from ϕi were 17, 36, and 70% larger. The soil-surface CO2 effluxes estimated with ϕV better matched the chamber values than did the estimates with ϕi or ϕf. We conclude that use of variable soil porosity improves estimations of soil-surface CO2 effluxes and soil CO2 production with the gradient method.

Behavior of carbon dioxide in soils affected by tillage systems

Agricultural management plays an important role in the storage of carbon in soils. The behavior of soil CO2 in an Andisol in two different tillage systems (no tillage and tillage) was studied. Soil-column incubation experiments were performed for a period of 150 days to conduct this study. Soil CO2 flux, under no-tillage and tillage treatments, was observed to be 0.557 and 0.616 gCO2-C m−2 d−1, respectively. The cumulative CO2 flux under tillage treatment was observed to be higher than that under no-tillage treatment, but no distinct difference in the soil carbon stock was observed between both treatments. The soil CO2 concentration under no-tillage treatment was clearly much higher than that under tillage treatment, except at a depth of 2.5 cm. Tillage decreased soil dry bulk density and enhanced soil air-filled porosity. Soil gas diffusivity, which depends on air-filled porosity, was increased by tillage at a depth of 0–15 cm, which contributed to their lower soil CO2 concentration. CO2 flux through the soil profile, calculated from the CO2 concentration, decreased with depth. Compared to the deep soils, the surface soil (0–5 cm) showed greater variation in CO2 flux. The CO2 production at depths of 0–10 cm accounted for 62.2 and 51.7 % of the whole CO2 production of the 0–30-cm soil profile for no-tillage and tillage treatments. CO2 production was higher for soil under no tillage at depths of 0–10 cm, but contrary results were observed for soil at depths of 10–30 cm.

Least-limiting water range of the soil seedbed submitted to mechanical and biological chiselling under no-till

Physical quality of the soil seedbed affects germination, seedling emergence and crop establishment. The aim of this work was to determine the least-limiting water range (LLWR) of a soil seedbed cultivated for 18 consecutive years under no-till (NT) and submitted to mechanical chiselling (NT-M) and biological chiselling by a forage radish cover crop (NT-B). The study was carried out in Ponta Grossa, Paraná, Brazil. The experimental design was randomised complete blocks with four replications. Soil samples at 0–5 and 5–10 cm depths were collected at 6 and 18 months after the start of the experiment which corresponded to maize (October 2009) and soybean (November 2010) planting. Water-retention curve, penetration-resistance curve, soil and relative bulk density, and LLWR were determined. Bulk density did not differ among treatments at 0–5 cm depth for both evaluation periods. At 5–10 cm soil depth, the NT-M treatment showed the lowest bulk density at the first sampling (2009), whereas NT-B showed the highest bulk density at the second sampling (2010). Soil penetration resistance was the most limiting factor of the LLWR, which was greater in NT-M for both soil layers at the first sampling. At the second sampling, the NT treatment had the greatest LLWR at 0–5 cm, but at 5–10 cm soil depth, both NT and NT-M had higher LLWR than NT-B. The efficiency of mechanical chiselling in improving soil seedbed physical quality lasted 18 months after its application. Biological chiselling was efficient in improving soil air-filled porosity in both periods as evaluated by the LLWR.

Forwarder traffic impacted over at least four years soil air composition of two forest soils in northeast France

Soil atmosphere composition results from a balance between biological activity and gas transfer, both likely to be affected by soil compaction following heavy traffic. We monitored soil atmosphere composition, temperature and moisture once a month for three years in the trafficked (by a wood-loaded forwarder) and undisturbed plots of two sites in the NE part of France. Our aim was to assess the impact of compaction on soil air composition and to test if soil restoration resulted in undisturbed levels of gas production/consumption and transfer. Soil air oxygen, O2 and carbon dioxide, CO2 contents were the two soil gasses most sensitive to compaction and climatic variations. During at least the first year following compaction at 30cm depth, heavy traffic resulted in an increase in soil atmosphere CO2 concentration ([CO2]) whatever the air-filled pore space. Following the first soil drought experienced at both sites, this initial impact disappeared toward an effect alternating between an increase in [CO2] when water filled pore space and a decrease when the soil was dry. The same interchanging impact was observed for soil air O2 content but with opposite trends. We assumed that soil cracks formed in the trafficked treatment due to lower resistance to stresses when the soil dried out drastically during summer droughts, resulting in an increase in soil gas diffusion while considering same soil temperature and air-filled porosity. However, three to four years following heavy traffic, soil air-filled porosity was still significantly decreased and gas production/consumption seemed to be still affected by compaction.

