Pore Continuity Index Research Articles

ATRIBUTOS FISICO-HIDRICOS COMO INDICADORES DA FUNCIONALIDADE DOS POROS DO SOLO SOB DIFERENTES NIVEIS DE COMPACTAÇÃO

ABSTRACT Compaction modifies the structural arrangement and essential functions of soil pores. In this context, the objective of this study was to evaluate the impact of different compaction levels in an Argissolo Amarelo (Ultisol) on the physical-hydraulic attributes that indicate the functionality of soil pores. The experiment was conducted using 0.05 x 0.05 m soil cylinders with 4 compaction levels (CL): 61, 71, 82 and 92%, and at each CL, the pore-size distribution, intrinsic soil air permeability (Kair), pore continuity index N, soil water characteristic curve and cumulative pore-size frequency were quantified under a completely randomized design. The increase in CL did not impact the amount of micropores, but reduced the amount of macropores to values lower than the minimum required from the CL of 82%. The increase in CL caused reductions in N index, Kair and aeration porosity, but with different amplitude depending on the CL and the water tension in the soil. CL above 61% reduced the water content at the tension range between 0 and 6 kPa and, as a consequence, increased the percentage of aeration pores, besides promoting greater water retention within the range between 10 and 1500 kPa. The evaluation of the physical-hydraulic attributes of the Argissolo Amarelo (Ultisol) revealed that the increase in the compaction level altered soil structure, reduced and formed pores that were poorly continuous and less permeable to air flow and, despite the higher water retention at the higher tensions, promoted lower available moisture content.

Soil tillage and machinery traffic influence soil water availability and air fluxes in sugarcane fields

Intensive soil disturbance by conventional tillage and heavy machinery traffic for planting, cultivation, and harvesting operations are the main causes of soil compaction and degradation of soil physical quality in sugarcane fields. However, the adoption of conservation soil tillage practices, such as no-till and reduced tillage associated with traffic control have been proposed as a key strategy to preserve soil physical quality, enhancing water availability and air fluxes. Nevertheless, the effects of reduced tillage associated with traffic control in sugarcane fields are still not well documented. A study was carried out to quantify soil water availability and air flux indicators in two soils with contrasting textures (named Sandy Loam and Clayey soil) under conventional and reduced tillage practices associated with and without controlled machinery traffic in central-southern Brazil. Soil physical parameters such as bulk density, total porosity, air-filled porosity, air permeability, pore continuity index, and the least limiting water range (LLWR) were measured. In both soils, there was no difference between conventional and reduced tillage in bulk density, air permeability, and LLWR. However, reduced tillage with non-traffic decreased bulk density and increased macroporosity, air-filled porosity, air permeability, pore continuity, and LLWR in Sandy Loam soil. In Clayey soil, the bulk density and LLWR did not change between tillage practices, but air permeability and pore continuity index were larger under reduced tillage with non-traffic. These results highlighted that soil disturbance by conventional tillage does not improve the water availability and air permeability during the crop cycle. However, the traffic control is essential to support the adoption of reduced tillage in sugarcane fields, preserving soil water availability and air fluxes for the subsequent ratoons. Reduced tillage and traffic control are two of the most important pillars for reducing soil compaction and promoting sustainability of sugarcane production in Brazil.

Effect of organic residues on soil properties of loamy topsoil of haplic Luvisol in Northern Germany

Summary The application of organic residues should ensure a sufficient air capacity (AC) and plant available water capacity (AWC) to improve the soil aeration and water supply for plant roots, whereas the air permeability (ka) primarily depends on the number of functional and, therefore, connected pores. The objective of the study was to investigate the effect of digestates derived from maize (Zea mays L.), sugar beet (Beta vulgaris L.), and wheat (Triticum aestivum L.) in ratios of 100%, 80%, and 20%, respectively; compost of shrub debris; and sewage sludge on AC, AWC, and ka values, including the pore continuity indices (c2, c3) of a loamy Ap horizon of a haplic Luvisol. The results indicate that AC values increase from 0.142 of up to 0.191 cm3 cm−3, but pore continuities and AWC values decrease from 0.143 down to 0.111 cm3 cm−3, except for wheat-containing digestate (20w80b), which shows an opposite trend. The application of organic residues can compensate low AC values but not the AWC values. The wheat-containing digestate should be preferred for improving the water-holding capacity and, therefore, the water supply for plant roots.

