Pore Size Class Research Articles

Geographic variations of pore structure of clayey soils along a climatic gradient

Soil pore structure over a wide range of pore sizes regulates soil functions and relies heavily on pedogenic processes. To date, limited knowledge is available on the spatial variations of soil pore structure and underlying mechanisms over a broad geographic scale. Herein, seven typical zonal soils with increased status of soil development in heavy-textures were collected at different genetic horizons under forests along the mid-temperate to tropic climate gradient in eastern China. Pore size distribution (PSD, 0.006 ∼ 800 μm) of aggregates (sized ∼ 1 cm3) and other pore structure characteristics (void ratio, pore area and average diameter) were determined by mercury intrusion porosimetry. Differential PSD displayed tri- or bi-modal patterns and different pore size classes followed diverse geographic trends. Textural pores most influenced by soil type exhibited larger void ratio and pore area in highly than in moderately developed soils; structural porosity most influenced by soil horizon overall decreased with increased soil depth, indicating the decreased levels of hierarchical aggregation whatever soil type. Relative to soil texture and total organic matter (0.70≤|r|≤0.92), total and textural pores were more related to inorganic cementing agents (0.71≤|r|≤0.95, p < 0.001), where kaolinite and free iron/aluminum oxides (mainly crystalline form) exerted the positive effects on void ratio and pore area through clayey pores, in contrast to vermiculite and exchangeable cations; void ratio and pore size of structural pores could be separately promoted by sand-silt size organic matter and vermiculite (r = 0.79 and 0.73, p < 0.001). Additionally, clay-sized organic matter, sand, and free manganese oxides played a secondary effect (R2 = 0.04 ∼ 0.30, p < 0.05) and their specific roles differed with pore size class. The overall results suggest that the dramatic geographic variations of soil pore structure are controlled by both inorganic cementing agents and specific organic components to a large extent at the regional scale.

Zero tillage has important consequences for soil pore architecture and hydraulic transport: A review

• Soil macroporosity decreases in ZT soils at the surface. • Soil structure changes are dependent on texture, depth, and time since conversion. • Changes in soil structure are not always reflected by hydraulic functionality. • ZT pore networks may increase losses of soluble material by preferential transport. Following the adoption of zero-tillage (ZT) from conventional tillage (CT), the soil pore network undergoes immediate and significant changes. As soil remains undisturbed for an extended period, a soil structure emerges that is primarily generated and stabilised by both biotic and abiotic processes. There is limited understanding concerning how the adoption of ZT influences the soil porous architecture and associated soil hydraulic properties, and specifically over what timeframe these changes occur. Since a previous synthesis of such information over 20-years ago, there has been a substantial number of new investigations aimed at addressing this knowledge gap. Here we review 34 papers that illustrate ZT can influence porosity depending on soil texture, pore size class, depth and time, and also influence important transport mechanisms likely to impact the fate of agrochemicals in soils. We found decreased macroporosity in surface layers of soil under ZT when compared with CT. In addition, soil pore connectivity tended to increase in soil under ZT though the associated effects on hydraulic transport were less clear. Our investigation reveals the value of a prospective examination of an evolving ZT pore network both visually and functionally across temporal and spatial scales. We also highlight the necessity for standardised methodology to aid in future data compatibility and quantitative analysis.

Poromechanical analysis of oil well cements in CO2-rich environments

Wells drilled in carbon storage sites could be converted to potential leakage pathways in the presence of CO2-bearing fluids and under the impact of the changes occurring in underground stress. To test this hypothesis, in this study, the behavior of Class G oil well cement in contact with supercritical CO2 has been investigated. The cement cores were cured under lime-saturated water for 28 days at a temperature of 20 ∘C and under atmospheric pressure. Subsequently, they were exposed to supercritical CO2 under a pressure of 20 MPa and at a temperature of 90 ∘C for 30 days. The penetration depth of the carbonation front and the change in the poromechanical properties of the cement core were measured against time. A numerical modeling exercise has also been conducted to simulate the alteration within the cement cores. The results presented in this study show that the precipitation of calcium carbonates reduces the porosity within the outermost layers of the cement cores. This phenomenon shifts the main pore size class towards smaller sizes. In contrast to expectations, the reduction in porosity does not improve the overall strength of the cement specimens. The observed reduction in the strength of the cement specimens might be associated with either the amorphous structure of the precipitated carbonates or the weak bonding between them and the solid walls of the pores and the high degradation of calcium silicate hydrates.

