A. Pohlmeier

Forschungszentrum Jülich, Jülich, North Rhine-Westphalia, Germany

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Publications (64)64.69 Total impact

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    ABSTRACT: Near surface soil moisture profiles contain important information about the evaporation process from a bare soil. In this study, we demonstrated that such profiles could be monitored non-invasively and with high spatial resolution using Nuclear Magnetic Resonance (NMR). Soil moisture profiles were measured in a column exposed to evaporation for a period of 67 days using a stationary Magnetic Resonance Imaging (MRI) high field scanner and a unilateral NMR sensor. The column was packed with medium sand and initially saturated. Two distinct shapes of soil moisture profiles that are characteristic for stage I (evaporation rate is controlled by atmospheric demand) and stage II (evaporation rate is controlled by the porous medium) of the evaporation process were followed by both, MRI and unilateral NMR. During stage I, an approximately uniform decrease of soil moisture over time was monitored, whereas during stage II, s-shaped moisture profiles developed which receded progressively into the soil column. These promising results and the specific design of the unilateral NMR system make it very well suited for determining soil moisture profiles in the field.
    Water Resources Research. 06/2014;
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    ABSTRACT: Water exchange between bare soil and atmosphere is controlled by evaporation. In the topmost soil layer moisture content and hydraulic conductivity may change strongly and capillary film flow (stage I) from saturated regions to the surface discontinues. Evaporation is now mainly driven by vapor diffusion through a dry layer (stage II). Water vaporizes in the unsaturated zone inside the soil what strongly reduces the evaporation rate and also soil surface temperature to a considerable amount. The dynamics of the transition from stage I to stage II as well as film flow and vapor diffusion at low water contents have received little attention. In this study we investigated water content changes in the uppermost soil layer with high spatial resolution using nuclear magnetic resonance (NMR). NMR is a feasible noninvasive method where the received signal of hydrogen protons allows conclusions on moisture and pore size distribution. The overall aim is to apply a mobile nuclear magnetic resonance surface sensor (NMR-MOUSE) directly for field measurements. This sensor has a max. measurement depth of 25 mm and operates at a Larmor frequency of 13.4 MHz. The general challenges of NMR in soils are the inherent fast transversal relaxation times of the soil matrix especially next to the residual moisture content. Therefore, as a first step of validation we applied and compared NMR-MOUSE measurements with magnetic resonance imaging (MRI) using an initially saturated sand column. The column was evaporated over 67 days and water content profiles were recorded by 1D-T2 relaxation measurements using the NMR-MOUSE as well as different 3D-MRI sequences during drying. Firstly, we report on the sensitivities and limits of the different devices and measurement sequences. Considering these data, we could monitor that over a period of 58 days the moisture decreased rather uniform until the onset of stage II. Thereafter, a dry surface layer developed and a retreating drying front was observed.
    04/2013;
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    ABSTRACT: Nuclear Magnetic Resonance (NMR) relaxation and NMR diffusion measurements are two of a series of fast and non-invasive NMR applications widely used e.g. as well logging tools in petroleum exploration [1]. For experiments with water, NMR relaxation measures the relaxation behaviour of former excited water molecules, and NMR diffusion evaluates the self-diffusion of water. Applied in porous media, both relaxation and diffusion measurements depend on intrinsic properties of the media like pore size distribution, connectivity and tortuosity of the pores, and water saturation [2, 3]. Thus, NMR can be used to characterise the pore space of porous media not only in consolidated sediments but also in soil. The physical principle behind is the relaxation of water molecules in an external magnetic field after excitation. In porous media water molecules in a surface layer of the pores relax faster than the molecules in bulk