Enhancing soil physical quality and cotton yields through diversification of agricultural practices in central Brazil

AbstractBrazil plays an important role in the global agricultural production, making Brazilian productive land use and management a critical issue. This study tested the hypothesis that soil physical quality (SPQ) of the Cerrado biome in central Brazil depends on appropriate crop rotation and tillage practices in cotton (Gossypium hyrsutum [L.])‐based cropping systems and is critical to achieve optimal cotton yields (COT). SPQ of an Oxisol was accessed for layers of 0.0–0.1, 0.1–0.2, and 0.2–0.4 m under conventional tillage (CT) and no‐tillage (NT) systems, in different farming systems, such as cotton monoculture, crop succession, and crop rotation in central Brazil. Variables studied were soil bulk density, total porosity, air permeability, pore continuity index, water content at field capacity, relative saturation at field capacity, the least limiting water range (LLWR), and COT. Soil sampling was performed in 2014, and COT was obtained in the 2013 and 2014 cropping seasons. Conservationist practices, such as crop diversity and absence of soil tillage, improved water availability, enlarged the LLWR, and indicated that brachiaria cultivation positively influences the SPQ, whereas continuous soil tillage and less crop diversity adversely affected SPQ. COT was positively influenced by SPQ aspects regarding the increased water access in the 0.2–0.4 m layer of NT and in the 0.0–0.1 m of CT management. Differences in SPQ, as indicated by the LLWR, were reflected in differences in COT, as well as in soil air permeability in the CT treatments for the 2014 cropping season. Other SPQ variables had no relation to COT.

Improvement of oxygen transport functions in grave soils due to quicklime application depending on soil texture

Quickliming (application of CaO) was tested as structure amelioration method in order to improve aeration properties in grave soils and thereby prevent decomposition problems due to an insufficient oxygen supply in the burial environment. CaO is expected to promote a stronger aggregation and stabilization in the backfilled soil and thereby enhance pore functionality. In this study, grave simulations (without buried corpses) were set up at three differently textured sites in Germany (S1: sandy loam, S2: silt loam, S3: clay loam texture) with clay contents ranging from 6 to 35%. Each site included two grave simulations: (i) excavation and backfill only (“Nil” treatment) and (ii) addition of 20 kg CaO m−3 to the soil backfill (“CaO” treatment) prior to backfilling. Undisturbed soil cores were repeatedly taken at specific time intervals to determine the aeration status in the simulated cover layer (L1, above potential coffin position in 50 cm depth) and the coffin layer (L2, 90–135 cm depth) by measuring the diffusivity and pore functionality parameters (air-filled porosity (θa), pore continuity index (C1) and tortuosity (Τ)) at field capacity (−6 kPa). Field measurements included a continuous monitoring of field moisture conditions (matric potential) and repeated measurements (2–4 week intervals) of O2 concentrations in both layers (L1 and L2).The addition of CaO to the backfill improved the aeration properties in the grave simulations; this was indicated by a higher diffusivity and mostly higher O2 concentrations at all sites, which was related to a more continuous (C1) and less tortuous (Τ) pore system compared to the “Nil” treatment. However, under field conditions, the effectiveness of soil-lime-mixtures was reduced with increasing clay and soil moisture content. A permanently high water saturation in L2 at S2 and S3 offsets the positive effects of CaO on gas transport ability and therefore restricts an undisturbed gas exchange from the atmosphere to the coffin layer. Consequently, for grave soils with high stagnic/ground water level an additional drainage is needed, while for cohesive, clay-enriched soils a more pronounced homogenization and lime mixing intensity is recommended.