Effects of land-use/cover change on soil hydraulic properties and pore characteristics in a semi-arid region of central Iran

The semi-arid regions of central Iran have vastly undergone the manipulation and conversion of rangelands to dry farmlands. Soil hydraulic properties and pore characteristics may differ in various land-use/cover types. This study evaluated the effects of good and poor rangeland, dry farmland and abandoned farmland on the soil hydraulic properties and pore characteristics in a semi-arid region of central Iran. A completely randomized design was used to analyze the effects of land-use/cover on soil hydraulic properties and pore characteristics. Water infiltration into the soil at inlet matric suction (h) values of 2, 5, 10 and 15 cm was measured using a tension infiltrometer in different land-use/cover types with 18 replications. Wooding's analytical method was used to model the infiltration data and the best-fit values for Gardner’s parameters of macroscopic capillary length (λc) and saturated hydraulic conductivity (Ks) were estimated. Pore characteristics were also estimated using the Watson and Luxmoore method. The results indicated that saturated and near-saturated hydraulic conductivity values [Kh] and λc were significantly influenced by the land-use/cover type. For h < 5 cm, good rangeland and dry farmland had the highest and lowest means of hydraulic conductivity, steady-state flux and sorptivity, respectively. The K values at h lower than 5 cm were found to be as follows: good rangeland > poor rangeland > abandoned farmland > dry farmland. Good rangeland had a greater number of large pore-size class (i.e., > 0.06 cm) and total porosity. Dry farmland and good rangeland had the lowest and highest proportions of large pore-size class (> 0.06 cm), respectively. Inappropriate management practices such as over-grazing of poor rangeland, cultivation and harvesting machinery stress and soil organic carbon decomposition in the dry farmland decreased the frequency of very large pore-size classes (i.e., > 0.15 cm). Although very large and large pores contributed to less than 1% of the soil volume, more than 50% of the total water flow would happen through these pore-size classes. Preserving rangelands in good condition can maintain soil structure and stability and would enhance water infiltration into the soil. These findings can be used by decision makers and land managers for holistic management in ecosystems of semi-arid areas.

Deep furrow and additional liming for coffee cultivation under first year in a naturally dense inceptisol

Naturally dense soils have been commonly managed for coffee plantations in Vertentes Fields physiographic zone of Minas Gerais state, Brazil. Such dense soils in association with the irregular rainfall in recent years have increased risks for crop productivity. Therefore, management practices aiming at improving subsoil root exploration have been recommended for enhancing absorption of water and nutrient uptake, thus mitigating the edaphic drought process and ensuring the development, productivity, and longevity of the coffee crop. A field experiment was carried out in an area of dense Inceptisols through the planting of the coffee crop to investigate the effect of deep furrowing with additional liming on soil quality indexes and as a function of normalized pore distribution by volume [S*(h)]. The experimental design was in randomized blocks with 5 treatments: 40 (conventional soil preparation with furrows at 0.40 m of depth); 60 (soil preparation with furrow to 0.60 m of depth with conventional (60S) and complementary liming application (60C)); 80 (soil preparation with furrow to 0.80 m of depth with conventional (80S) and complementary liming application (80C)). These treatments were compared to an area under native Cerrado vegetation. Undisturbed soil samples were collected at different depths in each treatment. On these samples, water content was measured at different matrix potentials, and physical quality indicators along with the S*(h) were calculated. The results indicated that soil management practices, compared to the native condition, promoted a better distribution of water and air (PAWC, RFC, PMAC and S index) as well S*(h) up to 0.40 m. Below this depth, none of the management practices were able to promote significant improvements (p < 0.05) of these variables in relation to the soil native condition. The 80C treatment had the best air and water availability, with increased of pore size class having functions of drainage and aeration. Moreover, this treatment stood out in comparison to the conventional management (40) for all measured variables, maintaining the adequate conditions of the native area in depth. For the managed areas, the depth of 0.60 m gave the best improved growth and vegetative vigor of the coffee plants.

