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Denitrification is sensitive to changes in soil physical properties that affect solute transport, air content and gas diffusion. Using lysimeters, containing intact soil from intensively tilled (IT) and no-tilled (NT) soil used to grow forage crops, we examined how simulated animal treading at different moisture contents (above and below field capacity; >FC and FC greatly increased denitrification, especially from IT soil and produced the greatest amount of [N.sub.2] (64 kg N [ha.sup.-1]), [N.sub.2]O (8.2 kg N [ha.sup.-1]), as well as the lowest [N.sub.2]O to [N.sub.2]O + [N.sub.2] ratio (0.08) and N[O.sub.3] leaching (136 kg N [ha.sup.-1] below 30 cm). In both the uncompacted or compacted soils FC. Treading at Additional keywords: denitrification, hydraulic conductivity, soil porosity, soil structure., Introduction Tillage and grazing can have a substantial impact on soil structural condition and especially pore geometry (Frede et al. 1994; Tebriigge and During 1999; Shipitalo et al. 2000; Buczko [...]

The gas transport parameters, diffusivity and air-filled porosity are crucial for soil aeration, microbial activity and greenhouse gas emission, and directly depend on soil structure. In this study, we analysed the effect of long-term tillage and irrigation practices on the surface structure of an arable soil in New Zealand. Our hypothesis was that topsoil structure would change under intensification of arable production, affecting gas exchange. Intact soil cores were collected from plots under intensive tillage (IT) and direct drill (DD), irrigated or rainfed. In total, 32 cores were scanned by X-ray computed tomography (CT) to derive the pore network >30 [micro]m. The cores were then used to measure soil-gas diffusivity, air-permeability and air-filled porosity of pores close to the resolution of the X-ray CT scans, namely [greater than or equal to] 30 [micro]m. The gas measurements allow the calculation of pore-network connectivity and tortuosity parameters, which were compared with the CT-derived structural characteristics. Long-term irrigation had little effect on any of the parameters analysed. Total porosity tended to be lower under IT than DD, whereas the CT-derived porosity was comparable. Both the CT-derived mean pore diameter (MPD) and other morphological parameters, as well as gas measurement-derived parameters, highlighted a less developed structure under IT. The differences in the functional pore-network structure were attributed to SOC depletion and the mechanical disturbance through IT. Significant correlations between CT-derived parameters and functional gas transport parameters such as tortuosity and MPD were found, which suggest that X-ray CT could be useful in the prediction of gas transport. Additional keywords: carbon depletion, Dexter index, intensive tillage, P-parameter, soil organic carbon, soil structure., Introduction Modern agriculture associated with increased inputs of agrichemicals, irrigation and intensive cultivation has led to soil compaction and erosion, which negatively affect the delivery of soil ecosystem services. Concomitant [...]

Mass transport in soil occurs through the soil pore network, which is highly influenced by pore structural parameters such as pore-size distribution, porosity, pore tortuosity, and coordination number. In this study, we visualised the networks of meso- and macro-pores (typical pore radius r [greater than or equal to] 10 [micro]m) using micro-focus X-ray computed tomography (MFXCT) and evaluated pore structural parameters of two loamy soils from Japan and New Zealand packed at different degrees of compaction. The effect of compaction on pore structural parameters and relationships between pore structural parameters and measured mass transport parameters were examined. Results showed a clear influence of compaction on pore structural parameters, with the MFXCT-derived mean pore radii and pore tortuosities decreasing and the mean pore coordination number increasing with increasing dry bulk density. Especially, pores with r > 80 [micro]m became finer or were not well formed due to compaction. The MFXCT-derived pore structural parameters were not well correlated with the equivalent pore radii from measured water retention curves. However, volumetric surface areas and pore-network connectivity-tortuosity factors derived from MFXCT allowed a fair prediction of several important mass transport parameters such as saturated hydraulic conductivities, soil-gas diffusion coefficients, and soil-air permeabilities. Further studies are needed to link micro-pores with radii smaller than the X-ray CT resolution to meso- and macro-pores visualised by X-ray CT to improve the prediction of mass transport parameters in soil. Additional keywords: air permeability, hydraulic conductivity, microfocus X-ray computed tomography (MFXCT), pore network, soil gas diffusion coefficient., Introduction A better understanding of gas transport in soil is important to elucidate the emission of greenhouse gases (Kruse et al. 1996; Smith 2003; Kawamoto et al. 2006; Wickramarachchi et [...]

