Soil Macropores Research Articles

Impacts of perforated sheet pipe installation on some paddy soil properties

Today, the world’s vision emphasizes the development of climate-adaptive agriculture for food security and provision of water with safe technology. Under extreme weather conditions, it is necessary to manage water effectively. We must not neglect drainage as a water management option for sustainable agricultural production. Specially designed perforated sheet pipe drains have been developed in Japan recently to control the underground water table and to upgrade croplands from lowlands to uplands. As soils with perforated sheet pipes installed could potentially improve soil aeration, and land conversion could also influence soil functions and properties, this study focuses on the importance of soil properties changed around perforated sheet pipes installed at a depth of 40 cm in former paddy soils. Using (3 × 3 × 2) factorial design with three replications, soils were sampled on farmland in the Japanese towns of Hisayama, Fukuoka and Usa, Oita, respectively, in 2017 based on three stream sites (upstream, midstream and downstream), three distances from the sheet pipe (center, 1 m and 2 m) and two soil depths of 10 cm and 25 cm, respectively. Thirteen potentially changeable soil properties were measured, and all recorded data were analyzed statistically by performing the F test. All means were also compared at least the 5% significantly different level. As the results, there was a major improvement in air-filled capacity and infiltration above the installed perforated sheet pipe. The nearer the sheet pipe, the more the soil bulked together, with the more significant increase in soil organic matter, and total carbonate content that promotes the formation of soil macropores, at 1 m of sheet pipe distance in the deep paddy soil layer. The increase in the porosity (f) of studied soils allows more water and air to pass through.

Quantification of Three Dimensional Characteristics of Macrofauna Macropores and Their Effects on Soil Hydraulic Conductivity in Northern Vietnam

Soil bioturbation is associated with the production of soil macropores that influence numerous ecological functions such as those associated with water infiltration and the generation of runoff water. This impact is especially important on sloping lands in the tropics that are highly susceptible to erosion. In this study, we questioned the influence of soil biodiversity on soil macropore properties (> 20 mm3) and saturated hydraulic conductivity (Ksat) on sloping land in northern Vietnam. Biostructures found at the soil surface (casts, sheetings and soil excavated on the ground) were used to identify areas colonized either by earthworms, termites or dung beetles, respectively. The influence of soil macrofauna on Ksat was measured in situ using the Beerkan method below bioturbated zones and compared to the surrounding soil without visible biostructures at the soil surface. Undisturbed soil columns were afterwards sampled and scanned by X-ray computed tomography (X-CT). Properties of macropores below each biostructure depicted a large variability, revealing the complexity of the macropore network. Further, galleries made by termites, dung beetles and earthworms were manually isolated from the rest of macroporosity. Galleries made by beetles, termites and earthworms were clearly differentiated on the basis of their diameter, verticality, sphericity, tortuosity, length and number of branches and the fraction of galleries in the top part of the column and. Ksat was most increased by dung beetles (45-fold), then by termites (30-fold) and to a lesser extent by earthworms (16-fold). Relationships between total macropore properties and Ksat showed that the most important properties explaining Ksat were (i) the volume of percolating macropores, (ii) the diameter, (iii) the critical macropore diameter and (iv) the number of macropores. In conclusion, this study confirmed not only the interest in using X-CT for the quantification of macroporosity but also the absence of a clear relationship between aboveground biostructures and macropore properties and functional impacts.