Stratigraphic and lithofacies control on pore characteristics of Mississippian limestone and chert reservoirs of north-central Oklahoma

We have determined how stratigraphy and lithofacies control pore structures in the Mississippian limestone and chert reservoir of north-central Oklahoma. There are 17 lithofacies and 29 high-frequency cycles documented in the Mississippian interval of this study. The high-frequency cycles have thicknesses ranging from 0.3 to 30.5 m (1–100 ft) and are mainly asymmetric regressive phases. The pore characteristics, measured through digital-image analysis (DIA) of thin-sections photomicrographs ([Formula: see text]100), exhibit unique correlations with core porosity, permeability, and lithofacies within a sequence-stratigraphic framework. There are five fundamental correlations observed. First, porosity from DIA and laboratory core measurements has a strong positive relationship ([Formula: see text]). However, some values from DIA porosity yield relatively higher values, specifically in spiculitic mudstone wackestones and argillaceous spiculitic mudstone wackestones. The difference is hypothesized due to the presence of isolated nanopores that are not accessible by helium during measurement of core porosity. Second, the relationship between pore circularity and permeability is indeterminate. The indeterminate relationship is related to a complex internal pore network, intensive diagenetic alteration, an unconnected microfracture network, and isolated pores. Third, positive moderate to strong correlations ([Formula: see text]) between porosity and permeability are observed only in four lithofacies. Fourth, coarse-grained lithofacies within the uppermost depositional sequence of the Mississippian interval have a heterogeneous pore-size distribution, whereas fine-grained lithofacies tend to exhibit a homogeneous pore-size distribution. Fifth, higher reservoir quality is associated with the upper intervals of high-frequency shallowing-upward cycles. This confirms that the sequence-stratigraphic variability of lithofacies is important to predict reservoir quality and its distribution. An alternative graphical method of pore-size distribution is also developed. To be a useful “technique,” examples of the plot are demonstrated using samples in this study. The plot successfully provides simple identification of pore-size classes, quantitative percentage of pore-size class, dominant pore class, and approximate minimum and maximum pore size.

Particle-size and organic matter effects on structure and water retention of soils

AbstractWater storage and flow in soils are highly dependent on soil structure, which strongly determines soil porosity. However pore size distribution can be derived from soil water retention curve (SWRC). Structural characteristics of cultivated arable fields (693 soil profiles, 1773 samples) and soils covered by treated forest stands (137 soil profiles, 405 samples) were selected from the MARTHA Hungarian soil physical database, and evaluated for expressing organic matter effects on soil structure and water retention. For this purpose the normalized pore size distribution curves were determined for the selected soils, plus the modal suction (MS) corresponding to the most frequent pore size class of the soil. Skewness of soils’ pore size distribution curves are found different. The quasi-normal distribution of sandy soils are transformed into distorted in clayey soils. A general growing trend of MS with the ever finer soil texture was shown. Sandy soils have the lowest average MS values, i.e. the highest most frequent equivalent pore diameter. Silty clay and clay soil textures are characterized by the highest MS values. A slight effect of land use and organic matter content is also observable in different MS values of soils under forest vegetation (’forest’) and cultivated arable land (‘plough fields’). MS values of the two land uses were compared statistically. The results of the analyses show that certain soil group’s MS are significantly different under forest vegetation and cultivation. However this difference can be explained only partly and indirectly by the organic matter of different plant coverage in the land use types.

