Air-filled Pore Space Research Articles

Physical Properties of Whole Fresh-ground Parboiled Rice Hulls for Use as a Horticultural Root Substrate

Horticultural root substrates are designed to provide the optimal physical properties for plant growth. These properties include bulk density (g·cm-3), air-filled pore space (% v/v), total pore space (% v/v), water-filled pore space (% v/v), water-holding capacity (% v/v and w/w), and wettability. Whole, fresh parboiled rice hulls were ground to produce four grades with varying particle size distributions. Particle sizes for the four grades ranged from <0.25 to >2.80 mm. Additionally, discrete particle sizes of <0.25, 0.50, 1.00, 2.00, 2.80, and >2.80 mm were produced. For all grade distributions and particle point sizes, physical properties were determined and contrasted against Canadian sphagnum peat. As the proportion of smaller particle sizes in the distributions increased or as the particle point sizes decreased, total pore space (% v/v) and air-filled pore space (% v/v) decreased, while, bulk density (g·cm-3) and water-holding capacity (% v/v and w/w) increased. Additionally, as the proportion of particle sizes from <0.25–0.50 mm increased, the wettabilty of the whole fresh parboiled rice hull material decreased. Particle sizes ranging from 1.00–2.80 mm possessed the physical properties most suitable for plant growth in containerized greenhouse crop production and were most similar to peat.

PREDICTIVE-DESCRIPTIVE MODELS FOR GAS AND SOLUTE DIFFUSION COEFFICIENTS IN VARIABLY SATURATED POROUS MEDIA COUPLED TO PORE-SIZE DISTRIBUTION

Accurate description of the soil-gas diffusion coefficient (DP) as a function of air-filled (ε) and total (Φ) porosities is required for studies of gas transport and fate processes. After presenting predictive models for DP in repacked and undisturbed soils (Part I and II), this third paper takes a more descriptive approach allowing for the inclusion of inactive air-filled pore space, εin. Three model-based interpretations of εin are presented: (1) a simple power-law model (labeled Millington-Call) with the exponent (V) taken from Millington (1959; Science 130:100-102), and expanded with a constant εin term (= 0.1 m3 m−3), (2) a model (SOLA) based on analogy with solute diffusion and assuming a linear increase in pore continuity from zero at the threshold air-filled porosity where gas diffusion ceases (εth) to a maximum at ε = Φ, (3) a power-law model (VIPS) assuming variable εin that linearly decreases from a maximum at ε = εth to zero at ε = Φ. Assuming εth = 0.1 m3 m−3, all three models satisfactorily predicted DP in 18 repacked soils. The difference between the three models is mainly pronounced for higher-Φ soils, and each model has its own advantage. The SOLA model together with similar models for solute diffusivity allows a direct comparison of pore continuity in the soil gaseous and liquid phases, suggesting large differences in tortuosity and inactive fluid-phase between the two phases. The low-parameter Millington-Call model could account for variability in measured DP along a field transect (Yolo, California) by varying εin with ±0.03 m3 m−3 and is applicable for stochastic gas transport simulations at field scale. The mathematically flexible VIPS model highly accurately fitted DP(ε) data for undisturbed soil, illustrating the large possible variations in εth and V. The VIPS model is coupled with the van Genuchten (vG) soil-water characteristic model, yielding a closed-form expression for DP as a function of soil-water matric potential. The VIPS-vG model is useful to illustrate the combined effects of pore size distribution and inactive pore space on soil-gas diffusivity.

