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  • Soils
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  • 1
    Language: English
    In: PLoS ONE, 01 January 2016, Vol.11(7), p.e0159948
    Description: Matter turnover in soil is tightly linked to soil structure which governs the heterogeneous distribution of habitats, reaction sites and pathways in soil. Thereby, the temporal dynamics of soil structure alteration is deemed to be important for essential ecosystem functions of soil but very little is known about it. A major reason for this knowledge gap is the lack of methods to study soil structure turnover directly at microscopic scales. Here we devise a conceptual approach and an image processing workflow to study soil structure turnover by labeling some initial state of soil structure with small garnet particles and tracking their fate with X-ray microtomography. The particles adhere to aggregate boundaries at the beginning of the experiment but gradually change their position relative to the nearest pore as structure formation progresses and pores are destructed or newly formed. A new metric based on the contact distances between particles and pores is proposed that allows for a direct quantification of soil structure turnover rates. The methodology is tested for a case study about soil compaction of a silty loam soil during stepwise increase of bulk density (ρ = {1.1, 1.3, 1.5} g/cm3). We demonstrate that the analysis of mean contact distances provides genuinely new insights about changing diffusion pathways that cannot be inferred neither from conventional pore space attributes (porosity, mean pore size, pore connectivity) nor from deformation analysis with digital image correlation. This structure labeling approach to quantify soil structure turnover provides a direct analogy to stable isotope labeling for the analysis of matter turnover and can be readily combined with each other.
    Keywords: Sciences (General)
    E-ISSN: 1932-6203
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  • 2
    Language: English
    In: Vadose Zone Journal, 2015, Vol.14(5), p.0
    Description: The guest editors introduce the seven contributions to the special issue on processes in capillary fringes, with a focus on the complex interaction of biological, chemical, and physical processes in this environemnt. Processes in capillary fringes (CFs) have a complex nature due to the interactions between the solid, liquid, and gaseous environments. Despite a considerable body of literature on CFs coming from different disciplines, the ongoing processes and their complex interactions are yet only partially understood.
    Keywords: Soils ; Solids ; Vadose Water ; Methods and Instruments ; General;
    ISSN: Vadose Zone Journal
    E-ISSN: 1539-1663
    Source: CrossRef
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  • 3
    Language: English
    In: Vadose Zone Journal, 2011, Vol.10(2), p.654
    Description: The unsaturated hydraulic conductivity function is the dominant material property for modeling soil water dynamics. Because it is difficult to measure directly, it is often derived from the water retention characteristic combined with a geometric model of the pore space. In this study, we developed an automated, simple multistep flux (MSF) experiment to directly measure unsaturated conductivities, K(psi (sub m) ), at a number of water potentials, psi (sub m) , using the experimental setup of classical multistep outflow (MSO) experiments. In contrast to the MSO experiment, the MSF experiment measures the conductivity directly at a spatially constant water potential assuming macroscopically homogeneous materials. Additionally, the proposed method reveals the hysteresis of K(psi (sub m) ) with respect to increasing and decreasing water potentials as well as the temporal dynamics of K(psi (sub m) ) during transient-flow conditions. This temporal behavior is explained by the dynamics of fluid configurations at the pore scale during drainage and imbibition leading to hydraulic nonequilibrium. It may provoke a systematic underestimation of hydraulic conductivity using inverse optimization of K(psi (sub m) ) based on classical MSO experiments. The new approach will improve the determination of K(psi (sub m) ) and it provides an experimental tool to quantify the effects of hydraulic nonequilibrium under transient conditions.