The effects of no-tillage with subsoiling on soil properties and maize yield: 12-Year experiment on alkaline soils of Northeast China

Soil sodicity and salinization are two major issues concerning agricultural production in Northern China and the affected areas are expanding at a rate of 1–1.5Mha/year. The effects of two treatments, i.e. no-tillage with subsoiling and straw cover (NTSC) and conventional tillage with ploughing and straw removal (CTSR), on soil physical and chemical properties and yields were compared from 1999 to 2011. The results showed that NTSC reduced soil bulk density in the 0–30cm soil layer, but more importantly the treatment increased total porosity by 20.9%, water stable aggregates and pore size class distribution. The enhance soil structure and improved infiltration in NTSC treatments contributed to reducing soil salinity by 20.3%–73.4% when compared with CTSR. Soil organic matter was significantly greater to 30cm in NTSC, while total soil nitrogen was lower than CTSR treatments; however, available P was significantly higher in the 0–5cm soil surface. During the first 3 years, there was no difference in spring maize yield between NTSC and CTSR, but yield significantly increased in NTSC compared with CTSR during the remaining years due to reduced salinity stress and increased soil health. In conclusion, NTSC soil management practices appear to be a more sustainable approach to farming than conventional methods that utilize intensive tillage and crop residue removal.

Anisotropy of Pore Size Classes’ Connectivity Related to Soil Structure Under No Tillage

No-tillage (NT) systems have been increasing during the last several decades. Development of a near-surface layer of laminar structure under NT has also been reported. The objectives of this work were to determine whether preferential connectivity of different pore size classes in two differently textured soils under NT exists and to propose a new index of pore connectivity based on water flux (Cw). The study was carried out at two sites in the Pampas Region of Argentina under long-term NT. Site 1 had a loamy A-horizon, and Site 2 had a sandy loam A-horizon. Undisturbed samples were extracted in vertical and horizontal directions within the first 8 cm of the soil. Saturated hydraulic conductivity (K s), near-saturated hydraulic conductivity (h: −50, −100, −200, −300, and −500 cm), pore size distribution from water retention curve, and total porosity were determined. The index of pore connectivity based on water flux, Cw, was developed as a ratio between K and pore volume for each pore size class. Site 1 showed laminar structure in the A-horizon, whereas Site 2 showed a blocky structure. We found a preferential connectivity in horizontal direction of macropores in Site 1. The index of pore connectivity was useful to assess the presence of preferential orientation of different pore size classes. In future studies, the connectivity of the different pore size classes should not be neglected.

Size distributions of pore water and entrapped air during drying‐infiltration processes of sandstone characterized by water‐expulsion porosimetry

The transport of pore water in a rock during drying‐infiltration processes was characterized by water‐expulsion porosimetry. In this method, pore water is expelled by increasing gas pressure in a stepwise fashion, and the amounts of water in each pore size class are determined. Fontainebleau sandstone, having an open porosity of 6% and main pore radii of 1–20 μm, was used. The change in size distribution of pore water was determined as a function of the degree of drying. As the drying proceeded, the amount of water decreased first in the larger pores followed by the smaller pores. Next, water was infiltrated into the sample preliminarily adjusted to various water saturations (Spre‐flow), and the entrapped air saturation, hydraulic conductivity (K), and size distribution of pore water were measured. As Spre‐flow decreased, the entrapped air saturation increased and K decreased as approximated by K ∝ Sflow4.5, where Sflow is the water saturation after the infiltration. A comparison of the size distributions of pore water before and after the infiltration revealed that the ease of water infiltration (air entrapment) depends on pore size. If pores of &gt;8 μm radii were emptied by drying, they were readily refilled with water, whereas smaller pores were only partially refilled because air was trapped in them. The experimentally measured K‐Sflowrelationship generally agreed with those predicted by the Mualem‐van Genuchten model and its modified version accounting for air entrapment, but some differences in the size distribution of pore water were observed between the experiment and the models.