A new method to trace colloid transport pathways in macroporous soils using X‐ray computed tomography and fluorescence macrophotography

The fast and deep percolation of particles through soil is attributed to preferential flow pathways, and their extent can be critical in the filtering of particulate pollutants in soil. Particle deposition on the pore walls and transport between the pores and matrix modulate the preferential flow of particulate pollutants. In the present research, we developed a novel method of combining fluorescence macrophotography and X‐ray computed tomography (CT) to track preferential pathways of colloidal fluorescent microspheres (MS) in breakthrough experiments. We located accumulations of MS by fluorescence imaging and used them to delimit the deposition structures along the preferential colloid pathways by superimposing these images on the 3‐D pore network obtained from CT. Advection–diffusion with transport parameters from the dual‐porosity equation correlated with preferential pathway features across different soil management techniques. However, management did not influence the morphology of the MS preferential pathways. Preferential flow occurred in only a small fraction of the total pore network and was controlled by pores connected to the soil surface and by matrix density.Highlights X‐ray tomography and epifluorescence images were combined to identify preferential pathways in soil. Preferential pathways constituted approximately 1% of the soil bulk volume. Correlations between descriptors of preferential pathways and colloid transport were found. Tillage influenced the morphology of pores, but not the shape of preferential pathways.

Reconfiguration of macropore networks in a silty loam soil following biochar addition identified by X‐ray microtomography and network analyses

Biochars are widely applied to improve soil pore structures. However, the quantitative effects of biochars on soil macropore networks still remain unclear. In this study, the effects and possible mechanism of biochar application on soil macropore networks were investigated quantitatively by industrial X‐ray microtomography (CT), a pore network model and network analyses. Soil cores of 5 cm in diameter and 10 cm in height were collected from a paddy soil (Typic Haplustalf) without biochar application (CK), and amended with rice straw biochar (RSB), corn straw biochar (CSB) and bamboo biochar (BB) at a rate of 25 Mg ha−1 (w/w). Results revealed that the total and connected macroporosity of the biochar‐amended soil was reduced significantly compared with CK, whereas the reverse was true for the isolated porosity. The three types of biochars exhibited different effects on macropores of the soil, depending on their size and shape. The pore network model and network analyses showed that the physical and topological structures of macropore networks in biochar‐amended soil were clearly reconfigured. The accessibility of these macropore networks was markedly reduced. Our results indicated that the incorporation of biochar did not necessarily increase the soil's macroporosity. The combination of pore network model and network analyses was effective for quantifying key properties of soil macropore networks affected by the incorporation of biochar. The changes in these properties may be used to evaluate the effects of biochar on water, air and nutrient transport.Highlights Effect of biochars on the soil macropore structure was studied by industrial CT. Macropore network structure was quantified by a pore network model and network analyses. Biochar incorporation markedly changed the accessibility of the soil macropore network. The macropore network of soil can be reconfigured following biochar addition.

Linking saturated hydraulic conductivity and air permeability to the characteristics of biopores derived from X-ray computed tomography

The different types of soil macropores (e.g., biopores and non-biopores) vary in the conductivity of water or air due to the difference in the 3D pore characteristics. The objectives of this study were to reveal which types of macropores and which macropore characteristics played the most important roles in regulating water or air flow. Intact soil columns sampled from the subsoil of a long-term fertilization experiment were scanned by medical X-ray computed tomography (CT), and subsequently, saturated hydraulic conductivity (Ks) and air permeability at −12 cm water potential (Ka12) were measured. The 3D characteristics of macropores were then analyzed with image analysis. The biopores and the percolating biopores that connected the top and the bottom of a soil column were separated for the biopore-dominated samples (with percolating biopores) and their 3D characteristics were also quantified. Our results showed that the mean macropore diameter of the limiting layer (MDLL) presented the best relationships with Ks and Ka12 compared with the other macropore characteristics for all the samples. The biopores and percolating biopores contributed 27.8–74.5% and 3.26–64.4% of the volume of total macropores, respectively, for the biopore-dominated samples. The hydraulic radius, mean diameter, compactness, global and local connectivities, and MDLL of biopores, especially those of percolating biopores, were generally larger than those of total macropores. The global connectivity (Γ) of biopores performed very well for estimating Ks and Ka12. The MDLL of percolating biopores could predict Ks much better than the MDLL of biopores and total macropores. Moreover, the performance of MDLL for estimating Ka12 was as good as the MDLL of biopores but was much better than the MDLL of total macropores. The findings of this study reveal that the MDLL is a more useful parameter in estimating saturated hydraulic conductivity and air permeability at low water potential than the other CT-derived pore characteristics.