Eleven Years' Effect of Conservation Practices for Temperate Sandy Loams: II. Soil Pore Characteristics

Core Ideas Long‐term effects of tillage, rotation, and residue management studied. Residue retention increased porosity, air permeability, and gas diffusivity. Residue retention alleviated negative effects of reduced tillage on soil pores. Moldboard plowing caused best topsoil (0‐20 cm depth) quality. Direct drilling resulted in highest air permeability at the 18‐ to 27‐cm depth. Conservation agriculture (CA) is regarded by many as a sustainable intensification strategy. Minimal soil disturbance in combination with residue retention are important CA components. This study examined the long‐term effects of crop rotation, residue retention, and tillage on soil pore characteristics of two Danish sandy loams. Rotation R2 is a rotation of winter crops (mainly cereals) with residues retained, rotation R3 a mix of winter and spring crops (mainly cereals) with residues removed, and rotation R4 the same mix of winter and spring crops, but with residues retained. Each rotation included the tillage treatments: moldboard plowing to 20‐cm depth (MP), harrowing to 8‐ to 10‐cm depth (H) and direct drilling (D). Soil cores were taken from the topsoil (4–8, 12–16, 18–27 cm) in mid‐autumn 2013 and early spring 2014. Water retention, air permeability, and gas diffusivity was determined for the first two depths (100‐cm3 cores), and air permeability for the 18‐ to 27‐cm depth (250‐cm3 cores). Moldboard plowing resulted in the best soil quality at two upper depths with higher total porosity, air‐filled porosity, air permeability, and gas diffusivity compared with reduced tillage. For instance, the volume of pores > 30 µm was more than 0.03 m3 m‐3 larger for MP than for D in spring 2014 at the 4‐ to 8‐cm depth. At the 18‐ to 27‐cm depth, direct drilling resulted in a better air permeability and pore continuity index (e.g., air permeability of 18.2 and 11.2 μm2 for D and MP, respectively at −10 kPa for the Foulum location). Residue retention, especially when combined with direct drilling, increased total porosity, air‐filled porosity, air permeability, and gas diffusivity and decreased blocked air porosities. Our results suggest residue management can be used to alleviate the negative effects of reduced tillage on soil structural quality.

Changes in soil aeration and soil respiration of simulated grave soils after quicklime application

AbstractThe decomposition of buried human remains on cemeteries can be delayed in poorly aerated graves due to high water levels and a low permeable pore system for oxygen and water transport. With aim to improve the soil aeration properties in the burial environment, the addition of quicklime (CaO) to the grave backfill was tested. Quicklime is expected to promote a stronger aggregation and stabilization of the backfilled soil mainly by forcing an immediate dehydration and particle cementation processes.Two different grave simulations (without buried corpses) were prepared: (1) mixing the grave backfill with 20 kg m−3 quicklime (“CaO”) and (2) backfill without CaO (“NIL”) on a cemetery in Northern Germany. The soil type was a Terric Anthrosol (Stagnic) with a loamy sand texture. Undisturbed soil cores were taken from two depths before and after excavation and backfill at regular intervals of 3 months in order to analyze changes in (1) gaseous transport functions expressed by air‐filled porosity, air permeability (air permeameter), gas diffusivity (double chamber method) and related pore continuity indices as well as in (2) soil respiration (alkali trap method) representing microbial activity.Results clearly demonstrated a more conductive pore system in the CaO variant reflected by higher gas diffusivity and air permeability over 1 year compared to the NIL variant. Pore continuity indices also indicated a more connective pore system for the CaO variant. Effects of CaO application on soil respiration rate differed between the quarterly sampling times. Results indicated that microorganism were still active under alkaline soil conditions induced by CaO application, but the quantitative determination of biologically produced CO2 is influenced by chemical reactions when hydrated quicklime [Ca(OH)2] was reformed to limestone under consumption of CO2. The experiments indicate that the application of quicklime is a promising approach to improve aeration properties of grave soils and is therefore proposed as an adequate method to improve the aeration of burials on cemeteries.