Modeling trophic pathways, nutrient cycling, and dynamic stability in soils

Summary Soil communities are compartmentalized into pathways of trophic interactions and nutrient flows that originate from plant roots, bacteria and fungi. The pathways differ in terms of the organisms that comprise them, the habitats that the organisms occupy and the rates by which the organisms process and transfer material and energy. The fungi, nematodes and arthropods within the fungal pathway live in air-filled pore spaces and water films, while the bacteria, protozoa, and nematodes within the bacterial pathway occupy water-filled pore spaces and water films. Organisms within the fungal pathway have longer generation times and process matter at slower rates than those within the bacterial pathway. Empirical studies have shown that under natural conditions the pathways co-exist in a stable manner. The relative sizes (indexed by the densities of organisms) and activities (indexed by nutrient-flow rates, excretion rates and respiration rates) of the pathways may change seasonally and in response to minor disturbances, but they persist. However, large anthropogenic and natural disturbances induce shifts in the relative sizes and activities of the pathways. Coincident with these shifts are reports of changes in the aboveground plant community and the availability and retention of plant limiting nutrients. We developed simple models of the bacterial and fungal pathways to explore the consequences of the observed shifts on the dynamic stability of the system. The more stable configurations occurred when there was a balance in the flow of nutrients between the two pathways. Large shifts in nutrient cycling and community structure towards either the fungal pathway or toward the bacterial pathway resulted in less stable or unstable configurations.

Review of Field Methods for the Determination of the Tortuosity and Effective Gas-Phase Diffusivity in the Vadose Zone

Modeling the gas exchange flux between soil and the atmosphere, risk assessment, and the evaluation of remediation strategies at contaminated sites require the knowledge of gas-phase diffusivities in the subsurface. We review methods to measure the tortuosity factor or the effective gas-phase diffusion coefficient in situ. The strong dependency of these parameters on the structure and volume of the air-filled pore space in the subsurface calls for an accurate and robust in situ measurement. A variety of approaches have been proposed during the last decades, each based on the observation and interpretation of gaseous tracer diffusion in near-surface soils or the deeper vadose zone under various initial and boundary conditions. We briefly describe the conceptual basis and experimental setup of each method and give insight into error propagation. We then discuss 115 effective diffusion coefficients D e compiled from the original method papers and applications. In situ methods and laboratory measurements on undisturbed soil cores yield comparable results. The Penman relationship, D e/ D m 0.66θa, sets an upper limit for the field-determined effective diffusion coefficient in the case of uniform porosity. The Moldrup relationship, \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(D\_{\mathrm{e}}/D\_{\mathrm{m}} = \mathrm{{\theta}}^{2.5}\_{\mathrm{a}}/\mathrm{{\theta}}\_{\mathrm{t}}\) \end{document}, originally proposed for sieved and repacked soils, gave the best predictions of several porosity-based relationships, but the relative deviation between observed and predicted D e can be substantial. Therefore, the application of such a relationship for the site-specific modeling of gas-phase diffusion should be justified with in situ measurements, especially in heterogeneous environments.

Ground Bovine Bone as a Perlite Alternative in Horticultural Substrates

Two grades of ground bovine bone were evaluated as potential alternatives to perlite in horticultural substrates. The bulk density of small and large bone-amended substrates was significantly higher than equivalent perlite-amended substrates. Large and small bone increased the air-filled pore space of sphagnum peat. However, at 10% and 20% (v/v), neither size of bone resulted in as high an air-filled pore space as equivalent amounts of perlite. At 30% and 40%, incorporation of small bone resulted in a similar air-filled pore space as incorporation of equivalent amounts of perlite, and incorporation of large bone resulted in a higher air-filled pore space than incorporation of equivalent amounts of perlite. Water-filled pore space and water-holding capacities of substrates were inversely related to air-filled pore space. When placed in a moist substrate, mineral elements within the bone were able to leach into the substrate over time. Substrates amended with 40% large and small bone had significantly higher concentrations of ammonium (NH4+), phosphorus (P), potassium (K), calcium (Ca), sodium (Na), and chloride (Cl-) than the 40% perlite-containing substrates. Substrates amended with 40% large bone had similar concentrations of magnesium (Mg), sulfur (S), iron (Fe), and copper (Cu) while substrates amended with 40% small bone had higher levels of these elements than perlite-amended substrates. Substrate concentrations of nitrate (NO3-), manganese (Mn), zinc (Zn), and boron (B) were not different among the substrates after 4 weeks in the greenhouse. The pH, electrical conductivity (EC) and NH4+ levels of bone-amended substrates increased to levels significantly higher than recommended and resulted in rapid mortality of `Orbit Cardinal' geranium (Pelargonium × hortorum), `Cooler Blush' vinca (Catharanthus roseus), and `Dazzler Rose Star' impatiens (Impatiens walleriana) plants grown in bone-amended substrates. Therefore, ground bovine bone was not a feasible alternative to perlite for use in horticultural substrates.