Volcanic soils are important resources because of their unique mineralogical and physical characteristics, and allophanic Andosols represent some of the world's most fertile soils. However, their unique properties can be lost when cultivated. Most soils in the Central Valley, Mexico, are derived from volcanic materials. This valley encompasses one of the largest water supply systems in the world by volume, but is affected by soil degradation and deforestation. Sustainably managing volcanic soils requires understanding how land use affects their hydrodynamic properties. Gas adsorption and mercury intrusion porosimetry, water retention curves, tension infiltrometry and X-ray tomography were used to describe pore structure characteristics. Two volcanic soils (one Andosol and one derived from indurated tuff--Tepetates), three land uses (maize monoculture, maize-wheat rotation and fallow) and two horizons (Ap and A2 for maize monoculture and maize-wheat rotation) were studied. Tillage affected topsoil by increasing the sand fraction by 38% and decreasing total porosity and macroporosity by 23% and 40% respectively. Macropore size was reduced and the number of isolated macropores was higher in the tilled layer under maize, compared with untilled subsoil. The plot under maize-wheat rotation had lower allophane content, and saturated hydraulic conductivity was reduced by nearly an order of magnitude and water retention by half, compared with maize and fallow plots. Compared with Andosols, Tepetates showed differences in mineralogical composition with lower contents of amorphous compounds and in its porous network characteristics with twice the total and percolating macroporosity compared with the maize plot. Its high content of organic carbon (3.5%) seemed beneficial for its hydrodynamic properties. Sustainable agricultural management of these volcanic soils requires reducing mechanised tillage, avoiding periods when soil is bare, not applying maize-wheat rotation and applying maize-fallow rotation allowing natural vegetation growth. Additional keywords: Andosol, pore size distribution, X-ray tomography., Introduction Volcanic soils are important land resources for agriculture because of their unique properties which place them among the most productive soils in the world (Shoji et al. 1993). They [...]

Sustainable horticulture depends on the integrity of soil functions, which directly depend on soil architecture affecting aggregation, root growth, as well as liquid and gas permeability. We hypothesised that changes in soil architecture resulting from feedback mechanisms between management, soil organic carbon contents (SOC), biota and vegetation can be captured with X-ray computed tomography (CT), and that these affect the soil filtering function, which thus, can be manipulated through orchard management. We compared the transport of copper, a widely used fungicide, through intact soil cores from vine rows of kiwifruit orchards under organic and integrated management. We first derived 3D-macropore characteristics from CT-images, followed by leaching a pulse of copper and a tracer through the same cores. The organic orchard soil had a significantly higher SOC content than the integrated orchard soil, and this was positively correlated with total porosity. Macropores (>92 [micro]m) were larger with a higher connectivity, but significantly fewer in the organic than the integrated orchard soil. This resulted in a lower macroporosity and a better copper filtering capacity of the organic than the integrated orchard soil. Copper distribution was reasonably predicted when combining SOC contents, pH and macropore characteristics. Significant relationships between soil parameters and indicators of the strength of preferential flow verified that CT-derived macropore characteristics can be used to predict functional solute transport parameters. The relevance of our results and relationships observed between macropore characteristics, functional indicators of preferential flow and the fate of copper needs verification with samples representing more soils and sites. Additional keywords: carbon, filtering function, kiwifruit orchard, soil architecture, X-ray computed tomography., Introduction Kiwifruit is the fruit of a perennial woody vine. New Zealand (NZ) is among the leading kiwifruit-producing countries worldwide. Kiwifruit production totalled ~1 412 351 MT in 2012 with [...]