Using undisturbed soil samples to study how rock fragments and soil macropores affect the hydraulic conductivity of forest stony soils: Some methodological aspects

Forest stony soils have rock fragments and many macropores, the latter arising mainly from the activity of soil fauna and plant root decay. We still lack knowledge about the hydraulic properties of this type of soil, especially when considering the influence of both factors – rock fragments and macropores – on the saturated hydraulic conductivity. This research aims to supplement the method we proposed before to test the hydraulic conductivity of undisturbed soil samples (Ilek and Kucza, 2014; Kucza and Ilek, 2016). We propose to enhance the method by taking into account the impact of rock fragments and macropores on the hydraulic conductivity of forest stony soils, particularly on the hydraulic conductivity of the medium conducting water in the soil, that is, soil matrix. We studied, using artificial samples, how rock fragments influence hydraulic conductivity. To test the enhanced method, we used 33 undisturbed samples taken from the top layers (0–12 cm) of forest stony soils. The results show that rock fragments in the soil can both increase and decrease the infiltration of water by the soil matrix. Rock fragments with diameters close to 20 mm had the least influence on water infiltration. Soil macropores in the samples always increased water infiltration. Empty macropores increased it from two to twenty-four times while filled macropores from three to five times. Our method gives the opportunity to study the hydraulic conductivity of macropores and their physical and chemical properties.

Characterising macropores and preferential flow of mountainous forest soils with contrasting human disturbances

Preferential flow can develop in soil macropores, and macropores are sensitive to human disturbances. This study investigated soil macropore features and the main factors controlling preferential flow at four sites with different levels of human disturbance in a mountainous area in Central China. The level of human disturbance decreased with increasing elevation, with the lowest elevation areas covered with coniferous trees (LF) > middle mountain areas covered with tea gardens (TG) > middle mountain areas covered with deciduous trees and mixed shrubs (MF) > subalpine areas covered with evergreen coniferous trees (HF). At each site, the soil macropore structure at 0–20 cm soil depth was analysed using computed tomography scans (0.6 mm resolution) and Image J software. Preferential flow was determined by analysing the breakthrough curve (BTC) of nitrate. The macroporosity, surface area density, mean macropore size, macropore number density, length density and node density were all ranked in the order of HF ≥ MF ≥ TG = LF. Less disturbed sites had stronger evidence of preferential flow as shown by faster breakthrough, longer tails and greater asymmetry of the BTCs. There were significant (P < 0.05) positive influences of soil macropore properties on pore water velocity and the solute dispersion coefficient. The dispersivity parameter was mainly affected by the macropore equivalent hydraulic radius. This study showed that human disturbance in the mountain forest areas significantly decreased soil macropores by changing soil physical properties (e.g. bulk density, texture and soil organic matter content) and root distribution, thus increasing the risk of surface runoff and nutrient losses.

A Dual‐Permeability Approach for Modeling Soil Water Flow and Heat Transport during Freezing and Thawing

Core Ideas Macropore flow may be enhanced in partly frozen soil. We developed a dual‐permeability modeling approach for frozen soils. Four test cases of increasing complexity were evaluated. Modeling results are in agreement with current process understanding. Measured data are lacking for a more quantitative evaluation of the model. Preferential flow may become significant in partially frozen soils because infiltration can occur through large, initially air‐filled pores surrounded by a soil matrix with limited infiltration capacity. The objectives of this study were to develop and evaluate a dual‐permeability approach for simulating water flow and heat transport in macroporous soils undergoing freezing and thawing. This was achieved by introducing physically based equations for soil freezing and thawing into the dual‐permeability model MACRO. Richards' equation and the heat flow equation were loosely coupled using the generalized Clapeyron equation for the soil micropore domain. Freezing and thawing of macropore water is governed by a first‐order equation for energy transfer between the micropore and macropore domains. We assumed that macropore water was unaffected by capillary forces, so that water in macropores freezes at 0°C. The performance of the model was evaluated for four test cases: (i) redistribution of water in the micropore domain during freezing, (ii) a comparison between the first‐order energy transfer approach and the heat conduction equation, (iii) infiltration and water flow in frozen soil with an initially air‐filled macropore domain, and (iv) thawing from the soil surface during constant‐rate rainfall. Results show that the model behaves in accordance with the current understanding of water flow and heat transport in frozen macroporous soil. To improve modeling of water and heat flow in frozen soils, attention should now be focused on providing experimental data suitable for evaluating models that account for macropore flow.