Impact of soil compaction upon soil water balance and maize yield estimated by the SIMWASER model

The capability of the soil water balance model SIMWASER to predict the impact of soil compaction upon the yield of maize ( Zea mays L.) is tested, using the results of a field experiment on the influence of soil compaction by wheel pressure upon soil structure, water regime and plant growth. The experimental site was located on an Eutric Cambisol with loamy silt soil texture at an elevation of 260 m in the northern, semi-humid sub-alpine zone of Austria. Within the experimental field a 7 m wide strip was compacted by a tractor driven trailer just before planting maize in May 1988. Compression effects due to trailer traffic resulted in distinct differences of physical and mechanical soil parameters in comparison with the uncompressed experimental plots down to a depth of about 30 cm: bulk density and penetration resistance at field capacity were increased from 1.45 to 1.85 g/cm 3, and from 0.8 to 1.5 MPa, respectively, while air-filled pore space as well as infiltration rate were appreciable lowered from about 0.08–0.02 cm 3/cm 3 and from 50 to 0.5 cm per day, respectively. The overall effect was a clear depression of the dry matter grain yield from 7184 kg/ha of the non-compacted plot to 5272 kg/ha in the compacted field strip. The deterministic and functional model SIMWASER simulates the water balance and the crop yield for any number of crop rotations and years, provided that daily weather records (air temperature, humidity of air, global radiation, wind and precipitation) are available. Crop growth and soil water regime are coupled together by the physiological processes of transpiration and assimilation, which take place at the same time through the stomata of the plant leaves and are both reacting in the same direction to changes in the soil water availability within the rooting zone. The water availability during rainless seasons depends on the hydraulic properties of the soil profile within the rooting depth and on rooting density. Rooting depth and density are affected by both the type of the crop and the penetration resistance of the soil, which depends on the soil moisture status and may be strongly increased by soil compaction. The model SIMWASER was able to simulate these effects as shown by the calculated grain yields, which amounted in the non-compacted plot to 7512 and to 5558 kg dry matter/ha in the compacted plot.

Growth of Ornamental Plants in Container Media Amended with Biosolids Compost

The use of biosolids compost, in the formulation of media used in the commercial production of container grown nursery crops, has been slow in the Northeast region of the United States. When biosolids compost is used in growing media, it is limited to small percentages. Regulations in Connecticut restrict the use of most biosolids compost to growing media for containerized ornamental plants and landfill cover. Information on the benefits of using biosolids compost, to grow a wide range of plant species in containers, could increase usage by nurseries. Seven species of flowering annuals, nine species of herbaceous perennials and eight species of woody ornamentals were grown in media containing 0, 25, 50, and 100 percent (by volume) biosolids compost, in combination with a mixture of bark, peat and sand. Biosolids compost came from the Metropolitan District Corporation (MDC) facility in Hartford, Connecticut. It was a mixture of wood chips and digested biosolids (3:1 by volume). Optimal plant growth generally occurred in media containing 50 and 100 percent compost. Plants growing in media high in compost were often somewhat stunted and chlorotic for several weeks after planting probably due to higher levels of salinity and ammonium nitrogen in their media. However, by the middle of the growing season these plants had recovered and at season's end, they were often superior to plants grown in media with less compost. Increasing proportions of compost generally increased the amounts of plant nutrients and heavy metals in media while decreasing air filled pore space. All heavy metal concentrations were below levels of concern.