In this study, the quality of soil permeability estimation based on computational fluid dynamics is discussed. Two types of three-dimensional geometries were considered: an image of Fontainebleau sand obtained from X-ray computed micro-tomography and a virtual pack of spheres. Numerical methods such as finite difference or lattice Boltzmann can conveniently use the image voxels as computational mesh elements. In this framework, the image resolution is directly associated with quality of the numerical computation. A higher resolution should promote both a better morphological description and discretisation. However, increasing the resolution may prevent the studied volume from being representative. Here, each sample was scaled and analysed at five resolutions. The dependence of soil properties with respect to the image resolution is discussed. As resolution decreased, the permeability and specific surface values tended to diverge from the reference value. This deterioration could be attributed to the shift of the pore size distribution towards badly resolved pores in the voxelised geometry. As long as granular soils are investigated, the volume fraction of pores smaller than six voxels in diameter should not exceed 50% to ensure the validity of permeability computation. In addition, based on an analysis of flow distribution, the volume fraction of pores smaller than four voxels should not exceed 25% in order to limit the flow rate occurring in badly discretised pores under 10%. For the Fontainebleau sand and virtual pack of spheres, the maximum voxel size meeting this criterion corresponded to 1/14 and 1/20 of the mean grain size respectively. Additional keywords: digital rock physics, image resolution, soil morphology, soil permeability., Introduction Understanding fluid flow and also mass and energy transport in soils remains a challenging problem. The difficulty lies in the complexity of the material porous microstructure. One key parameter [...]

Water retention in granular soils is a key mechanism for understanding transport processes in the vadose zone for various applications from agronomy to hydrological and environmental sciences. The macroscopic pattern of water entrapment is mainly driven by the pore-scale morphology and capillary and gravity forces. In the present study, the drainage water retention curve (WRC) was measured for three different granular materials using a miniaturised hanging column apparatus. The samples were scanned using X-ray micro-computed tomography during the experiment. A segmentation procedure was applied to identify air, water and solid phases in 3D at the pore-scale. A representative elementary volume analysis based on volume and surface properties validated the experimental setup size. A morphological approach, the voxel percolation method (VPM) was used to model the drainage experiment under the assumption of capillary-dominated quasi-static flow. At the macro-scale, the VPM showed a good capability to predict the WRC when compared with direct experimental measurements. An in-depth comparison with image data also revealed a satisfactory agreement concerning both the average volumetric distributions and the pore-scale local topology. Image voxelisation and the quasi-static assumption of VPM are likely to explain minor discrepancies observed at low suctions and for coarser materials. Additional keywords: hanging column experiment, marker-controlled watershed, representative element volume, voxel percolation method., Introduction Water distribution and transfer in the vadose zone plays a major role in a wide range of applications including agronomy for plant growth response to water availability (Couvreur et [...]