Methodology to Assess the Effects of Rice Cultivation Under Flooded Conditions on van Genuchten’s Model Parameters and Pore Size Distribution

The effects of rice (Oryza sativa L.) cultivation under flooded conditions on soil’s physical-hydraulic properties were studied in this article, using a new methodology based on a combined analysis on soil water retention curve (WRC) and pore size distribution (PSD). WRC analysis was carried out through the changes of van Genuchten’s model parameters, the characteristics of WRC at the inflection point, and the specific water capacity curve. Analysis of PSD was performed on the volume changes of porosity fractions through a detailed pore size classification, while different pore size classifications based on their hydraulic and structural characteristics were also used. The methodology was applied using a small dataset obtained from fine-textured Entisol soils which were subjected to rice cultivation under flooded conditions in Axios River plain (Northern Greece). Measurements of WRC were obtained at four depths of the soil profile from two fields, before and after the growing season of rice. The analysis indicated that the van Genuchten’s model parameters (θ s, a, and n) and the WRC characteristics at the inflection point (pressure head h i , pore equivalent diameter D i , and slope S i ) significantly changed after the growing season following similar patterns, along the soil profile in both fields. The parameters θ s, a, D i , and S i were decreased, while n and h i were increased. The h i and a were the most sensitive parameters, while the values of (h i and 1/a) in each layer before and after the growing season for each field were linearly correlated and shifted to higher values because of compaction, indicating that it could be applied as a tool to evaluate the degree of soil compaction to similarly textured soils. The peaks of the specific water capacity curves were compressed (lower values of slope S i ) and shifted to lower water potentials (h i ) that corresponded to pores of equivalent diameter D i between 2 and 6 μm. The soils had few structural pores (>9 μm) and low air-filled porosity (>30 μm) before the growing season, which presented accessory reduction after the growing season in both fields. Total porosity was reduced at the expense of structural porosity along the soil profile, while the pore size class of 5–3 μm was identified as the threshold where the smaller pores’ volume started to increase in all layers of both fields. The results indicated that the changes in the WRC and the PSD follow specific trends, which can be used in future studies to model temporal variability of soil’s physical-hydraulic properties.

Comparison of different nano- and micro-focus X-ray computed tomography set-ups for the visualization of the soil microstructure and soil organic matter

This study explores the potential of different X-ray computed tomography (X-ray CT) set-ups for the discrimination of soil mineral matter, soil organic matter (OM) and the pore phase. Different X-ray sources, detectors and filtering methods were investigated. Use of a low-energy detector as well as a Cu-filter decreased the potential for phase segmentation in X-ray CT images of an artificial sand–OM mixture. A mid-range X-ray detector showed to hold more potential. Results obtained for an artificial sand–OM sample showed that an attenuation coefficient (AC) grey-value histogram-based single threshold was unsuitable for automated phase segmentation. A dual thresholding approach enabled a better separation of the different phases. Secondly, the minimal measurable pore size class in a clay loam soil aggregate was compared using micro-focus and state-of-the-art nano-focus X-ray CT. The resolution of the scanned images depends on the spot size of the X-ray source, the resolution of the detector and the magnification used. Reliable discrimination of pore and solid phases was expected to be limited by the X-ray tube's focal spot size to 2.5 μm, in contrast to the voxel size of 0.84 μm actually obtained using nano-focus CT. Although this study was limited in its extent, indications were found that more fine porosity is visible at higher resolutions and that large connected pore spaces may be observed. This fine porosity seems to be very locally autocorrelated. Further fundamental research into AC grey-value automated segmentation of OM from the mineral and pore phases, as well as the truly achievable minimal pore size class using artificial calibration samples, is necessary.

Comparison of porosity in a Chernozemic clay loam soil under long-term conventional tillage and no-till

Tillage practice can alter soil structure, porosity and the size distribution of pores. Consequently, this study was conducted to compare the long-term (since 1968) effects of conventional tillage (CT) using a heavy-duty cultivator and no-till (NT) on structure and porosity of a Dark Brown Chernozem (clay loam) soil in southern Alberta. Number and porosity of total, round, intermediate and elongated pores, maximum equivalent circular diameter (MECD), and mean circularity of pore sizes 50–500 µm (P-150), 500–1000 µm (P-500) and &gt;1000 µm (P-1000) diameter were quantified. These parameters were determined for three depths (0–10, 10–20 and 20–30 cm) and two orientations (vertical, horizontal) using UV-dye impregnated soil sections and image analysis. The surface soil (0–10 cm) of CT was dominantly granular-spongy, but for NT it was dominantly crack to massive. Since tillage treatments were not replicated, only general trends on the effect of tillage and its interaction with depth and orientation are given and probability levels are not reported. Tillage treatment had an influence on P-150 and P-500 pores but little or no influence on P-1000 pores. Mean number and porosity of total and intermediate P-500 pores, which are the most important pore size class for seedling root growth in these soils, were higher for CT than NT for the surface soil, but were higher for NT than CT for the subsoil. For example, the total porosity of P-500 pores was 2.21% for CT and 1.95% for NT at the 0- to10-cm depth, 3.80% for NT and 2.27% for CT at the 10- to 20-cm depth, and 3.18% for NT and 2.80% for CT at the 20- to 30-cm depth. These results suggested a higher potential for seedling root growth in the surface soil of CT, but a greater potential for root growth in the subsoil of NT. Key words: structure, porosity, image analysis, tillage