Diffusion and seasonal dynamics of O2 in woody debris from the Pacific Northwest, USA

O2 is an important regulator of physiological processes involved in the decomposition of woody debris, yet O2 levels and diffusion rates within decomposing logs are largely unknown. We examined how O2 diffusion rates in decayed and sound wood varied with moisture and density, and we compared predicted with observed seasonal changes in oxygen concentration in logs in a Pacific Northwest old-growth Pseudotsuga menziesii forest. In the laboratory, the oxygen diffusion coefficient (DO2) was determined in the longitudinal and radial (or tangential) directions on wood cores of varying moisture content and density. In the field, O2 was measured in tubes inserted to three radial depths (2, 6 and 15 cm) within logs of two species (Pseudotsuga menziesii and Tsuga heterophylla) and five decay classes (where class 5 = most decayed). In both the radial and longitudinal directions, DO2 increased exponentially as the air filled pore space (AFPS) increased and as density decreased. In the field, mean O2 concentrations in logs were not significantly different between species. Mean O2 concentrations were significantly lower in the least decayed logs as compared to the most decayed logs. Mean O2 concentrations decreased with radial depth only in decay class two logs. Seasonal O2 levels did not consistently vary with log moisture, respiration, or air temperature. The comparison of the results from a model that assumes oxygen diffuses only in the radial direction to field data indicates that laboratory measurements of oxygen diffusion may underestimate field oxygen concentrations. Cracks, insect galleries and other passages in decayed logs, and longitudinal oxygen diffusion may account for this discrepancy. In the field, log oxygen concentrations were rarely as low as 2%, indicating anaerobic conditions may not be as common in logs as we previously thought. Oxygen limitations on decomposition may occur in relatively sound and/or water soaked wood, but probably not in decayed logs in a terrestrial setting.

Characterization of the physico-chemical properties of environmentally friendly organic substrates in relation to rockwool

SummaryTrials over two years aimed to characterize the physico-chemical properties and high-temperature responses of ecologically sound untreated organic substrates coconut (Cocos nucifera L.) coir and carbonated rice husk in relation to that of rockwool using tomatoes (Lycopersicon esculentum Mill.) as a test crop. In all substrates, water-filled pore space and water holding capacities were larger and air filled pore space was smaller. Bulk densities, water holding capacity and water-filled, air-filled and total pore spaces were lower in carbonated rice husk than coconut coir and rock wool. These values in coconut coir and carbonated rice husk were much increased with use. Most of the chemical properties, namely cation exchange capacity, electrical conductivity, pH, nitrate-N, P, K, Ca and Mg of these substrates were within recommended levels as growing media. There were no significant differences in plant height, stem diameter, individual fruit weight, percent fruit set, harvest index, ascorbic acid, total soluble solid, fruit pH, leaf chlorophyll contents, root dry matter percentage, fruit dry matter percentage and shoot/root ratio. There was less fluctuation in absolute growth rate, relative growth rate, net assimilate rate and leaf area ratio among the treatments. It also appeared that carbonated rice husk and coconut coir gave better crop performance and yield of tomatoes than rock wool under high temperature stress conditions namely 30°C and 35°C as compared with 25°C. Furthermore, crop productivity from the organic substrate coconut coir gave more profit than the popular rock wool. Thus, carbonated rice husk and coconut coir can be used successfully as a growing media amendment for producing fruit vegetables. .

Greenhouse Growth of Marigolds in Three Leached Sources of Spent Mushroom Compost over a 3-Year Period

Though high electrical conductivity (EC) levels are commonly held to be the primary limiting factor for using spent mushroom compost (SMC) as a growing substrate, EC can be reduced by leaching. This allowed SMC to be successfully used for growing plants. Leaching reduced EC of the substrate solution from as high of 30 dS·m-1 (mmhos·cm-1) to 2 to 3 dS·m-1, a level acceptable for growing plants. The initial EC and container capacity determined the number of leachings and total volume of water required to lower EC of SMC substrates to acceptable levels. As the concentration of SMC was increased, a higher number of leachings or larger volume of water were required to adequately reduce EC levels. In trials spanning 2.5 years, SMC was effectively used as a substrate in the production of marigold (Tagetes patula) `Yellow Girl'. Benefits to plant growth from SMC incorporation included a slow release of nutrients as the SMC decomposed and a good air-filled pore space/water-holding capacity when amended with a commercial nursery mix. From these trials, it is recommended that SMC be incorporated at rates of 25% to 50%. It is not recommended that SMC be used in concentrations over 50% because the EC may be too difficult to manage and the high levels of air-filled pore space of SMC. Though season may affect the initial EC level of SMC, such variation is minimized by leaching while differences in plant response are more likely to be attributed to environmental conditions. No differences in plant growth were observed among SMC sources.