Mucilage produced by myxodiaspores has been mainly studied for its role in seed ecology. This work investigates changes due to mucilage in soil structure and stability in the spermosphere of the myxodiaspore crop chia (.Salvia hispanica L.). In sandy-loam (S) repacked soil sown with chia and subjected to wetting-drying (DW), Computer Assisted X-ray micro tomography showed that extrusion of a mucilage capsule formed a pore of volume twice that of the seed, surrounded by a shell of particles with higher porosity and lower pore size than the bulk soil. In three soils (S; loam, L; and clay-loam, C) the aggregate stability index to wet sieving (Stw) decreased with one and two DW cycles. Application of 2% w/w mucilage increased Stw before and after DW, from 29% in C to more than 60-fold in S. Mucilage-amended samples after DW had higher or equal Stw compared with unamended soil before DW. Soil retained at the surface of hydrated diaspores exposed to water flow changed with soil texture (11.3, 90.5 and 91.7 mg on each hydrated seed for S, L and C respectively) and chia genotype (6.7, 9.9 and 12.8 mg per hydrated seed in BC and G8 long-day flowering mutants and commercial Black chia respectively). Our results showed that myxodiaspores affected soil structure by providing a microenvironment of altered porosity and high stability around the mucilage capsule extruded by the hydrated seed. This finding characterises a transient but crucial time of crop production around sowing when physical properties of soil surrounding seeds are relevant to stability, germination and plant-microorganism relations. Additional keywords: aggregate stability, germination, soil-plant interactions, soil porosity., Introduction Soil structure and related properties are affected by plants (Roose et al. 2015). Many studies focus on the effect of roots or polysaccharides produced by roots and associated microorganisms [...]

A fragipan is a diagnostic subsurface soil, not a cemented horizon, which is characterised by high density, and so restricts root penetration and water percolation. Although fragic horizons are considered to be pedogenic, the exact genesis of this phenomenon is not well understood. Quantitative study of pore space characteristics in a profile with a fragipan could help in understanding its origin and its pedogenic links to the other diagnostic horizons. Micromorphological and morphometric study of the porous network in soil thin sections and computed tomography in an Albic Fragic Retisol (Cutanic), formed in glacial till of Valday (Wurm) Glaciation in the South Karelia region in the north of Russia, showed a differentiation of pores by shape and distribution for each soil horizon controlled by the type of soil-forming processes. In particular we detected a difference in pore space organisation in the fragic EBx compared with other horizons. The pore space in the EBx was mainly represented by closed micropores, spread homogenously in the soil horizon body, independent of fissure and packing pores. Thus we propose that the pore system in this horizon was heterochronous, with micropores formed at the time of structural collapse and the fissures and other pores formed later. Additional keywords: Holocene pedogenesis, soil computed tomography, pore space descriptors., Introduction A fragipan is a subsurface soil horizon that restricts the penetration of roots and water; it has a specific coarse prismatic blocky structure and high bulk density. This horizon [...]

Tepetates are indurated subsoil horizons developed in tephra-derived materials in various parts of the Trans-Mexican Volcanic Belt. The term 'tepetate' includes fragipans, duripans, pedosediments and saprolites, derived from vitric rhyolitic tuff, Pleistocene volcanic ashes or pyroclastic flows. All soils with tepetates are at high risk of erosion and so they have been intensively studied for decades. The tepetates are common in Tlaxcala State of central Mexico, being formed under Ustic Isomesic soil climate. The aim of this investigation was to characterise the pore space of fragipan-type tepetates and the role of clay components in their formation. We studied porosity of tepetate from a tephra-palaeosol sequence in the north of Tlaxcala State, in undisturbed soil samples. Observations of pore space were made in 2D and 3D by analysing microscope images of thin sections and cross-sectional images from a computed tomography scanner. In the thin sections we also identified and described clay illuvial pedofeatures. Micromorphological observations showed two main pore types. Small rounded pores had a homogeneous distribution and were probably formed before the clay illuviation process that took place in a palaeosol formed on the tepetate material. The distribution pattern of the small pores in the studied tepetate was similar to that in the fragic horizon, which was probably formed by a hydro-consolidation process. Large crack-pores were formed during the palaeosol formation. Later these large pores were filled by illuvial clay coatings and so we conclude that each tepetate was part of the set of Bt horizons in the palaeosols of Luvisol type. Additional keywords: 3D computer tomography, clay illuviation, fragipan., Introduction Soils with indurated horizons, known as tepetate, present the most vulnerable component of the central Mexican soil mantle, with high risk of erosion and badland formation that has motivated [...]