Relationships of soil microarthropod biomass with organic matter and pore size distribution in soils under different land use

Soil microarthropods were sampled every 3 months for 1 y at 10 sites in the northern Netherlands, varying in soil type and land use. Microarthropods were divided into seven functional groups and biomass-C ha −1 was calculated for the top 10 cm soil layer. The four quantitatively principal functional groups were: cryptostigmatic mites; non-crytostigmatic mites; predatory mites; and omnivorous collembola. Possible relationships between the mean annual biomass of these groups and soil type, land use or soil organic matter were studied. Microarthropod biomass was larger in sandy soil than in loamy and generally larger in meadows than in wheat fields; the mineral layer of forest soils being intermediate. Non-cryptostigmatic mite, omnivorous collembola and predatory mite biomass showed strong positive correlations. Cryptostigmatic mite biomass correlated with lower organic matter input quality, while omnivorous collembola and non-cryptostigmatic mites showed a positive correlation with the amount of input. Omnivorous collembola were negatively affected by a discontinuous input of organic matter to the soil. We found relationships between functional group biomass and either soil organic matter density fractions or soil pore size distribution only when the grassland sites were analyzed separately. Both analyses showed correlation patterns for cryptostigmatic mites to deviate from those of the other three main functional groups. Cryptostigmatic mites showed a positive correlation with the lightest organic matter density fraction, while the non-cryptostigmatic mites and omnivorous collembola were correlated to the heavier fractions. The cryptostigmatic mites correlated with the 6–90 μm pore size class, while the other three groups showed strong correlation to the 1.2–6 μm, as well as the largest (> 90 μm) pore size class. Both observations lead to the conclusion that omnivorous collembola and non-cryptostigmatic mites are related to fungal growth (in the largest pores and on the heavy organic matter fraction), while the cryptostigmatic mites show a more saprovorous feeding mode.

The influence of the soil matrix on nitrogen mineralisation and nitrification. II. The pore system as a framework for mapping the organisation of the soil matrix

Large numbers of small undisturbed soil volumes (1·7 cm3 ) were collected from the surface layer of a 2 m by 3 m field plot on a red earth near Wagga Wagga, New South Wales. The hypothesis tested was that an analysis of relationships between the volume of different pore size classes and various soil properties, measured on these soil volumes, can provide an understanding of soil organisation within the framework of the pore system. Three discrete findings were presented in confirmation of the hypothesis. (1) A non-uniform distribution of organic N through the pore system was indicated by the data analysis. Soil organic N tended to be concentrated in pores &lt;0·6 µm and in pores 10-30 µm, but not in the intermediate pore size class (pores 0·6-10 µm). Concentrations of organic N in pores &lt;0·6 µm are probably because of physical protection from microbial decomposition, but concentrations of organic N in pores 10-30 µm are probably because these pores are infrequently water-filled, and this limits bacterial activity more severely than in the pores 0·6-10 µm. Currently available assays for potentially mineralisable N cannot account for the effect of substrate location within the pore system, and a characterisation of the soil for the distribution of N in pores may enhance their utility. Soil disturbance is likely to alter organic matter distribution through the pore system and alter mineralisation dynamics. (2) Observations of pore size distributions before and after wetting suggested that soils which were high in organic matter and clay tended to have a greater volume of pores 0 ·6-30 µm which are unstable to drying and rewetting. It is proposed that these unstable pores 0 ·6-30 µm had been produced by the movement and alignment of clay particles during the growth of microbial colonies. (3) The volume of pores &lt;0·6 µm had a relatively strong negative correlation with pH and a relatively strong positive correlation with Mn2+ . A mechanism based on redox chemistry principles was proposed to explain these relationships. It was suggested that the volume of pores &lt;0·6 µm is related to the potential anaerobicity of the soil volume. In anaerobic conditions, the terminal electron acceptor for organic C oxidation may be MnO2 instead of O2, and in these circumstances, considerably more H+ would be consumed than in aerobic conditions. It is suggested that this alkaline effect extends into regions of the matrix where N mineralisation and nitrification are occurring, and stimulates these processes. The greater nitrification which may result from such a chain of events may, over time, effect greater acidification in those soil volumes which have greater microporosity.