Physically Based Model for Extracting Dual‐Permeability Parameters Using Non‐Newtonian Fluids

Core Ideas We present a cost‐effective and simple method for dual‐permeability model parameterization. Physical characterization is done with only experimental results of saturated flow experiments. Model inputs require only saturated flow results with water and a non‐Newtonian fluid. Dual‐permeability models simulate flow and transport within soils characterized by preferential (macro) and matrix (micro) pore domains, with each exhibiting distinct hydraulic properties. The lack of suitable methods for determining appropriate and physically based model parameters remains a major challenge to applying these models. Here, we present a method that characterizes dual‐permeability model parameters using experimental results of saturated flows from water and a non‐Newtonian fluid. We present two submodels that solve for the effective pore sizes of micropores and macropores, with macropores represented either with cylindrical (for biological pores) or planar (for shrinkage cracks and fissures) pore geometries. The model also determines the percentage contribution (wi) of the representative macro‐ and micropores to water flow. We applied the model to experimental soil samples complemented with capillary tubes simulating the macropores and showed its ability to derive the bimodal pore size distributions in dual‐domain soils using only two fluids. As such, we present this method of characterization of dual structures for improved modeling of nonuniform preferential flow and transport in macroporous soils.

Reactive Transport of Manure‐Derived Nitrogen in the Vadose Zone: Consideration of Macropore Connectivity to Subsurface Receptors

Core Ideas Manure N transformation was modeled with different soil pore connectivities to tile drains. Macropores facilitated NH4+ oxidation reaction products deeper in the profile. Nitrate losses to tile were best predicted by models using pores connected to tile drains. Macropores can be important conduits for surface‐derived nutrients to reach subsurface receptors. Accordingly, nutrient reactive transport processes in macroporous soils need to be well understood. In this study, steady‐state two‐dimensional reactive transport simulations with MIN3P‐THCm (version 1.0.519.0) were used to elucidate how soil macropore connectivity to tile drains can influence N transformations following liquid swine manure (LSM) applications to soil. Four different soil scenarios were considered: homogeneous sand, homogeneous clay loam, and clay loam with discrete macropores connected to or disconnected from the bottom boundary used to represent tile drain outflow. In relation to the homogeneous soils, macropores, overall, facilitated chemical diffusion into the adjacent soil matrix along their length and broadly augmented O2 ingress into the soil profile. These processes combined to critically control the spatial distribution of NH4+ oxidation reaction products. When used in transient simulation mode with field data observed at experimental tile‐drained plots that received LSM application, the model showed that simulated nitrate mass losses to tile are considerably higher and most realistic under the connected macropore scenario compared with the homogeneous or disconnected macropore scenarios.

THE USE OF SOIL HYDRAULIC PROPERTIES AS INDICATORS FOR ASSESSING THE IMPACT OF MANAGEMENT PRACTICES UNDER SEMI-ARID CLIMATES