Within cropping season changes in soil physical properties under no-till in Southern Brazil

Although opening a narrow furrow for seed placement and fertilizer application is the only mechanical disturbance in soils under no-till (NT) production systems, the persistence of effects associated with that disturbance on physical properties throughout the growing season has not been widely quantified. This study tested the hypotheses that soil physical properties within the row (R) differ from those in the interrow (IR) zone under long-term NT, and that the disturbance effects on soil in the R zone persist during the soybean (Glycine max) growing season. The objective of this study was to determine soil physical properties under R and IR zones under NT soybean. Thirty undisturbed samples in each zone were obtained during the growth cycle at 15, 52 and 115days after sowing. Soil properties monitored were bulk density (BD), total porosity (TP), soil water content and air-filled porosity at field capacity (FC and εa, respectively), air permeability (Ka), pore continuity index (K1) and saturated hydraulic conductivity (Ksat). There were significant differences in all soil physical properties among R and IR zones when measured 15days after sowing (DAS). At the 52 and 115 DAS samplings, K1 and Ksat did not differ between R and IR positions, but significant differences persisted for BD, FC, εa, Ka and TP. BD in R and IR remained different during the soybean growing season. Despite that, the large difference for BD found in 15DAS (1.34 and 1.04Mgm−3 for IR and R, respectively), tended to disappear, resulting in values of 1.15 and 1.07Mgm−3 for IR and R at 115DAS, respectively. A long-term NT soil thus had different physical properties at R versus IR over a short term after sowing. Soil resilience was evident even over a short term, improving soil physical quality at IR position. The large contrast in soil physical parameters between R and IR positions was observed right after sowing, gradually decreasing over time and no longer being detectable by the end of the growing season.

Subsoil compaction assessed by visual evaluation and laboratory methods

Subsoil compaction is one of the major causes of land degradation worldwide and therefore a major threat to future crop productivity. The objective of this contribution was to evaluate the effects of compaction treatments on soil structure based on the numerical visual evaluation of subsoil structure (SubVESS) method and on quantitative measurements of soil pore characteristics. The effect of soil compaction was evaluated using treatments from a compaction experiment initiated in 2010 at Research Centre Flakkebjerg, Denmark, on a sandy loam soil using five levels of compaction. In this study we used i) non-compacted reference, ii) Treatment M3, where soil was subjected to multiple passes (five wheel passes per compaction event annually) of a tractor-trailer combination with max. wheel load of ∼3Mg, and iii) M8, with multiple passes (four wheel passes per compaction event annually) of a tractor-trailer combination with max. wheel load of ∼8Mg. The tire inflation pressure was generally above the recommended pressure in order to mimic the inflation pressures commonly used in practice. The treatments were applied track-by-track in the spring of 2010–2013 when the soil water content was close to field capacity. Spring barley (Hordeum vulgare L.) was established every year after a shallow secondary tillage to ∼0.05m depth to loosen the uppermost layer. Sampling and field evaluation were done on May 7, 2014, i.e. after four years of compaction treatments (2010–2013) and one year of recovery. The soil profiles were evaluated at the same time as soil cores were sampled at 0.3, 0.5 and 0.7m depth. In the laboratory, we measured water content, total porosity, air-filled porosity (εa), air permeability (ka) and calculated pore organization indices (PO1=ka/εa and PO2 = ka/εa2) on the soil cores. We estimated the blocked air-filled porosity and pore continuity index from the relationship between air permeability and air-filled porosity for −30 to −300hPa matric potentials. Assessment using the SubVESS method showed a marked effect of the M8 treatment on soil structural quality down to ∼0.65m depth, but the effects of the M3 were not significantly different from the control at any depth. This was confirmed by the laboratory-measured data, which showed that the M8 treatment drastically reduced total porosity, air-filled porosity, air permeability, pore size distribution, pore tortuosity and continuity, especially at 0.3 and 0.5m depths.Detailed measurements of the anisotropy of soil pore characteristics at 0.3–0.4m depth showed that for PO2 (pore size distribution) and blocked air-filled porosity the control soil was significantly anisotropic. Although compaction with the ∼8Mg wheel load affected the vertically and horizontally-oriented pores differently, it did not significantly affect the anisotropy of the different pore characteristics. Our results showed that in general, there was a good agreement between the field and laboratory methods and thus, the two can be combined to evaluate the effects of compaction in the subsoil.

