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Isotopic labeling techniques have the potential to minimize the uncertainty of plant root water uptake (RWU) profiles estimated through multi-source (statistical) modeling, by artificially enhancing soil water isotopic gradient. Furthermore, physical models can account for hydrodynamic constraints to RWU if simultaneous soil and plant water status data is available. In this study, a population of tall fescue (Festuca arundinacae cv Soni) was grown in a macro-rhizotron setup under semi-controlled conditions to monitor such variables for a 34-hours long period following the oxygen stable isotopic (18O) labeling of deep soil water. Aboveground variables included tiller and leaf water oxygen isotopic compositions as well as leaf water potential (ψleaf), relative humidity, and transpiration rate. Belowground profiles of root length density (RLD), soil water content and isotopic composition were also sampled. While there were strong correlations between hydraulic variables as well as between isotopic variables, the experimental results underlined the discrepancy between variations of hydraulic and isotopic variables. In order to dissect the problem, we reproduced both types of observations with a one-dimensional physical model of water flow in the soil-plant domain, for 60 different realistic RLD profiles. While simulated ψleaf followed clear temporal variations with little differences across plants as if they were “on board of the same rollercoaster”, simulated δtiller values within the plant population were rather heterogeneous (“swarm-like”) with relatively little temporal variation and a strong sensitivity to rooting depth. The physical model thus suggested that the discrepancy between isotopic and hydraulic observations was logical, as the variability captured by the former was spatial and may not correlate with the temporal dynamics of the latter. For comparison purposes a Bayesian statistical model was also used to simulate RWU. While they predicted relatively similar cumulative RWU profiles, the physical model could differentiate spatial from temporal dynamics of the isotopic signature, and supported that the local increase of soil water content and formation of a peak of labelled water observed overnight were due to hydraulic lift.

The need for improvements in the water use efficiency by agricultural ecosystems requires a holistic assessment of the hydraulic functioning of cropped soils, taking into consideration the most relevant interactions and feedbacks that control the soil water budget. We implemented a mechanistic approach to isolate the effects of soil hydraulic properties (K-θ-h) of layered soils on water balance components and land and water productivity, adopting comprehensive scenarios of soil water availability and requirements. The agro-hydrological simulations were performed using the SWAP model integrated with the WOFOST crop growth module. The simulated scenarios included the rainfed crop growth of maize and soybean in three climate zones, evaluating the current climate scenarios as well as two future scenarios, a wetter and a drier one, totaling 108 scenarios simulated for 30 years each. Simulations were performed for six soils, grouped pairwise (3 × 2), where each pair represented the same soil group with two different long-term land uses: natural forest (proxy of a no-tillage system) and conventional agricultural use. The K-θ-h relationships were obtained simultaneously by inverse modeling for the full range of soil water contents commonly found in the domain of crop available water. The agro-hydrological simulations showed that the soil hydraulic properties affect dynamically water balance components and land productivity by relating soil hydraulic functioning to climate patterns and crop water requirements. In general, maize productivity was more sensitive to soil hydraulic properties under future climate scenarios than soybean. While land productivities of maize and soybean increased under the wetter climate scenario, water productivity of both crops was consistently reduced by both future climate scenarios. The K-θ-h of soils under conventional agricultural use over-performed their counterparts under long-term natural forest use, especially regarding land productivity during growing seasons with pronounced dry spells. Depending on the length and timing of drought stress during the growing season, the yield response is determined by soil-specific conditions strictly related to water availability. The long-term average revealed that the sampled loamy sand soils have more favorable hydraulic properties for crop growth; moreover, the reduced unproductive water losses, especially runoff, increased the dynamic water storage of those soils.