Quantifying the structure of soil is essential for developing effective soil management for farming and environmental conservation efforts. One approach to quantify soil structure is to scan intact soil cores by X-ray computed tomography (CT), which allows using computer vision algorithms to identify internal components within the soil. One commonly used approach is the colour-based segmentation of CT-scan soil images into two regions--matter and void for the purpose of determining the soil porosity. A key problem with this approach is that soil CT images tend to be rather complicated, and thus this type of bi-region segmentation is a non-trivial problem, with algorithms following this type of bi-region approach typically performing unreliability across a variety of image sets. In this work, a technique is proposed that identifies an optimal number of regions present in the soil, rather than just two. It is claimed that this more sophisticated representation of soil structure leads to a more accurate representation than traditional bi-region segmentation; however, it is reducible to a bi-region segmentation yielding the required estimation of porosity with more accuracy and robustness than traditional methods. It is also proposed that segmentation is performed using a multiregion kriging algorithm, which establishes relationships between distance and regions that allows the segmentation to overcome many of the artefacts and noise issues associated with CT scanning. Our experiments focused on layer-by-layer segmentation and results demonstrated that the proposed approach produced segmentations consistent across a variety of scanned cores and were visually more correct than current state-of-the-art bi-region techniques., Introduction Soil management is essential for improving yield in agriculture preventing water pollution from pesticides and fertilisers used in agriculture (Bonilla 1997; Havlin et al. 2005), improving soil fertility (Thompson [...]

Soil organic matter (SOM) plays a key role in soil structure formation, although the bidirectional relationship between SOM and the soil pore network is complex and needs further investigation. Despite great advances provided by X-ray computed microtomography ([micro]CT), it has only been used in a few studies to investigate the organic matter 3D-arrangement within the soil matrix. Results are based on the X-ray linear attenuation coefficient (a), and mixtures of organic and mineral soil fractions could imply overlapping of information that makes any segmentation procedure difficult. The aim of this study was to visualise, segment, and quantify the particulate organic matter fraction (POM) within the soil matrix through X-ray [micro]CT in combination with contrast agents (phosphomolybdic acid and silver nitrate). Two series of repacked soil cores, 'dry' and 'wet', were scanned through X-ray [micro]CT at a 7-[micro]m resolution. Different segmentation approaches were tested to separate POM from other soil phases: manual, global, and local thresholding methods. Reported algorithms were also compared with a supervised grey value-based (GV) approach followed by morphological operations. Results showed contrast agents increased [alpha] of POM, simplifying its identification and the following segmentation on dry cores. The POM was discriminated from the mineral fraction and its content correctly estimated. This was particularly accurate when applying manual thresholding or GV approach with respect to indicator kriging, suggesting that operator-based ability to set threshold level is still the best solution for accurate POM segmentation. Beyond single-phase accounting, different thresholding algorithms and morphological operations also affected POM morphological characteristics. In particular, the simpler was an object shape, the easier was its segmentation. Improvements are thus required to increase the efficiency of automated thresholding algorithms. Moreover, wet cores were exposed to washing-out phenomena that compromised any digital image processing and further POM quantification, implying that more effort should be made to find other suitable staining agents. Additional keywords: soil image analysis, soil organic matter, X-ray computed microtomography., Introduction Understanding soil organic matter (SOM) spatial arrangement is essential for developing theoretical models on soil carbon fluxes and SOM-related properties. The SOM is recognised as a key driver leading [...]