Tensile strength of soil cores in relation to aggregate strength, soil fragmentation and pore characteristics

Tensile failure of soil is desirable in tillage. Soil tensile strength estimates most often are obtained from compression tests of dry aggregates. As tillage is performed under moist conditions, it would be highly relevant to measure tensile strength at high water contents. Plough-layer soil was sampled in a compacted soil (PAC) and in a non-compacted reference soil (REF). Tensile strength was measured in a new direct tension test using undisturbed soil cores (4.45 cm in diameter and 5.00 cm in height) adjusted to either −50 or −100 hPa matric potential. The air-filled pore space, ε a, was determined from water retention measurements. Air permeability, K a, was determined at −30, −100 and −300 hPa matric potentials and from these measurements an index of pore organization (PO= K a/ ε a) was calculated. Soil behaviour in the field was evaluated at approximately −300 hPa matric potential by measuring soil strength using the torsional shear box method and soil fragmentation using a simple soil drop test. The direct tensile strength results showed that the PAC soil had significantly higher tensile strength than the REF soil (e.g. 3.2 and 2.0 kPa, respectively at −100 hPa matric potential). This finding was in accordance with the aggregate tensile strength results and also agreed well with soil fragmentation in the field (i.e. geometric mean diameter (GMD) equal to 38.7 and 14.2 mm, respectively, for PAC and REF samples dropped from 75 cm height). The tensile strengths of the soil cores were close to the predicted values determined from the aggregate tensile strength results. The energy input in the soil drop test (i.e. approximately 8.9 J kg −1 dry soil) was low in comparison with the energy input in tillage but high compared with the specific rupture energy of soil aggregates (e.g. 0.4 and 0.3 J kg −1, respectively for PAC and REF aggregates adjusted to −100 hPa matric potential). The relatively poor fragmentation in the soil drop test indicated that a substantial amount of the energy input was stored as volumetric strain energy and/or lost to processes such as plastic deformation. The tensile strength of soil cores was negatively correlated to the macroporosity of the soil, whereas the ease of soil fragmentation was positively correlated to PO.

Continuous on-line calibration of diffusive soil-atmosphere trace gas transport using vertical 220Rn- and 222Rn-activity profiles

Continuous monitoring of 220Rn- and 222Rn-activities above and below the soil surface combined with sporadic direct 222Rn-flux measurements is used to quantify diffusive trace gas transport in the air-filled pore space of soil, through the soil-atmosphere interface and in the lowest layers of the atmosphere. In a calm night, 222Rn-activities above the surface first build-up near the ground (z<10 cm) and subsequently with a delay of 2-3 hours at higher altitudes (z<5 m). Knowing (1) the 222Rn-flux from activity profiles measured in soil gas, (2) from direct flux determinations and (3) using information about atmospheric diffusion parameters from 220Rn-activities measured near the surface it is possible to model the temporal evolution of the vertical 222Rn-profiles in a night with stable weather and constant soil conditions. The system operates automatically for extended periods of time in the field enabling a better understanding of transport processes in response to changing environmental conditions (wind, rain, soil humidity).

Soil respiration and methane flux in adjacent forest, grassland, and cornfield soils in Hokkaido, Japan

Soil respiration and methane flux from adjacent forest, grassland, and cornfield were measured by using the closed chamber method from June to November, 1999 in Shizunai, Hokkaido, Japan, where the soil was an Aquic Humic Udivitrands derived from volcanic ash. The forest soil absorbed methane, at arate ranging from -0.12 to -0.02 mg C m-2 h-1, while the grassland soil emitted methane, at a rate ranging from undetectable levels to 0.18 mg C m-2 h-1. In the cornfield soil methane flux ranged from -0.01 to 0.04 mg C m-2 h-1. The soil respiration rate varied from 3 to 230 mg C m-2 h-1, 27 to 372 mg C m-2 h-1, and 29 to 156 mg C m-2 h-1 for the cornfield, grassland, and forest soils, respectively. Linear regression analysis demonstrated that the methane flux rate was positively correlated with the soil water-filled pore space (WFPS), and negatively correlated with the relative gas diffusion coefficient (D/D o) and air-filled pore space (AFPS). Soil respiration rates were positively correlated with the soil temperature at all the sites. The Q 10 value was 4.8, 3.3, and 1.9 for the cornfield, grassland, and forest soils, respectively.