water because of interactions with the pore wall. Thus, the relaxation in smaller pores is generally faster than in bigger pores resulting in a relaxation time distribution for porous media with a range of pore sizes like soil [4]. In NMR diffusion experiments, there is an additional encoding of water molecules by application of a magnetic field gradient. Subsequent storage of the magnetization and decoding enables the determination of the mean square displacement and therefore of the self-diffusion of the water molecules [5]. Employing various relaxation and diffusion experiments, we get a measure of the surface to volume ratio of the pores and the tortuosity of the media. In this work, we show the characterisation of a set of sand and soil samples covering a wide range of textural classes by NMR methods. Relaxation times were monitored by the Carr-Purcell-Meiboom-Gill sequence and analysed using inverse Laplace transformation. Apparent self-diffusion constants were detected by a 13-intervall pulse sequence and variation of the storage time. We correlated the results with various soil properties like texture, water retention parameters, and hydraulic conductivity. This way we show that we can predict soil properties by NMR measurements and that we are able use results of NMR measurements as a proxy without the need of direct measurements. [1] Song, Y.-Q., Vadose Zone Journal, 9 (2010) [2] Stingaciu, L. R., et al., Water Resources Research, 46 (2010) [3] Vogt, C., et al., Journal of Applied Geophysics, 50 (2002) [4] Barrie, P. J., Annual Reports on NMR Spectroscopy, 41 (2000) [5] Stallmach, F., Galvosas, P., Annual Reports on NMR Spectroscopy, 61 (2007)
    04/2013;
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    ABSTRACT: The interface between roots and soil plays a key role in water transport in the Soil-Plant-Atmosphere-Continuum (SPAC). The transport which changes with the degree of dehydration is influenced by both the hydraulic conductivity of roots and the soil. One important factor in plant growth is the amount of available water in the soil, which correlates directly with soil texture. Water uptake of plant roots and water uptake patterns in soil can be monitored using non-invasive 1H Nuclear Magnetic Resonance Imaging (MRI). In a preceding study the effect of root water uptake and uniform desiccation patterns under drought conditions were observed for Ricinus communis grown in a model medium (Pohlmeier et al. 2008). Continuing these studies, the new aspect is the determination of water uptake patterns and root system architecture in a natural soil. The general challenge of MRI in soils are the inherent fast relaxation times T2* and T2 of the soil matrix. With the use of conventional sequences only water in macropores can be determined. The loss of sensitivity can be overcome by MRI sequences with sufficiently short detection times. In this work we employed and assessed two methods: SPI (Single Point Imaging) detects the T2* relaxation with a dead time of < 0.05 ms and SE3D (Spin Echo 3D) probes T2,eff with an echo time of about 0.8 ms. Zea mays, planted in a cylindrical container filled with a natural soil was completely sealed after 4 weeks of growth to avoid evaporation, so water loss took place via transpiration only. The water content of the soil was determined gravimetrically and by means of MRI each 2nd day over a period of 14 days. Furthermore a SEMS (Spin Echo Multi Slice) sequence was used to visualize the growth of root system architecture. This study shows that SPI3D and SE3D are feasible for the determination of water content in a natural soil up to a certain detection limit. We observed quite uniform water uptake patterns during drying of the soil until water content was less than 0.15 cm^3/cm^3, which is the detection limit of both sequences for the used soil material. Accordingly, this indicates an always sufficiently high hydraulic conductivity of the soil to sustain water supply for the plant. The growth of the root system architecture could reliably been visualized with SEMS sequence where the best differentiation between soil and roots is obtained by the choice of long echo time and small voxel size. During the whole drought period we observed an increase in root growth what is an effect of the high water supply. Pohlmeier, A., Oros-Peusquens, A., et al. (2008) " Changes in Soil Water Content Resulting from Ricinus Root Uptake Monitored by Magnetic Resonance Imaging" Vadose Zone J. 7: 1010-1017.