Water retention capacity and hydraulic conductivity are major soil hydraulic properties that influence soil quality and the environment. The objective of this work was to assess major soil hydraulic characteristics in response to the effect of soil management and tillage practices. Eight soil treatments were considered: conventional tillage, reduced tillage, no-tillage, fallow no-tillage, abandoned soil, compacted soil, plough on compacted soil, and application of super absorbent polymers. The experiments were conducted at Koohin station, Iran. The equality of field saturated hydraulic conductivity (Kfs) and estimated Kfs was studied. Moreover, the contribution of the porosity to soil water flux was investigated and quantified. At the -1500 kPa potential, water content was influenced by the compacted treatment at soil depth of 0 to 25 cm. In this study, van Genuchten water retention curve parameters n, a, and .r values were not significantly different among treatments. The fit of the van Genuchten equation to the soil water retention data resulted in low sum of squared errors. No distinct trend was observed for estimated sorptivity in the studied area. The sorptivity values were not significantly different among soil management practices. Moreover, estimated Kfs values by DISC software were lower than measured Kfs. Soil treatments did not influence the soil water-conducting medium and macropores. Unsaturated hydraulic conductivity may partly be affected by soil texture and soil structure which were uniform under all treatments. Soil water retention and transmission may be manipulated with changes in pore size distribution under different tillage and management practices in long-term.

Separation of Soil Macropore Types in Three‐Dimensional X‐Ray Computed Tomography Images Based on Pore Geometry Characteristics

Core Ideas For preferential flow modeling, different macropore types must be considered. A method was developed for separating biopores and cracks in 3D images from XRCT. The method enabled a more objective determination of structuring element sizes. The voxel‐based approach was found useful for quantification of macropore types. In structured soils, earthworm burrows, root channels, shrinkage cracks, and interaggregate spaces form complex macropore networks. Depending on the type and morphological properties, each macropore surface type is coated with specific organo‐mineral compounds, differently affecting sorption and mass exchange during preferential flow and turnover processes. For a quantitative, macropore type–specific analysis using X‐ray computed tomography (XRCT) with subsequent three‐dimensional (3D) image analysis, a discrimination of biopores from cracks and interaggregate spaces is necessary. We developed a method that allows separating biopores from other larger macropores in 3D images from XRCT of intact soil cores. An image‐processing workflow using the MAVI (Modular Algorithms for Volume Images) software framework ToolIP (Tool for Image Processing) was created to handle XRCT 3D images. Masking steps enabled to retain the surface roughness in the resulting two images of separated biopores and cracks. As a key point, the sizes of the structuring elements used in the spherical opening and dilation were objectively determined. For this purpose, maximum differences in the pore shapes between the 3D images of cylindrical biopores vs. more flat cracks and unregularly interaggregate spaces were focused. At the given resolution of 231‐μm voxel edge length, an optimum size of 2.5 voxels was found for both processing steps. The voxel‐based approach is applicable to XRCT 3D images of different spatial resolution and appears useful for the quantification of physicochemical surface properties of different macropore types for soil volumes, enabling a more precise description of preferential flow and transport.

Influence of shrub roots on soil macropores using X-ray computed tomography in a shrub-encroached grassland in Northern China

The relationship between soil macropore and plant roots warrants special attention because it influences the behavior of water in soil. However, the influence of shrub roots on soil macropores in shrub-encroached grasslands is poorly understood. The objectives of this study were to quantify soil macropores and root architecture in a shrub-encroached grassland in northern China, and to reveal the relationship between shrub roots and soil macropore. In this study, treatments were performed that corresponded to three successional states of the shrub C. microphylla L. with three different shrub densities. At each site, three undisturbed soil cores were excavated under the shrub canopies, and a Philips medical scanner was used to simultaneously visualize and quantify the soil and root architectures. Strong positive correlations between root volume, length, and surface area and the solid surface/solid volume ratio were found, and greater root growth was noted in more porous soil. The results highlighted that the soil macropore characteristics corresponded well with the root characteristics of the soils for the three treatments. Soil macroporosity and macropore volume increased with increasing shrub root network density. In addition, the influence of plant roots on soil macropores increased with increasing shrub encroachment. The study confirmed that the large number of macropores found in the soils under shrubs was attributed to the great degree of root development there. The greater degree of macroporosity under shrubs was attributed to the larger root network density, which might cause greater amounts of water to be concentrated in the deep soil layer by macropore flow under shrub patches. The influence of plant roots on soil macropores increased with increasing shrub encroachment.