Liquid Bovine Biofertilizer and Cultivation Effects on the Porosity of a Typic Haplocambids as a Function of Cultivation and Dose

The soil porous space is where processes related to gases and soil solution occur, and provides an adequate guide for agricultural practices. This study aimed to evaluate the integrated effects of cultivation and the application of liquid bovine biofertilizer on the porous network of a Typic Haplocambids (Cambissolo Haplico Ta Eutrofico) cultivated with figs (Ficus carica L.). Four treatments were evaluated (under fig cultivation with the application of 20, 40 and 60 % of biofertilizer through irrigation, a control treatment without biofertilizer and an additional treatment – soil under natural vegetation). Disturbed and undisturbed soil samples were collected in three soil layers (0.0-0.1, 0.1-0.2 and 0.2-0.3 m) with four replicates. The following physical properties were analyzed: particle-size distribution, soil bulk and particle densities, and soil water retention curve. The coefficient of intrinsic soil air permeability was calculated based on the equation that considers the decreasing pressure method. Soil porosity, pore continuity index and blocked porosity were calculated and pore length was estimated. Compared with the native forest, pore network quality is improved, if not maintained, when the soil is cultivated under the conditions described in this experiment. In the conditions of cultivation, the application of bovine biofertilizer, for supplying sediments that block or reduce the size of the pores, did not improve soil air permeability. The cases where pore network quality was worsened in soil porosity as a result of the applied treatments (Biofertilizer 20 %, Biofertilizer 40 % and Biofertilizer 60 % for the layer of 0.0-0.1 m and Biofertilizer 60 % for 0.1-0.2 m), although not considered critical to plant development, point to the need for specific management practices (for instance, avoid coarse residues in the biofertilizer before its application) to avoid soil degradation.

Temporal dynamics of the physical quality of an Andisol under a grazing system subjected to different pasture improvement strategies

The south of Chile presents adequate conditions for the development of pastoral ecosystems in volcanic soils. The improvement strategies of these pastures consider soil tillage and fertilization. Andisols have special physical properties, however, their ability to support high stress levels affecting the soil physical quality should be defined. The aim of this study was to evaluate the evolution of the effect of different pasture improvement strategies on the physical quality of an Andisol. The study was carried out in the Estación Experimental Agropecuaria Austral (39°46′ S, 73°13′ W), on a Duric Hapludand. Four treatments were randomly distributed corresponding to a mixed binary species (T1: Lolium perenne L.+Trifolium repens L.), a multiple species mixture (T2: Bromus valdivianus Phil., L. perenne, Dactylis glomerata L. and T. repens) both of which were tilled, and another two that had no mechanical intervention of the soil, which was degraded naturalized pasture without fertilization (T3) and, finally, one that represented a naturalized degraded pasture with fertilization (T4). Additionally, a naturalized, degraded pasture without intervention was analyzed, which corresponded to the initial situation (IS) of the pasture. Undisturbed samples were taken between 1 and 10cm in December 2010 and October 2011. The bearing capacity of the soil (BC), the water retention curve (WRC), the saturated hydraulic conductivity (Ks) and the air conductivity (kl) were determined. Soil physical quality indicators were derived such as plant available water (PAW), pore continuity indexes (C2 and C3) and the S index for physical quality. The tillage of the soil separated the pastures, grouping them into either disturbed (T1 and T2) or non-disturbed (T3 and T4). In general, parameters that were derived from the pore volume never reached the critical levels (e.g., PAW>20%, S Index>0.035) proposed in the literature. On the other hand, parameters that define the functionality of the porous system (kl), showed differences in one order of magnitude between pastures that maintained their structure (T3 and T4) compared to the tilled ones (T1 and T2), so that for T4 the pore continuity index (C2 and C3) increased by 300% and the volume of blocked pores (ϵb) did not reach 1%. At the same time, properties that define the performance of the pores evolved positively over time in a more intense manner in pastures that maintained their structure (T3 and T4). However, this does not mean that in the long term these pastures would not present conditions of physical degradation.