Treading damage inflicted by grazing animals degrades soil physical quality, increases sediment and nutrient loss to water, and reduces pasture yield. The extent or severity of treading damage can be assessed using several methods such as roller chain, depth of pug, and visual scoring. However, the ability of these methods to assess treading damage, and their relative merits, have not previously been compared. Four methods of measuring treading damage are compared in this study including the three methods mentioned and a new tool called the pugometer, which was developed to measure treading damage in a spatially explicit manner. These comparisons were conducted on three paddocks at Massey University's Dairy 4 farm near Palmerston North, Manawatu, New Zealand which had sustained treading damage of varying levels during grazing in wet conditions. All four methods were able to identify varying degrees of treading damage competently with strong correlations between them ([R.sup.2] = 0.72-0.87). Therefore, the selection of the most appropriate method to assess treading damage will depend on the circumstances. Visual scoring was the quickest and simplest method to use over a large area, and so could be employed easily by a farmer. The pugometer provides a quantitative measure of the spatial variability of treading damage and so would be a useful research tool. The depth of pug and roller chain methods are reliable but much more time consuming and therefore are only practicable as research tools for small plot studies. Following a treading event, the pugometer was able to monitor the recovery of surface roughness in a spatially explicit and rapid manner. Additional keywords: depth of pug, quantifying pugging, roller chain, visual assessment., Introduction Treading damage of paddocks by grazing livestock degrades soil quality and increases the loss of nutrients and the transport of pathogens in surface runoff (Drewry et al. 2003; Kurz [...]

Concern about soil physical quality has grown in recent years, particularly in view of serious problems caused by intensive soil use. We hypothesised that improper soil management in irrigated areas damages the structure of sensitive soils in some regions in North-eastern Brazil. The aim of the study was to evaluate the physical quality of irrigated soils planted with annual and perennial crops, compared with soils under natural vegetation in Ceara State, Brazil. Measurements were made of least limiting water range (LLWR), the S index, and relative density. Undisturbed soil samples were collected at two depths (5-10 and 20-25 cm) in four cultivated areas (banana, guava, pasture, and maize/bean in succession) and two natural vegetation areas (NV1, NV2) adjacent to the cultivated areas. All sites were in the JaguaribeApodi Irrigated District, Limoeiro do Norte, Ceara, Brazil. The LLWR was determined using the water retention curve, soil resistance to penetration, and soil bulk density, which are parameters needed to obtain the upper and lower limits of LLWR. The S index was obtained from the water retention curve. The relative density was obtained from the relationship between bulk density and maximum density obtained from the Proctor test. The S index varied as a function of soil management. The variation in LLWR differed between the studied areas as a function of soil bulk density. The relative densities for NV1 and NV2 were lower than for cultivated areas, showing that intensive soil use has caused compaction. The studied parameters seem to be good indicators of soil physical quality, and it was noticed that soils under cultivation suffer an alteration of their structure relative to soils under natural vegetation. Additional keywords: least limiting water range, Proctor test, relative density, S index, soil degradation, soil water retention curve., Introduction Agricultural establishment in most regions involves deforestation of native vegetation to provide suitable conditions for land development and crop production. In arid and semi-arid regions, provision of irrigation is [...]

A laboratory investigation was undertaken on the mechanical behaviour of two sandstones that are commonly used as analogous hydrocarbon reservoir sandstones, using triaxial apparatuses with a capacity of up to 70 MPa confining pressure. Both materials are lightly cemented, but show a mechanical behaviour at large strains that is similar to that seen in rocks with much stronger cementation. At small strains, however, these materials behave more like uncemented sands in that they show a strong stress dependency of the shear modulus and yield takes places at relatively small strains. It is believed that fabric plays a fundamental role in the observed behaviour and this is supported by the experimental evidence, so that the concept of strong fabric in sandstones is introduced. Key words: fabric-structure of soils, soft rocks, stiffness, laboratory tests, sands, rock mechanics. Une etude en laboratoire a ete realisee sur le comportement mecanique de deux gres qui sont couramment utilises comme gres de reservoir d'hydrocarbures analogues. Les essais ont ete effectues a l'aide d'appareils triaxiaux ayant une pression de confinement jusqu'a 70 MPa. Les deux materiaux sont legerement cimentes mais demontrent un comportement mecanique a grandes deformations similaire a celui rencontre dans les roches ayant une cimentation beaucoup plus forte. Cependant, a faibles deformations, ces materiaux se comportent plutot comme des sables non cimentes puisque leur module de cisaillement presente une forte dependance aux contraintes, et l' ecoulement se produit a des deformations relativement faibles. La structure semble jouer un role fondamental dans le comportement observe, et ceci est supporte par des evidences experimentales, alors le concept de structure forte dans les gres est introduite. Mots-cles : structure des sols, roches molles, rigidite, essais en laboratoire, sables, mecanique des roches. [Traduit par la Redaction], Introduction The framework of critical state soil mechanics has been used for the interpretation of laboratory test results for a range of artificially and natural cemented sandstones (e.g., Airey 1993; [...]