Fluxes of water and solute in a coastal wetland sediment. 2. Effect of macropores on solute exchange with surface water

We investigated interactions between sediment physical structure and solute transport in an intertidal coastal wetland. Two distinct pore-size classes in the sediment were identified. Macropores had effective diameters greater than 100 μm and a normalized volume of 5%; matrix pores had effective diameters smaller than 100 μm and were the volumetrically dominant pore-size class (95%). We found that infiltration and evaporation-driven water fluxes were segregated between macropores and matrix pores, respectively, which had the effect of enhancing diffusive effluxes of chloride from the sediment to surface water. Chloride was highly concentrated relative to seawater in matrix porewater but was comparatively dilute in macropores. Concentration differences in pore-size classes declined with depth until indistinguishable below 10 cm. The segregated chloride distribution can be explained if recharge to the sediment occurred by downward infiltration in macropores and discharge occurred by an upward flux in matrix pores to satisfy evapotranspiration. Without disturbance by the downward infiltration flux in macropores, upward advection of chloride in matrix pores and evapoconcentration increased chloride concentrations in matrix pores to a level well above the concentration in seawater. The resulting high concentrations of chloride in matrix pores induced a large diffusive efflux of chloride into surface water that was sufficient to balance new input of chloride by infiltration of seawater in macropores (0.085 mmol Cl cm -2 day-'). Transport models that were constrained by water balance measurements at the field site explained both the exponential form of the vertical distribution of chloride in matrix pores and the rate of change in storage of chloride in sediment porewater over a 1 month period. We conclude that segregation of water and solute fluxes in two pore-size classes strongly influences sediment salinity of coastal wetlands, which has direct bearing on primary productivity of dominant vegetation and on exchange of dissolved nutrients and contaminants between intertidal wetlands and open water.

Effect of an introduced inoculum on soil microbial diversity

A laboratory study was carried out to evaluate the impact of the introduction of genetically modified microorganisms into soil, in terms of effect on the diversity of the indigenous microflora, and at the process level. The impact on microbial phenotypic diversity, and on soil denitrification of an inoculum of a lux-modified denitrifier, Pseudomonas fluorescens, was examined using two different soil types in re-packed soil microcosms. The effect on diversity was found to depend on the soil pore size class into which the modified inoculum was introduced. The introduction of lux-modified cells into the 15–30 μm pore neck size class caused a short-term reduction in the overall microbial diversity. There was no significant change in the diversity of the indigenous microbial community, however, when cells were introduced into the 40–60 μm pore class. Partial chloroform fumigation proved useful in differentiating cell populations with respect to pore location. No change in diversity was observed when dead cells (either heat killed or glutaraldehyde fixed) were introduced into either pore size class. At the process level, the effect on soil denitrification of introduction of lux-modified P. fluorescens was not significantly different from introduction of the equivalent inoculum of the parental wild-type, although denitrification was found to be dependent upon both soil structure and pore size location of the introduced inoculum.