Variability of Furrow Infiltration and Estimated Infiltration Parameters in a Macroporous Soil

AbstractUnderstanding the spatial and temporal variations of infiltration in furrows is essential for the design and management of furrow irrigation systems. A key difficulty in quantifying the pro...

Influence of exclosure on CT-measured soil macropores and root architecture in a shrub-encroached grassland in northern China

Grassland exclosures with fencing are commonly used to prevent shrub encroachment. However, few studies have been conducted on the effect of exclosures on soil macropores and root architecture during shrub encroachment. This study aimed to quantify the effects of exclosures on soil macropore and root characteristics beneath the encroached shrub C. microphylla L. in the Inner Mongolia grassland. Two exclosure treatments, 9EX (shrubland enclosed for 9 years) and 2EX (shrubland enclosed for 2 years), were designed, with free grazing plots serving as controls. A total of twelve soil cores (0–50 cm deep) were excavated, which included three replicates of each treatment and control. Soil and root architecture of the soil cores were explored using X-ray computed tomography method. Results indicated that macroporosity was 1.03–1.66 times greater in the soil under the enclosed C. microphylla L. than in the soil under the grazed C. microphylla L., and the soil macroporosity increased with exclosure age. Macropores were abundant over the 0–400 mm soil layers under the enclosed shrubland but were mainly present at 0–250 mm soil depth under freely grazed shrubland. Exclosure led to significant proliferation of shrub roots compared with grazing. Root density was 1.04–1.86 times greater in the soil under the enclosed C. microphylla L. than in the soil under the grazed C. microphylla L. Roots were concentrated in the 0–250 mm soil layer in the grazed shrubland, whereas roots were abundant over 0–400 mm in the enclosed shrubland. The high number of macropores under the enclosed shrubland was closely associated with a high root density. These results suggest that exclosure enhances shrub root proliferation and therefore increases soil macroporosity and soil pore connectivity, which would be favourable for shrub encroachment.

The effect of freezing and thawing on water flow and MCPA leaching in partially frozen soil

Limited knowledge and experimental data exist on pesticide leaching through partially frozen soil. The objective of this study was to better understand the complex processes of freezing and thawing and the effects these processes have on water flow and pesticide transport through soil. To achieve this we conducted a soil column irrigation experiment to quantify the transport of a non-reactive tracer and the herbicide MCPA in partially frozen soil. In total 40 intact topsoil and subsoil columns from two agricultural fields with contrasting soil types (silt and loam) in South-East Norway were used in this experiment. MCPA and bromide were applied on top of all columns. Half the columns were then frozen at −3 °C while the other half of the columns were stored at +4 °C. Columns were then subjected to repeated irrigation events at a rate of 5 mm artificial rainwater for 5 h at each event. Each irrigation was followed by 14-day periods of freezing or refrigeration. Percolate was collected and analysed for MCPA and bromide. The results show that nearly 100% more MCPA leached from frozen than unfrozen topsoil columns of Hov silt and Kroer loam soils. Leaching patterns of bromide and MCPA were very similar in frozen columns with high concentrations and clear peaks early in the irrigation process, and with lower concentrations leaching at later stages. Hardly any MCPA leached from unfrozen topsoil columns (0.4–0.5% of applied amount) and concentrations were very low. Bromide showed a different flow pattern indicating a more uniform advective-dispersive transport process in the unfrozen columns with higher concentrations leaching but without clear concentration peaks. This study documents that pesticides can be preferentially transported through soil macropores at relatively high concentrations in partially frozen soil. These findings indicate, that monitoring programs should include sampling during snow melt or early spring in areas were soil frost is common as this period could imply exposure peaks in groundwater or surface water.