ESTIMATIVA DE VARIAÇÕES NO COEFICIENTE RELATIVO DE DIFUSÃO DOS GASES NO SOLO COM BASE NA ANÁLISE DA CURVA DE RETENÇÃO DA ÁGUA

O objetivo desta pesquisa foi verificar a possibilidade do uso da curva de retenção de água para estimar os incrementos no coeficiente relativo de difusão dos gases no solo, Ds/Do, durante o processo de drenagem em um Cambissolo. Numa primeira etapa, utilizou-se a curva de retenção de água para obter informações sobre os incrementos no espaço aéreo, durante a drenagem do solo. Verificou-se que os maiores incrementos, no espaço aéreo no solo, ocorreram no intervalo de tensão matricial de 0 a 20kPa. Na segunda etapa da pesquisa, utilizando gás metano (CH4), estudou-se os incrementos em Ds/Do durante a drenagem, a partir da determinação do coeficiente de difusão dos gases no solo (Ds) numa célula de difusão de duas câmaras. Essas determinações foram feitas em amostras com tensão matricial de 10kPa, 20kPa e secas ao ar, ou seja, com tensão matricial maior que 1500kPa. Os resultados obtidos evidenciaram que tanto os incrementos médios no espaço aéreo, quanto os incrementos médios em Ds/Do, foram cerca de 100 vezes maiores no intervalo de tensão matricial de 0 a 20kPa, do que no intervalo de tensão matricial de 20 a 1500kPa. Concluiu-se que, a partir da análise da curva de retenção, é possível identificar o intervalo de valores de tensão matricial em que ocorrem os maiores incrementos no espaço aéreo e em Ds/Do.

257 Characterization of the Physico-chemical Properties of and Plant Response to Ecologically Sound Organic Substrates in Relation to Rockwool

Characterization of physico-chemical properties of ecologically sound unprocessed coconut (Cocos nucifera L.) coir and carbonated rice husk in relation to rockwool were investigated to examine the crop performance along with productivity of tomatoes (Lycopersicon esculentum Mill.). In all substrates, the water-filled pore space and water-holding capacity were larger and air-filled pore space was smaller. Bulk densities, water-holding capacity, and water-filled, air-filled, and total pore spaces were lower in carbonated rice husk than coconut coir and rockwool. These values in coconut coir and carbonated rice husk were increased by use. Most of the physical properties, EC, pH, and inorganic elements, of these natural organic substrates were within appropriate levels as growing media. There were little differences in plant height, stem diameter, percent fruit set, harvest index, ascorbic acid, total soluble solid, fruit pH, and leaf chlorophyll ratio. But, number of nodes, internode length, leaf number and area, days to first anthesis, flower number, and fruit number and weight differed significantly among treatments. There was smaller fluctuation in absolute growth rate, relative growth rate, net assimilate rate, and leaf area ratio among the treatments. It appeared that carbonated rice husk and the coconut coir gave better crop performance than rockwool under moderate high temperatures (30 and 35 °C compared to 25 °C). Furthermore, crop productivity from the organic substrate coconut coir and carbonated rice husk gave more profit than that of rockwool. Thus, carbonated rice husk and coconut coir substrates can be used successfully as a bag culture media amendment for producing vegetables, especially in tropical and subtropical areas.