Effect of an introduced inoculum on soil microbial diversity

A laboratory study was carried out to evaluate the impact of the introduction of genetically modified microorganisms into soil, in terms of effect on the diversity of the indigenous microflora, and at the process level. The impact on microbial phenotypic diversity, and on soil denitrification of an inoculum of a lux-modified denitrifier, Pseudomonas fluorescens, was examined using two different soil types in re-packed soil microcosms. The effect on diversity was found to depend on the soil pore size class into which the modified inoculum was introduced. The introduction of lux-modified cells into the 15–30 μm pore neck size class caused a short-term reduction in the overall microbial diversity. There was no significant change in the diversity of the indigenous microbial community, however, when cells were introduced into the 40–60 μm pore class. Partial chloroform fumigation proved useful in differentiating cell populations with respect to pore location. No change in diversity was observed when dead cells (either heat killed or glutaraldehyde fixed) were introduced into either pore size class. At the process level, the effect on soil denitrification of introduction of lux-modified P. fluorescens was not significantly different from introduction of the equivalent inoculum of the parental wild-type, although denitrification was found to be dependent upon both soil structure and pore size location of the introduced inoculum.

Relationships between habitable pore space, soil biota and mineralization rates in grassland soils

The hypotheses that the accessible soil pore volume determines the biomass of bacteria and their grazers, and that the activity of bacteria and the mineralization rates of C and N are affected by grazing pressure on bacteria were tested. We determined the biomass of bacteria, fungi, protozoa and nematodes, the pore-size distribution, and the potential mineralization rates of C and N in grassland soils with different textures. Bacteria constituted by far the largest biomass pool. Fungi, protozoa and nematodes together comprised only 10% of the total biomass. It was found that in loams and clays, most pores had diameters <0.2 μm and between 0.2 and 1.2 μm, whereas in sandy soils most pores had diameters from 6 to 30 and 30 to 90 μm. A close positive correlation was found between the bacterial biomass and the soil volume of pores with 0.2–1.2μm dia and between the biomass of nematodes and the soil volume of pores with 30–90 μm dia. The biomass of fungi and protozoa showed no relationship with a specific pore-size class. Bacterial activity [measured as the frequency of dividing-divided cells (FDDC); the number of viable cells; and the amount of CO 2 produced per cell] were not affected by grazing intensity. The amount of N mineralized bacterium −1, however, was much higher in soils with a high grazing pressure of bacterivorous nematodes and flagellates than in soils with a low grazing pressure of these groups. This indicates that grazing of bacteria by bacterivorous nematodes and flagellates may considerably increase N mineralization. No relationship was found between the grazing pressure of amoebae and the amount of N mineralized bacterium −1.

Characterization of Tillage and Traffic Effects on Unconfined Infiltration Measurements

AbstractTillage and wheel‐traffic compaction alter pore structure and hydraulic properties of agricultural soils. These alterations will affect root growth and movement of water and solutes. This study was conducted to develop new methods needed to characterize the effects of wheel traffic and tillage on pore structure as measured by water flow through macropores. Unconfined (three‐dimensional) saturated and unsaturated infiltration measurements (0‐, 30‐, 60‐, and 150‐mm water tension) at the soil surface were taken sequentially at field sites to determine the steady‐state rate of water flow through different pore‐size classes on a Tama silty clay loam (fine‐silty, mixed, mesic Typic Argiudoll). A tension infiltrometer was used to obtain the unsaturated infiltration rates. Sites were selected on trafficked and untrafficked interrow positions in two tillage systems (chisel plow and no‐till). Steady‐state infiltration rates from 2 to 300 µm s−1 were measured. Increasing the tension of applied water resulted in decreasing infiltration rates for both tillages and traffic treatments because, as tensions increased, larger pores emptied. Wheel traffic reduced infiltration rates in both tillages, but caused a greater decrease in infiltration rates in the chisel‐plow system than in no‐till. Increasing tension caused proportionately smaller decreases in infiltration rates for wheel‐trafficked positions. This suggests that larger macropores were transporting a greater proportion of the total water flow in untrafficked soil than in trafficked soil and reinforces the concept that larger, as opposed to smaller, pores are more easily destroyed by wheel traffic. Unconfined infiltration measurements were shown to be useful in quantifying the effects of tillage and compaction on soil macropores.