Stress responses of spring rape plants to soil flooding

Abstract Stress responses of spring rape to soil hypoxia were investigated during 8-days flooding. Soil air-filled porosity decreased from 25-30% to 0%, oxygen diffusion rate - from 2.6-3.5 to 0.34 μmol O2 m-2 s-1, and redox potential - from 460 to 150mVwithin few hours. Alcohol dehydrogenase activity in roots increased up to 7-fold after one day of flooding and then decreased to 170% of control. Superoxide dismutase activity in roots increased by 27% during first 3 days and then dropped to 60% of control; in the leaves superoxide dismutase activity increased in average by 44%. Ascorbate peroxidase activity in leaves increased by 37% during first 3 days and then decreased to control value. Glutathione reductase activity increased by 45% in roots of flooded plants but did not change in leaves. Proline concentration in leaves increased up to 4-fold on the 3d day of flooding and then decreased to control value. Thus soil flooding induces increase of alcohol dehydrogenase activity and subsequent increase of superoxide dismutase and glutathione reductase activities in roots while the leaves display a few days increase of free proline concentration and ascorbate peroxidase activity, and a long-term increase of superoxide dismutase activity.

Effect of soil physical properties on the long-term performance of planted Scots pine in Finnish Lapland

Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies L. Karst.) forests dominate in Finnish Lapland. This study examined the long-term effects of soil physical properties and conditions measured in intact intermediate areas, as well as site preparation, on the survival and height growth of planted pine on eight experimental sites, 25-27 yr after reforestation. On the four originally spruce-dominated sites, pine survival was the highest on sites with a high soil air-filled porosity (AFP) near saturation (at -1 kPa), a high van Genuchten parameter, and a low soil water content (SWC) in situ, and height growth was the fastest on sites with a high soil AFP in situ and a high van Genuchten parameter n, and on sites reaching a soil AFP of ca. 0.20 m3 m-3 at a high matric potential after saturation. Survival, but not mean height, was enhanced on the spruce sites by intensive site preparation methods such as ploughing instead of lighter site preparation methods. On the four originally pine-dominated sites, site preparation affected the mean height but not survival. The use of SWC as a sole criterion for sites suitable for pine reforestation was tested and found to be uncertain. Key words: Boreal forest soils, soil water content, air-filled porosity, van Genuchten function, site preparation, reforestation, Scots pine

Variability of soil potential for biodegradation of petroleum hydrocarbons in a heterogeneous subsurface

Quantifying the spatial variability of factors affecting natural attenuation of hydrocarbons in the unsaturated zone is important to (i) performing a reliable risk assessment and (ii) evaluating the possibility for bioremediation of petroleum-polluted sites. Most studies to date have focused on the shallow unsaturated zone. Based on a data set comprising analysis of about 100 soil samples taken in a 16 m-deep unsaturated zone polluted with volatile petroleum compounds, we statistically and geostatistically analysed values of essential soil properties. The subsurface of the site was highly layered, resulting in an accumulation of pollution within coarse sandy lenses. Air-filled porosity, readily available phosphorous, and the first-order rate constant ( k 1) of benzene obtained from slurry biodegradation experiments were found to depend on geologic sample characterization ( P < 0.05), while inorganic nitrogen was homogenously distributed across the soil stratigraphy. Semivariogram analysis showed a spatial continuity of 4–8.6 m in the vertical direction, while it was 2–5 times greater in the horizontal direction. Values of k 1 displayed strong spatial autocorrelation. Even so, the soil potential for biodegradation was highly variable, which from autoregressive state-space modeling was partly explained by changes in soil air-filled porosity and gravimetric water content. The results suggest considering biological heterogeneity when evaluating the fate of contaminants in the subsurface.