Surficial processes and CO 2 flux in soil ecosystem

Global warming as a result of rising levels of atmospheric CO 2 concentration has become an issue of increasing environmental concerns. This study is to estimate the role of surficial processes, including solar radiation, air temperature, relative humidity, rainfall, soil water movement and heat flux, and soil respiration, on CO 2 diffusive flux into the atmosphere from a soil ecosystem. An existing one-dimensional mathematical model for the simultaneous movement and transport of water, heat, and CO 2 through the unsaturated soil is modified for the purpose of this study. Two simulation scenarios (i.e. daily and monthly) are performed to estimate the CO 2 flux through the soil ecosystem. Simulation results show that surficial processes have decisive effects on CO 2 flux through the soil ecosystem. Of the processes examined in this study, solar radiation is one of the most important processes. It governs the daily cycles of soil temperature and water evaporation, which in turn controls the soil CO 2 production rates, and thereby the CO 2 flux into the atmosphere. Rainfall is another important process that controls the monthly CO 2 flux. It determines the soil water content available for biological respiration and the air-filled pore spaces available for CO 2 flux. Daily cycles of the soil CO 2 production rate are similar to those of the surface temperature, but the overall magnitude decreases consecutively in response to the increase in soil water content. Soil CO 2 production rate is controlled by both soil temperature and soil water content. As the soil water content decreases, the overall CO 2 production rate is expected to decrease. The pattern of CO 2 flux is more or less similar to that of the surface soil temperature (i.e. increasing during the day and decreasing during the night), but the overall magnitude decreases consecutively over time. The daily variations of surface CO 2 flux are driven by soil temperature, whereas the overall increase in CO 2 flux rate is due to the increase in CO 2 concentration immediately beneath the soil surface. This study suggests that surficial processes play an ultimate role in soil CO 2 flux into the atmosphere.

A New Explanation for the Behavior of Saturated Soils Subject to Vigorous Vibration

The high effectiveness of the deep compaction of saturated soils by percussive loads (heavy tampers , piles, deep-seated blasting), the cause of which had not been explained until now, is a basic peculiarity o f their behavior . Much information about this problem is cited in [1] . The effectiveness of the percussive compaction of gas-containin g soils has never been in doubt. As fo r the compaction of saturated soils, many soil engineers have referred to use of this method for some time wit h skepticism, and have placed the validity of the results obtained in doubt. This is explained by the existin g notion of a saturated soil as a medium that is essentially incompressible under short-lived loads. Specialists consider that the compaction of saturated soils should be accompanied by the pressing o f water from the pores in the soil. Moreover, the pressing is represented as the compressive squeezing of a sa t urated soil due to squeezing of the water-free and air-filled pore space, which usually amounts to 1-3% of th e volume of the pores (internal pressing), and also as an outflow of fluid through drainage interlayers into th e peripheral sections of the soil being compacted (external pressing). The hypothesis of soil liquefaction, which consists in the fact that the soil is liquefied each time in th e impact phase, and is consolidated in the at-rest phase, was proposed to explain the mechanism responsible fo r the consolidation of soil during its compaction by repeated impacts. After comprehensive analysis, Rabinovic h and Khavin [1] drew the following conclusions . The high effectiveness of the compaction of saturated soils by percussive loads can be explained, if w e adopt the acoustic cavitation that develops in the pore water under vigorous vibration as an intermediate lin k in the formulation of an opinion . Acoustic cavitation (a short-lived pressure pulse to 1 0 3 MPa and higher, and the heating of gas in bu b bles to 10°C) is the reason that the pore water is excited, and electromolecular interactions between the wate r molecules and the surface of the soil particles are disrupted one with the other. As a result, all pore wate r becomes free water and is less viscous; as compared with what one would expect, this appreciably increase s the water’s mobility in the soil. This, in turn, increases the volume of externally pressed water, which co n tributes to the soil’s compaction . The cavitation initiates and accelerates many physico-chemical reactions [2], which in some manner ,

Continuity of air-filled pores and invasion thresholds for a soil-borne fungal plant pathogen, Rhizoctonia solani