Restoration of wetlands: successes and failures on scalds comprising an iron oxide clogged layer with areas of acid sulfate soils

This paper reports on successes and failures experienced over nearly two decades while attempting to remediate degraded scalds typified by iron clogged layersinterspersed with patches of acid sulphate soil on the Eastern Dundas Tablelands in Victoria, Australia. In 2004 (Gardner et al. I Overview Plant Soil 267:51–59, 2004a, Plant Soil 267:85–95, b), it was suggested that redox processes similar to acid sulfate soil reactions were acting to exacerbate and amplify salinity effects in the landscape. Soil permeability to 1 m depth was measured and compared to that at 3-5 m depths. Additional analysis of soils in the discharge zone were undertaken (mineralogy, SEM, XRF, XRD) to seek evidence of typical acid sulfate reactions which had not been found in 2004. Water quality (EC, pH) and hydraulic pressures were measured in 2005- 2006, after shallow trenchs were dug across the site, and revegetation attempted with a range of native species. Following observations of improved growth at leaking peizometers, those with above ground water levels were drilled to allow water to escape. Assessment of revegetation outcomes were conducted in 2006 and 2016. Soil permeability was lower in the upper 1 m layer of the soil than at 3-5m depth, Clear evidence of acid sulfate soil mineralogy was found, however small scale variation was the norm. Clogging of soil macropores was observed, which could be manually cleared. Fracturing the soil to increase discharge either with explosives or excavation of both shallow and deep trenchs failed because the increased discharge was insufficient to overcome evaporation and salt concentration, and redox processes continued which would eventually clog the soil again. Success was achieved with leaking piezometers to 4.5 m depth, which allowed the groundwater to reach the surface without reducing ferric oxides. Rushes planted at the discharge point maintained an oxidised environment thereby halting the redox processes. Evaporation effects were prevented because the increased discharge occurred at a point. This combination allowed a range of native species to flourish and after 10 years, to spread beyond what we estimate the leaking piezometers would support. Evaporation causing concentration of salts is very important, but this is linked to the redox processes, which cause soil clogging and reduced permeability, and thus the evaporation and salt issue. Success was achieved when the reduced groundwater could access the surface in an oxidised environment, at a sufficient rate to prevent salt concentration by evaporation. The revegetation has expanded beyond what could be supported by the original discharge, suggesting that the plants are breaking the clogged soil layers and increasing discharge. The successful colonising species typically produce specialised structures such as dauciform and proteoid roots which are able to reduce and chelate iron oxides.

Soil wet aggregate distribution and pore size distribution under different tillage systems after 16 years in the Loess Plateau of China

In the Loess Plateau of China, conventional tillage is defined as the tillage without crop residues left on the soil surface and ploughed twice a year. The use of alternative practices is a way to reduce soil erosion. Our objectives were to assess the long-term impacts of different soil tillage systems on soil physical and hydraulic characteristics, emphasizing management practices to improve the soil physical qualities (reduce bulk density and increase stability of aggregate) under the conservation tillage system in the Loess Plateau of China. Conventional tillage (CT), no tillage (NT), and sub-soiling (SS) were applied in this experiment. Soil wet aggregates distribution and stability, soil organic carbon (SOC) content, soil water retention curves and pore size distributions were measured. The results showed that in the 0–10 cm and 10–20 cm depth soil layers, NT and SS treatments showed a significantly higher proportion of wet aggregates >250 μm (macroaggregates) compared to CT. In these two layers, the proportion of wet aggregates <53 μm (microaggregates) was significantly higher in CT with respect to NT and SS. SOC content increased as the aggregate fraction size increased, and was higher within wet aggregates >250 μm than within the 250–53 μm and < 53 μm (silt + clay) fractions at both depths. In addition, the conservation tillage (NT and SS) can result in improved total porosity and reduced soil bulk density compared with CT in the surface layer. Pore size distribution in CT soil was unimodal, with the maximum in the 10–30 μm matrix pores of the surface layer. However, in the surface layer the pore size distributions from NT and SS showed a dual porosity curve, with two peaks in the matrix and structural pore areas. The 10–20 cm layer showed similar pore size distributions in each treatment. After scanning the soils by micro-computed tomography, we visualized the pore characteristics. The images showed that CT reduced the long and connected macropores compared with conservation tillage. Overall, soil aggregate stability and soil macropores are most improved under conservation tillage. Conservation tillage with crop residues should be adopted instead of conventional tillage, as an effort to improve crop yield and control soil erosion in the Loess Plateau of China.