    04/2012;
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    ABSTRACT: Water fluxes in soils control many processes in the environment like plant nutrition, solute and pollutant transport. In the last two decades non-invasive visualization methods have been adapted to monitor flux processes on the small scale. Magnetic resonance imaging (MRI), also well known from medical diagnostics, is one of the most versatile ones. It mostly probes directly the substance of interest: water, and it offers many opportunities to manipulate the observed signals for creating different contrasts and thus probing different properties of the porous medium and the embedded fluids. For example, one can make the signal sensitive to the total proton density, i. e. water content, to spatial distributions of relaxation times which reflect pore sizes, to spatial distributions of transport coefficients, and to concentration of contrast agents by using strongly T1 weighted MRI pulse sequences. In this presentation we use GdDTPA2- for monitoring flux processes in soil columns in an ultra-wide bore MRI scanner. It offers the opportunity for monitoring slow water fluxes mainly occurring in soil systems which are not monitorable with direct MRI flow imaging. This contrast agent is most convenient since it behaves conservatively, i.e. it does not sorb at different soil materials and it is chemically stable. Firstly, we show that its mode of action in natural porous media is identical to that known from medical applications as proved by the identical relaxivity parameters [1]. Secondly, the tracer is applied for the visualization of flux processes during evaporation-driven flow. Theoretical considerations by forward simulation predicted a lateral redistribution of solutes during evaporative upward fluxes from highly conductive fine material to neighbouring domains with low water content and conductivity. Here we could prove that such near-surface redistribution really takes place [2]. Thirdly, this tracer is applied for the investigation of water uptake by root systems. Depending on the transpiration conditions slow uptake in the dark is present, where the tracer moves directly into the xylem. When fully illuminated, the tracer uptake is limited by the Caspari band, and it is enriched strongly in the roots cortex. The results so far show that this tracer offers a new window for monitoring slow water fluxes in bare and grown soil columns. [1] Haber-Pohlmeier S, Bechtold M, Stapf, S, Pohlmeier, A. (2010) Vadose Zone Journal 9, 835-845 [2] Bechtold M, Haber-Pohlmeier S, Vanderborght J, Pohlmeier A, Ferré T, Vereecken H. (2011) Geophysical Research Letters 38, L17404
    04/2012;
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    ABSTRACT: An automated method for root system architecture reconstruction from 3D volume datasets obtained from magnetic resonance imaging is developed and validated with a 3D semi-manual reconstruction using virtual reality and a 2D reconstruction.
    Vadose Zone Journal 01/2012; · 2.20 Impact Factor
  • Organic Geochemistry 09/2011; 42(8):865–866. · 2.52 Impact Factor
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    ABSTRACT: Characterization and quantification of root water uptake processes play a key role in understanding and managing the effects of global climate change on agricultural production and ecosystem dynamics. Part of this understanding is related to the flow of water towards plant roots in soils. In this study we demonstrate for the first time, to our knowledge, that fluid flow in the voids of the pore space of a model soil system (natural sand) can be detected and mapped to an NMR image for mean flows as low as 0.06 mm/s even under the influence of internal magnetic field gradients. To accomplish this we combined multi-slice imaging with a 13-interval pulse sequence to the NMR pulse sequence 13-interval stimulated echo multi-slice imaging (13-interval STEMSI). The result is a largely reduced influence of the internal magnetic field gradients, leading to an improved signal-to-noise ratio which in turn enables one to acquire velocity maps where conventional stimulated echo methods fail.
    Journal of Magnetic Resonance 07/2011; 212(1):216-23. · 2.30 Impact Factor
  • Natascha Spindler, Andreas Pohlmeier, Petrik Galvosas
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    ABSTRACT: Understanding root water uptake in soils is of high importance for securing nutrition in the context of climate change and linked phenomena like stronger varying weather conditions (draught, strong rain). One step to understand how root water uptake occurs is the knowledge of the water flow in soil towards plant roots. Magnetic Resonance Imaging (MRI) in combination with q‐space imaging is potentially the most powerful analytical tool for non‐invasive three dimensional visualization of flow and transport in porous media. Numerous attempts have been made to measure local velocity in porous media by combining velocity phase encoding with fast imaging methods, where flow velocities in the vascular bundles of plant stems were investigated. In contrast to water situated in the cellular structure of plants, NMR signal arising from water in the pore space in soil may be much more affected by the presence of internal magnetic field gradients. In this work we account for the existence of these gradients by employing bipolar pulsed field magnetic gradients for velocity encoding. This enables one to study flow through sand (as a model system for soil) at flow rates relevant for the water uptake of plant roots.
    AIP Conference Proceedings. 03/2011; 1330(1):73-76.