Fundamental knowledge of the way fungi explore the pore volume within a soil is crucial if we are to understand how soil physical conditions affect population dynamics and invasion of many important fungal parasites and saprophytes within soil. In this study, spread of the fungus Rhizoctonia solani Kühn (AG4), an economically important soil-borne plant pathogen and saprophyte, was quantified in relation to tortuosity and continuity of the air-filled pore space. Samples containing fine or coarse sand were equilibrated at matric potentials ranging from −1 to −7 kPa and inoculated with R. solani. We quantified the colony extent and biomass of R. solani spreading from a localized source of inoculum using microscopy and a monoclonal antibody-based immunosorbent assay. Air permeability rapidly increased when the air-filled pore space became continuous below a threshold matric potential of −2.0 kPa for the fine sand and −1.0 kPa for the coarse sand. Fungal spread was limited at high matric potentials with a colony extent of 6.6 and 4.6 mm for the fine and coarse sand, respectively, but increased to 32.6 and 28.5 mm, with a sharp transition below a threshold matric potential of −2.2 and −1.3 kPa. The average colony biomass dropped markedly at the same threshold matric potential. We conclude that the ability of the fungus to invade soil depends on the connectivity and tortuosity of the air-filled pore volume. At near-saturated conditions, the spread was spatially constrained to a relatively small volume, forming small dense colonies. In a well connected air-filled pore volume, larger colonies with low biomass density were formed. The broader implications for invasion of soil by a fungus and for transmission of fungal diseases are discussed.

Processes controlling the oxygen isotope ratio of soil CO 2: analytic and numerical modeling

The 18O/ 16O ratio of soil CO 2 is important for both global and ecosystem scale budgets of the 18O/ 16O ratio of atmospheric CO 2, as well as for using this ratio as a proxy for the isotopic ratio of soil water. The 18O/ 16O ratio of soil CO 2 reflects that of soil water due to the isotope exchange which occurs during the CO 2 hydration reaction. The rate of this isotope exchange reaction strongly influences the oxygen isotope ratio of soil CO 2, and may be about two orders of magnitude slower than the isotope exchange reaction between CO 2(aq) and water (effective first order rate constant, k eff, of 10 −4 versus 10 −2 s −1 at 25°C). This difference in rate reflects transport across the soil water/soil air interface, which limits the overall rate of reaction. A diffusion-production-reaction model (Hesterberg and Siegenthaler, 1991) suggests that the 18O/ 16O ratio of soil CO 2 at depth deviates from the equilibrium value by ∼1–2‰ if k eff = 10 −4 s −1, negating the assumption that soil CO 2 is in isotopic equilibrium with soil water. A sensitivity analysis of this model indicates the following factors affect 18O/ 16O ratio of soil CO 2 (in order of decreasing importance): (1) the isotope ratio of soil water; (2) the rate constant of the isotope exchange; (3) soil air-filled pore space and tortuosity. Notably, soil respiration rate has only a minor effect. Advection of soil gas due to wind pumping has a negligible effect on the δ 18O value of soil CO 2 except in most extreme cases. Likewise, transient effects on the oxygen isotope ratio of soil CO 2 due to changes in soil respiration rate are small.

The effects of chamber pressurization on soil‐surface CO2 flux and the implications for NEE measurements under elevated CO2

AbstractSoil and ecosystem trace gas fluxes are commonly measured using the dynamic chamber technique. Although the chamber pressure anomalies associated with this method are known to be a source of error, their effects have not been fully characterized. In this study, we use results from soil gas‐exchange experiments and a soil CO2 transport model to characterize the effects of chamber pressure on soil CO2 efflux in an annual California grassland. For greater than ambient chamber pressures, experimental data show that soil‐surface CO2 flux decreases as a nonlinear function of increasing chamber pressure; this decrease is larger for drier soils. In dry soil, a gauge pressure of 0.5 Pa reduced the measured soil CO2 efflux by roughly 70% relative to the control measurement at ambient pressure. Results from the soil CO2 transport model show that pressurizing the flux chamber above ambient pressure effectively flushes CO2 from the soil by generating a downward flow of air through the soil air‐filled pore space. This advective flow of air reduces the CO2 concentration gradient across the soil–atmosphere interface, resulting in a smaller diffusive flux into the chamber head space. Simulations also show that the reduction in diffusive flux is a function of chamber pressure, soil moisture, soil texture, the depth distribution of soil CO2 generation, and chamber diameter. These results highlight the need for caution in the interpretation of dynamic chamber trace gas flux measurements. A portion of the frequently observed increase in net ecosystem carbon uptake under elevated CO2 may be an artifact resulting from the impact of chamber pressurization on soil CO2 efflux.