    Keywords: Hydrogeology ; Experimental Studies ; Geometry ; Ground Water ; Hydraulic Conductivity ; Hysteresis ; Inverse Problem ; Mathematical Methods ; Measurement ; Models ; Movement ; Optimization ; Phase Equilibria ; Soils ; Unsaturated Zone;
    ISSN: Vadose Zone Journal
    E-ISSN: 1539-1663
    Source: CrossRef
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  • 4
    Language: English
    In: Vadose Zone Journal, 2011, Vol.10(3), p.1082
    Description: Predicting solute transport through structured soil based on observable structural properties of the material has not been accomplished to date. We evaluated a new approach to predicting breakthrough curves (BTCs) of dissolved chemicals in intact structured soil columns based on attributes of the pore structure at hierarchical spatial scales. The methodology centers on x-ray computed microtomography of a hierarchic suite of undisturbed soil samples (diameters 1, 4.6, 7.5, and 16 cm) to identify the network of pores 〉10 mu m in diameter. The pore structure was quantified in terms of pore size distribution, interface area density, and connectivity. The pore size distribution and pore connectivity were used to set up an equivalent pore network model (PNM) for predicting the BTCs of Br (super -) and Brilliant Blue FCF (BB) at unsaturated, steady-state flux. For a structured silt loam soil column, the predictions of Br (super -) tracer breakthrough were within the variation observed in the column experiments. A similarly good prediction was obtained for Br (super -) breakthrough in a sandy soil column. The BB breakthrough observed in the silt loam was dominated by a large variation in sorption (retardation factors between R = 2.9 and 24.2). The BB sorption distribution coefficient, k (sub d) , was measured in batch tests. Using the average k (sub d) in the PNM resulted in an overestimated retardation (R = 28). By contrast, breakthrough of BB in the sandy soil (experimental R = 3.3) could be roughly predicted using the batch test k (sub d) (PNM simulation R = 5.3). The prediction improved when applying a sorption correction function accounting for the deviation between measured interface area density distribution and its realization in the network model (R = 4.1). Overall, the results support the hypothesis that solute transport can be estimated based on a limited number of characteristics describing pore structure: the pore size distribution, pore topology, and pore-solid interfacial density.
    Keywords: Soils ; Bad Lauchstadt Germany ; Boundary Conditions ; Breakthrough Curves ; Bromine ; Central Europe ; Central Germany ; Chemical Dispersion ; Chernozems ; Computed Tomography ; Convection ; Density ; Dye Tracers ; Equations ; Europe ; Experimental Studies ; Fuhrberg Germany ; Germany ; Halogens ; Image Analysis ; Laboratory Studies ; Lower Saxony Germany ; Microtomography ; Minckowski Functions ; Morphology ; Networks ; Podzols ; Porosity ; Quantitative Analysis ; Saxony-Anhalt Germany ; Simulation ; Soils ; Solute Transport ; Spectra ; Tomography ; Topology ; Transport ; X-Ray Spectra;
    ISSN: Vadose Zone Journal
    E-ISSN: 1539-1663
    Source: CrossRef
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  • 5
    Language: English
    In: Vadose Zone Journal, 2013, Vol.12(4), p.0
    Description: The hydraulic behavior of soil is determined by its hydraulic properties and their variability in space. In agricultural soils, this heterogeneity may stem from tillage or may have natural origin. The root distribution of plants will adapt to some extent to this soil heterogeneity. However, the combined impact of soil heterogeneity and root water uptake (RWU) on long-term soil water budgets has not received much attention. Numerical experiments helped identify how soil heterogeneity affects plant transpiration, soil evaporation, and groundwater recharge. Two-dimensional virtual soils with hierarchical heterogeneity, both natural and tillage induced, served as a basis for modeling soil water dynamics for a 10-yr climate record from two weather stations in Germany that vastly differ in annual precipitation. The complex interactions between soil and vegetation were explored by (i) comparing different RWU strategies (depth-, structure-, and time-dependent root profiles), (ii) land use types (perennial grass and annual winter crops), (iii) a combination of textures (silt above sand and sand above loam), and (iv) RWU with or without a compensation mechanism. The simulations were repeated with one-dimensional, effective representations of these virtual soils. In the framework of hydropedology, this study shed some light on the interaction between plants and pedological features and its impact on the macroscopic soil water budget. We demonstrated that land use has a major impact on the annual water balance through the partitioning of evapotranspiration into bare soil evaporation and plant transpiration. Compensational RWU becomes important for the annual water balance when the root zone comprises contrasting materials with respect to water holding capacity. Soil heterogeneity has in fact a minor impact on long-term soil water budgets. As a consequence, the relative contribution of plant transpiration, soil evaporation, and groundwater recharge to the total soil water loss was well reproduced by simulations in one-dimensional effective soil profiles. This advocates the application of one-dimensional soil-atmosphere-vegetation transfer (SVAT) models at larger scales. These findings only hold for assumptions made in our numerical simulations including flat area without lateral flow and no macropore flow.