Changes in soil pore network in response to twenty-three years of irrigation in a tropical semiarid pasture from northeast Brazil

Irrigation is a technique in agri-, vini- and horticulture which consists in the controlled supply of water and which has been used for many years in order to ensure agricultural productivity in many regions of the world. In regions where this water application technique is used there is the possibility of changes in soil properties; amongst others, soil structure, a physical attribute that is related to several other soil attributes such as density, porosity and aggregate stability. Soil structure is also greatly affected by wetting and drying cycles, which are provided by irrigation in these regions. This study aims at evaluating changes in the physical attributes of a soil caused by 23 years of irrigation. The study area is located on the Apodi Plateau, Ceará, Brazil. Two areas were evaluated, one irrigated by a central-pivot sprinkler system and a non-irrigated control. For the study, eight disturbed soil samples were collected in each area, in the layers of 0–20, 20–40 and 40–60cm, and four undisturbed soil samples, using cylinders of 5cm×5cm, in the layers of 0–20cm and 20–40cm. The analyzed physical attributes were: particle density, soil bulk density, particle size distribution, total porosity, macroporosity (pores with diameter≥50μm, applying matric potential of −6kPa), microporosity (pores with diameter<50μm, total porosity minus macroporosity), soil air permeability, soil resistance to penetration, aggregate stability and pore continuity indices. The results indicated that irrigation contributed to changes in soil physical attributes directly related to soil structure. Also, the consequent wetting and drying cycles were sufficient to improve aggregate stability; however, these cycles, combined with processes of translocation and compaction, have increased microporosity, causing the formation of less continuous and more tortuous pores and lower soil air permeability.

Effects of the stress field induced by a running tyre on the soil pore system

This study investigated the impact of vehicle traffic on soil physical properties by systematically collecting samples in a transect running from the centreline to the outside of the wheel rut in a wheeling experiment conducted on a clay loam soil at Suberg near Bern, Switzerland, in 2010. Four repeated wheelings were performed by a forage harvester (wheel load 6100kg; tyre width 80cm). Mean normal and horizontal stresses were measured with Bolling probes (at 10, 20 and 40cm depth) and load cells (at 40, 50, 60cm lateral distance from the centreline of the wheel rut at 10, 30 and 50cm depth), respectively. Intact soil cores of 100cm3 sampled at 10, 30 and 50cm depth in a soil transcet running from the centreline of the wheel rut to the unwheeled part of the field were used for measurements of water retention and air permeability (ka) at −30, −100 and −300hPa matric potential. The complete stress state in the soil profile beneath the harvester tyre was calculated using the SoilFlex model. Pore continuity index (N) and blocked air-filled porosity (εb) were estimated from the relationship between ka and air-filled porosity (εa) for a range of matric potentials. Calculated and measured stresses agreed well at all depths. At −100hPa, εa was consistently lower under the centreline of the wheel rut than at the lateral edge of the rut or outside the wheel rut, while ka was lowest at the lateral edge of the wheel rut and highest outside the wheel rut, with intermediate values under the centreline of the wheel rut. Simulations of the stress field in the soil beneath the tyre indicated that the trends in ka were determined by both the mean normal stress and the shear stress, while the trend in εa was determined by the mean normal stress only. At 10cm depth, the index of pore continuity (N) supported the interpretation that soil pores under the centreline of the wheel rut are primarily reduced in size, while pore continuity is highly affected at the lateral edge of the wheel rut, as indicated by a higher value of εb than in other locations. These results indicate that sampling along the wheel track transect can provide better information about traffic-induced changes on soil physical properties than random sampling in lateral locations relative to the centreline of the wheel rut.

Gas transport and subsoil pore characteristics: Anisotropy and long-term effects of compaction