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    ABSTRACT: Water and solute movement as any other transport processes through soil are influenced by the hydraulic properties of the soils. The heterogeneities of the soils imply heterogeneous spatial distribution of the hydraulic properties leading to heterogeneous distribution of soil water content. This may affects the water availability for plant growth, the groundwater contamination and nutrients losses within the root zone. The measurement techniques available today for the estimation of soil hydraulic parameters do not account for the heterogeneity of the sample and treat each measurement sample as a homogeneous representative volume. On the other side natural soils contain large heterogeneities mostly in terms of inclusions of different materials. Therefore the purpose of this study is to estimate soil hydraulic properties of a heterogeneous sample by combining classical multi-step-outflow (MSO) with magnetic resonance imaging (MRI) experiments. MSO experiments were performed on a sample filled with sand and sand-clay mixture in a coaxial structure. During each pressure application MRI images at 4.7 T (200 MHz) were recorded using a pure phase-encoding MRI sequence in order to provide information about the soil water content at specific locations within the coaxial sample. The recorded cumulative outflow and water content data were used as input data in the inversion of the MSO experiment. For the simulation and inversion of the MSO experiment we used the hydrological model HYDRUS-2D3D in which the initial hydraulic parameters of the two materials were estimated based on CPMG-T2 relaxation measurements on homogeneous sub-samples. The results show conclusively that the combination of the two MRI and MSO methods leads to a unique estimation of the hydraulic properties of two materials simultaneously.
    03/2011;
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    ABSTRACT: Under stage-1 evaporation solutes accumulate in zones of lowest hydraulic headConsidering cycles of infiltration and evaporation this leads to mass flux loopsThe solute redistribution has important consequences for the leaching of solutes
    Geophysical Research Letters 01/2011; 38, L17404. · 3.98 Impact Factor
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    ABSTRACT: A low-field NMR sensor has been built to measure partial saturation of soils in a non-invasive way. The sensor was deployed in a set-up where a one-step outflow experiment was carried out. Partial saturation before, during and after the experiment was acquired for two model soils. Hydraulic characteristics of the soils were obtained through inverse analysis. NMR signal was analyzed to gain information about the microscopical conditions of the liquid phase.
    Journal of Hydrology 01/2011; 406(1):30-38. · 2.96 Impact Factor
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    ABSTRACT: Magnetic resonance imaging (MRI) was used to study the process of infiltration and solute transport in an undisturbed soil sample of coarse sandy loam. The sample was subjected to the recurrent ponded infiltration (RPI) experiment, which was carried out in order to assess the changes in the entrapped air volume and its impact on steady state flow rates and solute breakthrough. The main stages of the first and second experimental RPI runs were monitored using an MRI sequence that follows both water density and magnetic relaxation. In a steady state stage of each experimental run a nickel nitrate pulse was injected in order to visualize the solute breakthrough. Effluent from the sample was collected for chemical analysis and a breakthrough curve of the nickel was constructed. To obtain information about the soil structure and to reveal potential preferential pathways, the soil sample was scanned using computed tomography. The local nickel ion transport breakthrough was evaluated from MR images in a series of local observation points distributed along the selected preferential pathways.The preferential flow instability phenomenon with the emphasis on air bubble formation was shown by detecting a 60% decrease of the steady state infiltration rate. The detailed analyses of MRI measurements at observation points revealed air bubble formation, producing a flow rate decrease accompanied by redirection of nickel ion transport trajectories. By analyzing M0 maps it was found that the volumetric water content decrease was 2.2%.
    Organic Geochemistry 01/2011; 42(8):991-998. · 2.52 Impact Factor
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    ABSTRACT: Water flux through soils is one of the most important control function with respect to the water supply for root and plant growth. The understanding of these processes bases generally on the interplay between experimental investigations and the development of theory and numerical models. In recent time detailed 3D models have been developed, but experimental information is mainly available from two-dimensional rhizotrons or only with coarse resolution from water content measurements by means of TDR probes and tensiometers. An emerging powerful tool for high resolution, non-invasive imaging of water content and fluxes in soils in saturated and unsaturated state is Nuclear Magnetic Resonance Imaging (MRI)[1,2]. With respect to water fluxes the slow flow velocities do not allow the direct monitoring by MRI flow imaging so indirect methods like transport of contrast agents should be applied. Therefore we have chosen Gd-DTPA [3], a negatively charged paramagnetic Gd-complex, as tracer for the investigation of water fluxes during i) infiltration and ii) injection experiments of unsaturated model soil with maize and lupin plants. The contrast of the NMR measurements was optimised using high resolution T1 weighted spin echo sequences. With respect to the infiltration experiment we observed that during rapid infiltration from the