    Keywords: Environmental Geology ; Soils ; Atmosphere ; Boundary Conditions ; Central Europe ; Eastern Germany ; Europe ; Field Studies ; Germany ; Grain Size ; Heterogeneity ; Hydrodynamics ; Hydrology ; Hydropedology ; Julicher Borde Germany ; Land Use ; Magdeburg Germany ; Mapping ; North Rhine-Westphalia Germany ; Numerical Models ; One-Dimensional Models ; Rhizosphere ; Saxony-Anhalt Germany ; Scale Factor ; Size Distribution ; Soil-Atmosphere-Vegetation Transfer ; Soils ; Topography ; Two-Dimensional Models ; Unsaturated Zone ; Vegetation ; Water Balance ; Western Germany;
    ISSN: Vadose Zone Journal
    E-ISSN: 1539-1663
    Source: CrossRef
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  • 6
    Language: English
    In: Computers and Geosciences, 2010, Vol.36(10), pp.1246-1251
    Description: For many analyses, grey scale images from X-ray tomography and other sources need to be segmented into objects and background which often is a difficult task and afflicted by an arbitrary and subjective choice of threshold values. This is especially true if the volume fraction of objects is small and the histogram becomes unimodal. Bi-level segmentation based on region growing is a promising approach to cope with the fuzzy transition zone between object and background due to the partial volume effect, but until now there is no method to properly determine the required thresholds in case of unimodality. We propose an automatic and robust technique for threshold selection based on edge detection. The method uses gradient masks which are defined as regions of interest for the determination of threshold values. Its robustness is analysed by a systematic performance test and finally demonstrated for the segmentation of pores in different soils using images from X-ray tomography.
    Keywords: Segmentation ; Thresholding ; Edge Detection ; Region Growing ; Tomography ; Geology
    ISSN: 0098-3004
    E-ISSN: 1873-7803
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  • 7
    Language: English
    In: Vadose Zone Journal, 2013, Vol.12(3), p.0
    Description: Soils are structured on multiple spatial scales, originating from inhomogeneities of the parent material, pedogenesis, soil organisms, plant roots, or tillage. This leads to heterogeneities that cause variability of local measurements of hydraulic state variables and affects the flow behavior of water in soil. Whereas in real-world systems, the true underlying structures can never be absolutely known, it is appealing to employ synthetic or "virtual" experiments for assessing general properties of flow in porous media and grasping the main physical mechanisms. With this aim, three two-dimensional virtual realities with increasing structural complexity, representing cultivated soils with hierarchical spatial heterogeneity on multiple scales were constructed by the interdisciplinary research group Virtual Institute of the Helmholtz Association (INVEST). At these systems, numerical simulations of water dynamics including a heavy rain, a redistribution, and a long-lasting evaporation period were performed. The technical aspects of the construction of the virtual soils and results of the forward simulations have been presented in a paper by Schluter et al. (2012). In this follow-up paper, we use inverse modeling to investigate measurements in virtual vertical soil profiles, mimicking typical field monitoring campaigns with moisture content and matric potential sensors placed at five depths. Contrary to the real situation, we can interpret observed data, their variability, estimated hydraulic properties, and predicted water balance in the light of the known truth. Our results showed that measurements, particularly those of water contents, varied strongly with measuring position. Using data from single profiles in systems similar to our virtual soils thus will lead to very different estimates of the soil hydraulic properties. As a consequence, the correct calculation of the water balance is rather a lucky coincidence than the rule. However, the average of the predicted water balances obtained from the one-dimensional simulations, and the estimated soil hydraulic properties agreed very well with those attained from the two-dimensional systems.
    Keywords: Soils ; Hydrogeology ; Boundary Interactions ; Evaporation ; Grain Size ; Heterogeneous Materials ; Hydrodynamics ; Infiltration ; Interpretation ; Inverse Problem ; Irrigation ; Matric Head ; Measurement ; Moisture ; One-Dimensional Models ; Quantitative Analysis ; Simulation ; Size Distribution ; Soils ; Spatial Distribution ; Tdr Data ; Two-Dimensional Models ; Unsaturated Zone ; Van Genuchten-Mualem Parameters ; Water ; Water Balance;
    ISSN: Vadose Zone Journal
    E-ISSN: 1539-1663
    Source: CrossRef
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  • 8
    In: Water Resources Research, June 2017, Vol.53(6), pp.4709-4724
    Description: The relaxation dynamics toward a hydrostatic equilibrium after a change in phase saturation in porous media is governed by fluid reconfiguration at the pore scale. Little is known whether a hydrostatic equilibrium in which all interfaces come to rest is ever reached and which microscopic processes govern the time scales of relaxation. Here we apply fast synchrotron‐based X‐ray tomography (X‐ray CT) to measure the slow relaxation dynamics of fluid interfaces in a glass bead pack after fast drainage of the sample. The relaxation of interfaces triggers internal redistribution of fluids, reduces the surface energy stored in the fluid interfaces, and relaxes the contact angle toward the equilibrium value while the fluid topology remains unchanged. The equilibration of capillary pressures occurs in two stages: (i) a quick relaxation within seconds in which most of the