Tracking the Transport of Silver Nanoparticles in Soil: a Saturated Column Experiment

Silver nanoparticles (AgNPs) can enter the environment when released from products containing them. As AgNPs enter soil, they are often retained in the soil profile and/or leached to the groundwater. This research assessed the transport of AgNPs in their “particle form” through the soil profile using a series of columns. Three soil types were put into soil columns: LSH (loam with high organic matter (OM)), LSL (loam with low OM), and Sand (no OM). The results showed that AgNP transport and retention in soil as well as particle size changes are affected by soil organic matter (OM) and the cation exchange capacity (CEC) of soil. OM affected the transport and retention of AgNPs. This was evident in the LSH columns where the OM concentration was the highest and the AgNP content the lowest in the soil layers and in the effluent water. The highest transported AgNP content was detected in the Sand columns where OM was the lowest. CEC had an impact on the particle size of the AgNPs that were retained in the soil layers. This was clear in columns packed with high CEC-containing soils (LSL and LSH) where AgNP particle size decreased more substantially than in the columns packed with sand. However, the decrease in AgNP sizes in the effluent water was less than the decrease in particle size of AgNPs transported through but retained in the soil. This means that the AgNPs that reached the effluent were transported directly from the first layer through the soil macropores. This work highlights the ability to track AgNPs at low concentrations (50 μg kg−1) and monitor the changes in particle size potential as the particles leach through soil all of which increases our knowledge about AgNP transport mechanisms in porous media.

Using the X‐ray computed tomography method to predict the saturated hydraulic conductivity of the upper root zone in the Loess Plateau in China

Core Ideas Computed tomography (CT)‐measured properties were in the order: soil under Q. liaotungensis &gt; P. tabuliformis &gt; C. korshinskii &gt; M. sativa. The parameters measured by CT could help estimate saturated hydraulic conductivity (Ksat) in the Loess Plateau. Macropore quantity has a higher R2 for predicting Ksat than the morphology of macropores. The recovery of natural vegetation significantly improves soil macroporosity through root decay and biological activity. Macropores are preferential pathways for the movement of water to deep soil. However, the quantification of soil macropore structure and its relationship with soil hydraulic conductivity (Ksat) are not well understood. We characterized the macropores under different vegetation types to evaluate the effects of macropores on Ksat. Undisturbed soil cores were collected from four treatments: areas dominated by Quercus liaotungensis Koidz.(QLI), Pinus tabuliformis Carrière (PTA), Caragana korshinskii Kom. (CKO), and Medicago sativa L.(MSA). A medical computed tomography (CT) scanner was used to acquire images with a voxel size of 0.977 by 0.977 by 1.000 mm for depths of 0 to 360 mm. We used ImageJ software to quantify the macropore properties. Soil structure and Ksat improved with the succession of vegetation. The mean macropore volume fraction across the soil cores for the QLI, PTA, CKO, and MSA treatments were 6.6, 3.5, 1.3, and 0.6% within a 4900‐mm2 area, respectively. Macropore quantity (volume fraction and number) had a higher R2 for predicting Ksat than macropore morphology (branch density, connectivity density, and junction density). Moreover, the grayscale values were negatively correlated with Ksat and accounted for 78.8% of the variation in Ksat. Grayscale values, volume fraction, and the number of macropores were the best combination of CT‐measured parameters for predicting Ksat, accounting for 81.9% of the variation in Ksat. The CT‐measured parameters could be used to estimate Ksat in the upper root zone in the Loess Plateau.