bottom the plume moved homogeneously into the bulk soil, but leaves out the immediate surrounding of the maize roots. After this initial period a continuously increasing enrichment of tracer in this region is monitored, but no uptake by the plant within one hour. Continuing these studies injection experiments have been performed where the tracer was placed in a small volume in the direct vicinity of the roots, and the entire water content changed only minimal even under quite dry conditions. For a well developed lupin root system we observed diffusive spreading followed by a very slow transporte of the plume to the root system over a period of two days. The important difference to the short term experiment is the observation of tracer uptake followed by an upward transport in the inner root tissues. This could also be proved by a following chemical analysis showing decreasing Gd content from the roots over the shoot to the leaves. In parallel a high resolution 3d image of the root system architecture was performed, in order to compare the experimentally observed motion of the plume with detailed D model calculations of water uptake and tracer transport. References: 1. Pohlmeier, A., et al., Imaging water fluxes in porous media by magnetic resonance imaging using D2O as a tracer. Mag. Res. Imag., 2008. 27(2): p. 285-292. 2. Pohlmeier, A., et al., Changes in Soil Water Content Resulting from Ricinus Root Uptake Monitored by Magnetic Resonance Imaging Vadose Zone Journal, 2008. 7: p. 1010-1017. 3. Haber-Pohlmeier, S., Stapf S. and Pohlmeier A., Waterflow monitored by tracer transport in natural porouse media using MRI. Vadose Zone Journal, submitted
    05/2010;
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    ABSTRACT: Combination of experimental studies with detailed deterministic models help understand root water uptake processes. Recently, Javaux et al. developed the RSWMS model by integration of Doussańs root model into the well established SWMS code[1], which simulates water and solute transport in unsaturated soil [2, 3]. In order to confront RSWMS modeling results to experimental data, we used Magnetic Resonance Imaging (MRI) technique to monitor root water uptake in situ. Non-invasive 3-D imaging of root system architecture, water content distributions and tracer transport by MR were performed and compared with numerical model calculations. Two MRI experiments were performed and modeled: i) water uptake during drought stress and ii) transport of a locally injected tracer (Gd-DTPA) to the soil-root system driven by root water uptake. Firstly, the high resolution MRI image (0.23x0.23x0.5mm) of the root system was transferred into a continuous root system skeleton by a combination of thresholding, region-growing filtering and final manual 3D redrawing of the root strands. Secondly, the two experimental scenarios were simulated by RSWMS with a resolution of about 3mm. For scenario i) the numerical simulations could reproduce the general trend that is the strong water depletion from the top layer of the soil. However, the creation of depletion zones in the vicinity of the roots could not be simulated, due to a poor initial evaluation of the soil hydraulic properties, which equilibrates instantaneously larger differences in water content. The determination of unsaturated conductivities at low water content was needed to improve the model calculations. For scenario ii) simulations confirmed the solute transport towards the roots by advection. 1. Simunek, J., T. Vogel, and M.T. van Genuchten, The SWMS_2D Code for Simulating Water Flow and Solute Transport in Two-Dimensional Variably Saturated Media. Version 1.21. 1994, U.S. Salinity Laboratory, USDA, ARS: Riverside, California. 2. Javaux, M., et al., Use of a Three-Dimensional Detailed Modeling Approach for Predicting Root Water Uptake. Vadose Zone J., 2008. 7(3): p. 1079-1088. 3. Schröder, T., et al., Effect of Local Soil Hydraulic Conductivity Drop Using a Three Dimensional Root Water Uptake Model. Vadose Zone J., 2008. 7(3): p. 1089-1098.
    05/2010;
  • Sabina Haber-Pohlmeier, Andreas Pohlmeier
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    ABSTRACT: Magnetic resonance imaging is most useful for 3D non-invasive visualization of subsurface structures and processes. Especially imaging of the rhizosphere is of great interest, since here soil properties can can be altered by the activity of the root. This leads to phenomena like locally enhanced water content, as pointed out by Carminati et al. 2009. On the other hand in the past depletion zone have been observed by MRI. The use of contrast agents is helpful for the further investigation of processes in the rhizosphere. Here we report about recent measurements on the motion of the conservative contrast agent GdDTPA in the vicinity of the root. Setup was a) a conventional cylindrical container, and b) a planar rhizotron in which the vicinity of selected roots is directly accessible by drilled holes in the wall. MRI Resolution was about 0.6 x 0.6 mm. For the optimal contrast a spin echo multisclice sequence was used where the contrast agent was highlighted by the choice of short repetition times combined with a short echo-time leading to strong T1 weighting. Additionally most recent results obtained in small growing vessels with a resolution of about 0.05mm will be presented. Starting point is always a water depleted root soil system ( < 0.12) into which a small amount of tracer is injected in the vicinity of a root. Frequently a zone of reduced intensity around the root is observed, which is passed by the contrast agent after sufficiently long time. This is proved by the appearence of the contrast agent inside the root, where it is transported slowly toward the shoot. We can conclude that the rhizosphere appears as zone of less intensity around the root, which was passed by the tracer to be monitored again in the inner part of the root system.