pressure drop that built up during drainage is dissipated, a process that is to fast to be captured with fast X‐ray CT, and (ii) a slow relaxation with characteristic time scales of 1–4 h which manifests itself as a spontaneous imbibition process that is well described by the Washburn equation for capillary rise in porous media. The slow relaxation implies that a hydrostatic equilibrium is hardly ever attained in practice when conducting two‐phase experiments in which a flux boundary condition is changed from flow to no‐flow. Implications for experiments with pressure boundary conditions are discussed. What happens to fluids in a porous medium after pumping is stopped? Fast X‐ray tomography shows that even in a sample smaller than a sugar cube fluid interfaces continue to move for hours until an optimal fluid configuration is reached. The pace is limited by slow relaxation of dynamic contact angles. Therefore hydrostatic equilibrium, which is the state at which all fluid interfaces come to rest, is hardly ever attained in practice when conducting two‐phase flow experiments where the flow is stopped in much larger soil or rock samples. Relaxation dynamics through internal redistribution of fluids after fast drainage occurs in two stages A quick dissipation within seconds is followed by slow relaxation within several hours due to relaxation of dynamic contact angles Fluid configurations during relaxation are very different from those during quasi‐static drainage and imbibition
    Keywords: Two‐Phase Flow ; Dynamic Effects ; Hydraulic Nonequilibrium ; Dynamic Contact Angle ; Fluid Configuration ; Fluid Topology
    ISSN: 0043-1397
    E-ISSN: 1944-7973
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  • 9
    In: Ecohydrology, September 2018, Vol.11(6), pp.n/a-n/a
    Description: By applying the newly developed flow cell (FC) concept, this study investigated the impact of small‐scale spatial variations (millimetre to centimetre) in organic matter (OM) composition (diffusive reflectance infrared Fourier transform spectroscopy), biological activity (zymography), and wettability (contact angle [CA]) on transport processes (tracer experiments, radiography). Experiments were conducted in five undisturbed soil slices (millimetre apart), consisting of a sandy matrix with an embedded loamy band. In the loamy band increased enzyme activities and OM (10 mm apart) were found compared with the sand matrix, with no interrelations although spatial autocorrelation ranges were up to 7 cm. CAs were increased (0–110°) above the loamy band and were negatively correlated with acid phosphatase. Missing correlations were probably attributed to texture variations between soil slices. A general correlation between CA and C content (bulk) were confirmed. Variability in texture and hydraulic properties led to the formation of heterogeneous flow patterns and probably to heterogeneously distributed interfacial properties. The new FC concept allows process evaluation on the millimetre scale to analyse spatial relations, that is, between small‐scale textural changes on transport processes and biological responses. The concept has been proved as a versatile tool to analyse spatial distribution of biological and interfacial soil properties in conjunction with the analysis of complex micro‐hydraulic processes for undisturbed soil samples. The concept may be improved by additional nondestructive imaging methods, which is especially challenging for the detection of small‐scale textural changes.
    Keywords: Drift Spectroscopy ; Extracellular Enzyme Activity ; Flow Cell ; Soil Water Repellency ; Transport Processes ; Undisturbed Soil ; X‐Ray Radiography
    ISSN: 1936-0584
    E-ISSN: 1936-0592
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  • 10
    In: Land Degradation & Development, September 2018, Vol.29(9), pp.3112-3126
    Description: Bioeconomy strategies have been adopted in many countries around the world. Their sustainable implementation requires a management of soils that maintains soil functions and avoids land degradation. Only then, ecosystem services can be maintained and resources used efficiently. We present an analytical framework for impact assessment that links policy and technology driving forces for soil management decisions to soil processes, soil functional changes, and their impacts on ecosystem services and resource use efficiency, both being targets that have been set by society and are anchored in bioeconomy policy strategies and sustainable development goals. Although the resource use efficiency concept has a long‐term tradition, most studies of agricultural management do not address the role of soils in their efficiency assessment. The concept of ecosystem services has received increasing attention over the last years; however, its link to soil functions and soil management practices is still not well established. This study is the first to conceptually link the socioeconomic processes of external drivers for soil management with the natural processes of soil functions and connect them back to impacts on the social system. Application of the framework helps strengthen the science‐policy interface and to systemically assess and compare the opportunities and threats of soil management practices from the perspective of goals set by society at different spatial and temporal scales. Insights gained in this way can be applied in stakeholder decision‐making processes and used to inform the design of governance instruments aimed at sustainable soil management within a bioeconomy.
    Keywords: Bioeconomy ; Ecosystem Services ; Impact Assessment ; Resource Use Efficiency ; Soil Management Practices ; Sustainable Development Goals
    ISSN: 1085-3278
    E-ISSN: 1099-145X
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