Winter annual pastures are traditionally established using traditional tillage (TT) on grazing areas that are steeply sloping and not suitable for row-crop production. This leads to increased erosion and rapid soil degradation. The objectives of this study were to determine the effects of tillage intensity on water-stable soil aggregates (WSA), their C and N content, and soil bulk density (BD). The study consisted of three tillage treatments; TT, disk tillage (DT), and no-tillage (NT). Total WSA was greater in the 0- to 5-cm soil layer than the 5- to 10-cm layer for the DT and NT treatments, but was similar in the TT treatment. Mean WSA in the four largest aggregate size classes was greatest in the 0- to 5-cm soil layer but similar for both sample depths in the smallest size class. As aggregate size decreased, relative aggregate content at the toeslope position increased. For all tillage treatments and soil depths, mean WSA decreased from the largest size class (>4 mm) to the next size class (4-2 mm), and increased thereafter as aggregate size decreased. Soil BD was unaffected by tillage treatment. Water-stable aggregate C and N content and weights were equivalent in the DT and NT treatments and greater than the TT treatment. Winter annual grass production using DT or NT practices is a viable means of maintaining or enhancing soil quality. doi: 10.2136/sssaj2009.0280

The paper brings out the influence of relative density of foundation soil on performance improvement of geocell reinforcement; through model load tests on geocell-reinforced and unreinforced sand beds. Tests were carried out for five different relative densities of sand (30, 40, 50, 60, and 70%). The test results indicate that the beneficial effect of geocell reinforcement, in terms of increase in stiffness, bearing capacity, and load dispersion angle of the foundation bed, is present over a wide range of relative density; however, it is higher for dense condition of foundation soil. With geocell reinforcement offering three-dimensional confinement, the dilation induced benefit is substantially high for dense soil fill. Therefore, for effective utilization of geocell reinforcement, the foundation soil should be compacted to higher density. DOI: 10.1061/(ASCE)MT.1943-5533.0000040 CE Database subject headings: Foundations; Soil stabilization; Reinforcement; Model studies; Relative density; Sand, soil type. Author keywords: Foundation bed; Reinforced soil; Geocell reinforcement; Relative density; Model studies.