    05/2010;
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    ABSTRACT: Magnetic resonance (MR) imaging and MR relaxometry measurement techniques were used to study the process of infiltration in two undisturbed soil samples of coarse sandy loam and loamy sand taken into the plexiglas cylinders (dia. 6.0 cm × h. 12 cm). For coarse sandy loam sample the repeated ponded infiltration (RPI) experiment was carried out, the first infiltration was conducted into a relatively dry sample and the repeated infiltration into the gravitationally drained sample. The RPI method for this sample was performed in order to assess the changes in entrapped air distribution and its impact on steady state flow rates. A single infiltration run was carried out for loamy sand. An automatic setup continuously monitoring fluxes and pressure head in one tensiometer was constructed for these experiments. The main stages of each experiment run - wetting, steady state flow, drainaige - were monitored by multi-echo multi-slice (MEMS) MR sequence. Multiple vertical slices at a spatial resolution of 0.53×2×5 mm covered the whole soil core to obtain 3D image. During steady state flow, axial slices at spatial resolution of 1×1×5 mm of T1 maps were acquired. Later the nickel nitrate pulse was injected with the aim to visualise the solute breakthrough. Effluent from the sample was collected into a fraction collector and breakthrough curve of the nitrate was developed. Soil samples were scanned with computed tomography (CT) at a spatial resolution of 0.2×0.2×0.6 mm. The CT images were obtained before and after magnetic resonance investigation. The novelty of this approach is the 3D monitoring of infiltration process in natural soil samples. It reveals its potential to study the complex flow dynamics. The research has been supported by GACR 103/08/1552 and SP/2E7/229/07.
    05/2010;
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    ABSTRACT: Magnetic resonance imaging (MRI) is applied to the study of flow processes in a model and a natural soils core. Since flow velocities in soils are mostly too slow to be monitored directly by MRI flow velocity imaging, Gd-DTPA was used as contrast agent for the first time for flow processes in soils. Apart from its chemical stability the main advantage is the anionic net charge in neutral aqueous solution. Here we can show that this property hinders the adsorption at soil mineral surfaces and therefore retardation. Gd-DTPA turns out to be a very convenient conservative tracer for the investigation of flow processes in model and natural soil cores. With respect to the flow processes in the coaxial model soil column and the natural soil column we found total different flow patterns: In the first case tracer plume moves quite homogeneously only in the inner highly conductive core. No penetration into the outer fine material takes place. In contrast, the natural soil core shows a flow pattern which is characterized by preferential paths avoiding dense regions and preferring loose structures. In the case of the simpler model column also the local flow velocities are calculated by the application of a peak tracking algorithm.