Reduction of bias in static closed chamber measurement of δ13C in soil CO2 efflux

The (13)C/(12)C ratio of soil CO(2) efflux (delta(e)) is an important parameter in studies of ecosystem C dynamics, where the accuracy of estimated C flux rates depends on the measurement uncertainty of delta(e). The static closed chamber method is frequently used in the determination of delta(e), where the soil CO(2) efflux is accumulated in the headspace of a chamber placed on top of the soil surface. However, it has recently been shown that the estimate of delta(e) obtained by using this method could be significantly biased, which potentially diminish the usefulness of delta(e) for field applications. Here, analytical and numerical models were used to express the bias in delta(e) as mathematical functions of three system parameters: chamber height (H), chamber radius (R(c)), and soil air-filled porosity (theta). These expressions allow optimization of chamber size to yield a bias, which is at a level suitable for each particular application of the method. The numerical model was further used to quantify the effects on the delta(e) bias from (i) various designs for sealing of the chamber to ground, and (ii) inclusion of the commonly used purging step for reduction of the initial headspace CO(2) concentration. The present modeling work provided insights into the effects on the delta(e) bias from retardation and partial chamber bypass of the soil CO(2) efflux. The results presented here supported the continued use of the static closed chamber method for the determination of delta(e), with improved control of the bias component of its measurement uncertainty.

Growth of rose roots and shoots is highly sensitive to anaerobic or hypoxic regions of container substrates

Commercial greenhouse cut rose plants commonly have shallow root systems, even in well-drained substrates. We studied rose plant responses to a wide range of soil air-filled porosity values to determine tolerance to low soil aeration. ‘Kardinal’ rose plants on ‘Natal Briar’ rootstock were grown for 11 weeks in containers holding 2.5 l of Yolo loam soil. The soil had been treated with a polyacrylamide soil conditioner to stabilize aggregates after they had been separated into three size fractions: coarse (1–2 mm), medium (0.1–1 mm), and fine (<0.1 mm). Plants were irrigated frequently to keep the soil at container capacity. Total new root growth was greatest in the medium and coarse soil fractions, which had average air-filled porosities of 7.6% and 15.9%, respectively. The fine soil fraction, which had an average air-filled porosity of 1.3%, had almost no root growth. In all soil fractions, roots were absent or scarce in regions of soil that were anaerobic or hypoxic. Highest root length densities occurred at air-filled porosity values of 12–17%, and no new roots grew at air-filled porosity values below 3%. Shoot yields were lower for plants in the substrate with low air-filled porosity, and their leaves had lower stomatal conductance, chlorophyll content, mid-day water potential, and macronutrient concentrations. Frequent irrigation that precludes aeration of the lower regions of the substrate profile will result in shallow root systems and could lead to decreased yields.

Linear Model to Predict Soil-Gas Diffusivity from Two Soil-Water Retention Points in Unsaturated Volcanic Ash Soils

Risk assessment and design of remediation methods at soil sites polluted with gaseous phase contaminant require an accurate description of soil-gas diffusion coefficient (Dp) which is typically governed by the variations in soil air-filled porosity (va). For undisturbed volcanic ash soils, recent studies have shown that a linear Dp(va) model, taking into account inactive air-filled pore space (threshold soil-air content, va, th), captured the Dp data across the total soil moisture range from wet to completely dry conditions. In this study, we developed a simple, easy to apply, and still accurate linear Dp(va) model for undisturbed volcanic ash soils. The model slope C and intercept (interpreted as va, th) were derived using the classical Buckingham (1904) Dp(va) power-law model, vaX, at two soil-water matric potentials of pF 2 (near field capacity condition) and pF 4.1 (near wilting point condition), and assuming the same value for the Buckingham exponent (X=2.3) in agreement with measured data. This linear Dp(va) prediction model performed better than the traditionally-used non-linear Dp(va) models, especially at dry soil conditions, when tested against several independent data sets from literature. Model parameter sensitivity analysis on soil compaction effects showed that a decrease in slope C and va, th due to uniaxial reduction of air-filled pore space in between aggregates markedly affects the magnitude of soil-gas diffusivity. We recommend the new Dp(va) model using only the soil-air contents at two soil-water matric potential conditions (field capacity and wilting point) for a rapid assessment of the entire Dp-va function.