Anisotropy of soil pore functions significantly affects the transport of gas and water in soil. This paper quantifies anisotropy of subsoil pores and investigates the long-term impact of soil compaction by agricultural machinery. Two long-term field experiments on soil compaction formed the basis for the investigation, one established in 1981 on a clay soil in Finland (60°49′N, 23°23′E) and another in 1995 on a sandy clay loam in Sweden (55°49′N, 13°11′E). In 2009/2010, soil cores were sampled in vertical and horizontal directions from 0.3, 0.5, 0.7 and 0.9m depth (the two lower depths only in Sweden). In the laboratory, water content, air-filled porosity (εa), air permeability (ka) and gas diffusivity (Ds/D0) were determined at selected matric potentials. For the sandy clay loam, morphological characteristics of pores (effective pore diameter, dB; tortuosity, τ; the number of effective pores per unit area, nB) were calculated using a tortuous tube model at −100hPa matric potential. Blocked air-filled porosity (εb) and a pore continuity index (N) were estimated from the relationship between ka and εa for a range of matric potentials. A factor of anisotropy (FA) was determined as the ratio of a given property measured in the horizontal direction to that in the vertical direction. ka showed anisotropic behaviour (FA<1) for the clay soil and for the 0.3m depth of the non-compacted sandy clay loam soil, while Ds/D0 displayed anisotropy for the clay soil (FA<1). In the sandy clay loam soil, dB and nB displayed significant anisotropy (FA<1) except at 0.9m. We interpreted this as effects of biological activities and physical processes in the B-horizon not being active in the C-horizon (0.9m depth). Compaction generally reduced ka, Ds/D0, dB, nB and increased τ for both sampling directions. Compaction had an effect on anisotropy for soil drained to −100hPa, but only for ka and dB in the sandy clay loam at 0.3m depth. Compaction reduced anisotropy for the N parameter, i.e. effects on soil pore continuity at the macropore scale, while it increased the anisotropy for εb. Our data thus indicate that compaction had persistent effect on soil physical properties and also affects anisotropy, especially that of macropores.

Direction-dependent behaviour of hydraulic and mechanical properties in structured soils under conventional and conservation tillage

The development of soil structure units with defined forms and dimensions (e.g. platy by soil compaction or prismatic up to subangular-blocky by swelling–shrinkage processes) can lead to direction-dependent behaviour of mechanical and hydraulic properties. However, little research has investigated direction-dependent behaviour directly. Undisturbed samples were collected at different horizons and orientations (vertical and horizontal) of Stagnic Luvisols derived from glacial till (Weichselian moraine region in Northern Germany). A direct shear test determined the cohesion ( c) and the angle of internal friction ( φ). The water retention curve (WRC), the saturated hydraulic conductivity ( k s) and the air permeability ( k a) were also measured. The air-filled porosity ( ɛ a) was determined and pore continuity indices ( N) and blocked porosities ( ɛ b) were derived from the relationship between k a and ɛ a. Although the pore volume as a scalar is isotrop, the saturated hydraulic conductivity and air permeability can be anisotropic. In the seedbed (SB) and plough pan (PP) of conventionally managed soils the effective porosity is non-direction-dependent, however, differences in k s as a function of sampling direction can reach one order of magnitude in PP ( k sh > k sv). The shear strength parameters do not present a significant anisotropy, although, a pronounced spatial orientation of soil aggregates (e.g. induced by soil compaction in a plough pan) lead to direction-dependent shear strength (by σ n: 10 kPa, σ tv: 12 kPa and σ th: 19 kPa). This behaviour was especially observed in pore continuity indices (e.g. vertical and horizontal oriented aggregates observed in Bvg and PP presented ɛ bv < ɛ bh and ɛ bv > ɛ bh, respectively) showing that the identification of soil structure can be used as the first parameter to estimate if hydraulic properties present a direction-dependent behaviour at the scale of the soil horizon, which is relevant in modelling transport processes.

Gas diffusion, fluid flow and derived pore continuity indices in relation to vehicle traffic and tillage

SUMMARYRelative gas diffusivity, air permeability and hydraulic conductivity were measured in undisturbed soil cores from tillage and traffic experiments. Continuity indices were taken as the quotient of relative diffusivity and air‐filled porosity, and of air permeability and air‐filled porosity (and the square of air‐filled porosity). These were applied to individual measurements or to treatment means. More general continuity indices were derived from the changes in flow or diffusion with porosity, where the variations in porosity were due to both field variability and applied changes of water potential. These indices were the exponent in the relationship between relative diffusivity and air‐filled porosity and the slope of log–log plots of air permeability and air‐filled porosity or hydraulic conductivity and degree of saturation. Some physical significance was attached to the exponents by comparison with models of soil porosity. Positive intercepts of the relative diffusivity or air permeability plots on the air‐filled porosity axes were taken as porosities blocked to gas movement.Continuity indices and flow measurements showed differences between tillage and traffic treatments which did not necessarily reflect differences in bulk density. Intrinsic permeability was better estimated from air permeability than from unsaturated hydraulic conductivity.