    05/2010;
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    ABSTRACT: Within Cluster A, Partial Project A1, the pore space exploration by means of Nuclear Magnetic Resonance (NMR) plays a central role. NMR is especially convenient since it probes directly the state and dynamics of the substance of interest: water. First, NMR is applied as relaxometry, where the degree of saturation but also the pore geometry controls the NMR signature of natural porous systems. Examples are presented where soil samples from the Selhausen, Merzenhausen (silt loams), and Kaldenkirchen (sandy loam) test sites are investigated by means of Fast Field Cycling Relaxometry at different degrees of saturation. From the change of the relaxation time distributions with decreasing water content and by comparison with conventional water retention curves we conclude that the fraction of immobile water is characterized by T1 < 5 ms. Moreover, the dependence of the relaxation rate on magnetic field strength allows the identification of 2D diffusion at the interfaces as the mechanism which governs the relaxation process (Pohlmeier et al. 2009). T2 relaxation curves are frequently measured for the rapid characterization of soils by means of the CPMG echo train. Basically, they contain the same information about the pore systems like T1 curves, since mostly the overall relaxation is dominated by surface relaxivity and the surface/volume ratio of the pores. However, one must be aware that T2 relaxation is additionally affected by diffusion in internal gradients, and this can be overcome by using sufficiently short echo times and low magnetic fields (Stingaciu et al. 2009). Second, the logic continuation of conventional relaxation measurements is the 2-dimensional experiment, where prior to the final detection of the CPMG echo train an encoding period is applied. This can be T1-encoding by an inversion pulse, or T2 encoding by a sequence of 90 and 180° pulses. During the following evolution time the separately encoded signals can mix and this reveals information about the connectivity of the pore system. Examples are given for T1-T2 correlation of some soil samples (Haber-Pohlmeier et al. 2010). Third, relaxometric information forms the basis of understanding magnetic resonance imaging (MRI) results. The general difficulty of imaging in soils are the inherent fast T2 relaxation times due to i) the small pore sizes, ii) presence of paramagnetic ions in the solid matrix, and iii) diffusion in internal gradients. The last point is important, since echo times can not set shorter than about 1ms for imaging purposes. The way out is either the usage of low fields for imaging in soils or special ultra-short pulse sequences, which do not create echoes. In this presentation we will give examples on conventional imaging of macropore fluxes in soil cores (Haber-Pohlmeier et al. 2010), and the combination with relaxometric imaging, as well as the advantages and drawbacks of low-field and ultra-fast pulse imaging. Also first results on the imaging of soil columns measured by SIP in Project A3 are given. Haber-Pohlmeier, S., S. Stapf, et al. (2010). "Waterflow Monitored by Tracer Transport in Natural Porous Media Using MRI." Vadose Zone J.: submitted. Haber-Pohlmeier, S., S. Stapf, et al. (2010). "Relaxation in a Natural soil: Comparison of Relaxometric Imaging, T1 - T2 Correlation and Fast-Field Cycling NMR." The Open Magnetic Resonance Journal: in print. Pohlmeier, A., S. Haber-Pohlmeier, et al. (2009). "A Fast Field Cycling NMR Relaxometry Study of Natural Soils." Vadose Zone J. 8: 735-742. Stingaciu, L. R., A. Pohlmeier, et al. (2009). "Characterization of unsaturated porous media by high-field and low-field NMR relaxometry." Water Resources Research 45: W08412
    04/2010; 12:13014.
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    ABSTRACT: Root water uptake by ricinus communis (castor bean) in fine sand was investigated using MRI with multiecho sampling. Before starting the experiments the plants germinated and grew for 3 weeks in a cylindrical container with a diameter of 9 cm. Immediately before the MRI experiments started, the containers were water-saturated and sealed, so water content changes were only caused by root water uptake. In continuation of a preceding work, where we applied SPRITE we tested a multi-echo multi-slice sequence (MSME). In this approach, the water content was imaged by setting TE = 6.76 ms and nE = 128 with an isotropic resolution of 3.1mm. We calculated the water content maps by biexponential fitting of the multi-slice echo train data and normalisation on reference cuvettes filled with glass beads and 1 mM NiCl2 solution. The water content determination was validated by comparing to mean gravimetric water content measurements. By coregistration with the root architecture, visualised by a 3D fast spin echo sequence (RARE), we conclude that the largest water content changes occurred in the neighbourhood of the roots and in the upper layers of the soil.
    The Open Magnetic Resonance Journal 2010 (2010) 3. 01/2010;

Publication Stats

115 Citations
64.69 Total Impact Points

Institutions

  • 2001–2012
    • Forschungszentrum Jülich
      • Institute of Bio- and Geosciences (IBG)
      Jülich, North Rhine-Westphalia, Germany
  • 2011
    • Università degli studi di Palermo
      • Department of earth and marine science (DiSTeM)
      Palermo, Sicily, Italy
  • 2010
    • RWTH Aachen University
      • Institute for Technical und Macromolecular Chemistry
      Aachen, North Rhine-Westphalia, Germany
  • 2002
    • IME Metallurgy
      Aachen, North Rhine-Westphalia, Germany