Tillage and cropping sequence may influence C and N sequestration, microbial activities, and N mineralization in dryland soil aggregates. We evaluated the 21-yr effect of tillage and cropping sequence combinations on C and N fractions in aggregates of a Dooley sandy loam (fine-loamy, mixed, superactive, frigid Typic Argiustolls) at the 0- to 20-cm depth in eastern Montana. Tillage and cropping sequences were no-tilled continuous spring wheat (NTCW) (Triticum aestivum L.), spring-tilled continuous spring wheat (STCW), fall- and spring-tilled continuous spring wheat (FSTCW), fall- and spring-tilled spring wheat-barley (Hordeum vulgare L.) (1984-1999) followed by spring wheat-pea (Pisum sativum L.) (2000-2004) (FSTW-B/P), and spring-tilled spring wheat-fallow (STW-F). Carbon and N fractions were soil organic C (SOC), total N (STN), particulate organic C and N (POC and PON), microbial biomass C and N (MBC and MBN), potential C and N mineralization (PCM and PNM), N[H.sub.4]-N, and N[O.sub.3]-N. Aggregate proportion was greater in NTCW than in FSTCW in the 4.75- to 2.00-mm aggregate-size class at 0 to 5 cm but was greater in STW-F than in STCW in the 2.00- to 0.25-mm size class at 5 to 20 cm. After 21 yr, STW-F reduced SOC, STN, POC, and PON concentrations in aggregates by 34 to 42% at 0- to 5-cm and by 20 to 32% at 5- to 20-cm compared with NTCW and STCW. The PCM and MBC were greater in NTCW and STCW than in STW-F in the Abbreviations: FSTW-B/P, fall- and spring-tilled spring wheat-barley (1984-1999) followed by spring wheat-pea (2000-2004); FSTCW, fall- and spring-tilled continuous spring wheat; MBC, microbial biomass C; MBN, microbial biomass N; NTCW, no-tilled continuous spring wheat; PCM, potential C mineralization; PNM, potential N mineralization; POC, particulate organic C; PON, particulate organic N; SOC, soil organic C; STW-F, spring-tilled spring wheat-fallow; STCW, spring-tilled continuous spring wheat; STN, soil total N.

Unsaturated soil interfaces exist where unsaturated soil is in contact with structures such as foundations, retaining walls, and buried pipes. The unsaturated soil interface can be defined as a layer of unsaturated soil through which stresses are transferred from soil to structure and vice versa. In this paper, the shearing behavior of unsaturated soil interfaces is examined using results of interface direct shear tests conducted on a low-plasticity fine-grained soil. A conventional direct shear test device was modified to conduct direct shear interface tests using matric suction control. Further, the results were used to define failure envelopes for unsaturated soil interfaces having smooth and rough counterfaces. Results of this study indicate that matric suction contributes to the peak shear strength of unsaturated interfaces; however, postpeak shear strength did not appear to vary with changes in matric suction. Variations in net normal stress affected both peak and postpeak shear strength. Failure envelopes developed using the soil-water characteristic curve (SWCC) appeared to capture the nonlinear influence of matric suction on shear strength of soil and interfaces. Key words: soil, unsaturated, interface, shear strength. Les interfaces de sol non satures existent lorsque le sol non sature est en contact avec des structures comme les fondations, les murs de soutenement et les tuyaux enfouis. L'interface de sol sature pent etre definie comme une couche de sol non sature a travers laquelle les contraintes sont transferees du sol a la structure, et vice versa. Dans cet article, le comportement en cisaillement des interfaces de sol non sature est examine a partir de resultats d'essais en cisaillement direct sur des interfaces d'un sol fin a faible plasticite. Un appareil conventionnel d'essais de cisaillement direct a ete modifie afin d'effectuer des essais de cisaillement direct sur des interfaces en contr6lant la succion matricielle. De plus, les resultats ont ete utilises pour definir l'enveloppe de fracture pour des interfaces de sol non sature ayant des surfaces correspondantes lisses et rugueuses. Les resultats de cette etude demontrent que la succion matricielle contribue a la resistance an cisaillement de pic des interfaces non saturee; cependant la resistance an cisaillement depasse le pic ne semblait pas varier selon la succion matricielle. Les variations des contraintes normales nettes affectent autant la resistance au cisaillement de pic et post-pic. Les enveloppes de fractures developpees avec les courbes de retention d'eau capturent l'influence non lineaire de la succion matricielle sur la resistance au cisaillement des sols et des interfaces. Mots-cles : sol, non sature, interface, resistance au cisaillement. [Traduit par la Redaction], Introduction When structural elements are in contact with unsaturated soil, there is transfer of stress between the two materials through a contact zone referred to herein as an 'unsaturated interface.' [...]

The writers appreciate the discussers (Ahmad ALsakran et al. 2016) for their interest in the paper and their contributions to the further interpretation of the vulnerability of soils to suffusion. [...]