Accurate knowledge of soil hydraulic properties (K-θ-h) for the entire range of crop available water is essential for the prediction of soil water movement and related processes by mechanistic models, including the partitioning of surface energy fluxes into transpiration and evaporation and the dynamics of root water uptake, mandatory processes for adjustments of crop water use efficiency. We implemented an experimental and numerical protocol to obtain K-θ-h of eleven soils with a broad spectrum of texture and land use. Measurements of the soil water content during evaporation experiments using gamma-ray beam attenuation, a non-invasive technique, were adopted as an alternative approach to conventional measurements of the soil water pressure head. Inverse parameter optimization was performed using Hydrus-1D. The optimized K-θ-h functions were interpreted with respect to crop available water, where results calculated by a proposed “dynamic” method were compared with those determined using the conventional “static” criteria with standardized pressure heads. The evaporation experiment protocol allowed the determination of the K-θ-h relationships by inverse modeling from near-saturation to the dry range (∼ −150 m) with satisfactory accuracy. Soil water retention curves of the fine-textured soils determined by the conventional method (pressure plates) deviated from those estimated by the inverse optimization near saturation and in the dry range, with the conventional method predicting larger water content values. In terms of crop available water, the “dynamic” method allowed incorporating system characteristics (atmospheric demand and crop properties) and K-θ-h in a process-based way, contrarily to the “static” method. Considering a specific scenario, for the fine-textured soils the “static” and “dynamic” approaches performed similarly, however, for the coarse-textured soils, they diverged significantly. No tendency could be revealed for crop water availability under different land uses, and, in general, crop available water for soils under forest use was very similar to their counterparts under agricultural use.

Seedling size is a very important issue when producing plants for restoration projects. Scientific evidence on the appropriate size for drylands is contradictory. Thus, the aim of this study was to evaluate the effect of seedling size during first establishment by conducting a short term greenhouse experiment with Pinus canariensis containerized seedlings. A selection of large (mean height: 33.7 cm) and small (14.3 cm) one-year-old seedlings were planted in pots under two volumetric water content regimes: dry (7%) and wet (15%). Midday shoot water potential was measured in two periods: 10 (prior to root protrusion) and 30 (once the roots had protruded from the plug) days after planting. The length of protruding roots was measured after 30 days. One month after planting, the large seedlings under the dry regime produced more new roots than the small seedlings, but also showed the highest midday water potential values. Therefore, the greater root growth of the former did not offset the higher transpiration demand when planted in dry soils. These results suggest that under uncertainty about the soil humidity levels of dry areas, using small seedlings can improve their short-term survival after planting. The study was supported by the AGL2011- 24296 ECOLPIN project (Department of Science and Innovation, Spanish Government) and the PhD educational program of the Technical University of Madrid, Spain.

Artificial capillary barriers (CBs) are used to improve root zone conditions as they can keep water and nutrients in the root zone by limiting downward percolation. Numerical analysis is one of the promising tools for evaluating CB systems’ performance during the cultivation of leafy vegetables. This study aims to investigate the effects of the CB system on soil water dynamics during spinach cultivation in a soil column under different irrigation scenarios using HYDRUS (2D/3D) by comparing uniform (UNI), line-source (LSI), and plant-targeted (PTI) irrigations combined with alternative irrigation schedules. Simulation results of volumetric soil water contents were generally corresponding to measured data. Simulation results with various hypothetical irrigation scenarios exhibited that the CB was an effective system to diminish percolation losses and improve the root zone’s soil water storage capacity. On the other hand, evaporation loss can be increased as more water is maintained near the surface. While this loss can be significantly minimized by reducing the water application area, the irrigation amount must be carefully defined because applying water in a smaller area may accelerate downward water movement so that the water content at the CB interface can reach close to saturation. In addition to the malfunction of the CB layer, it may also cause a reduction of plant root water uptake (RWU) because the root zone is too wet. Among evaluated irrigation scenarios, irrigating every two days with PTI was the best scenario for the spinach as water use efficiency was greatly improved.

The plasticity in root system development (RSD) is a key trait for the adaptation of rice to mild drought. However, the enhanced RSD due to the plasticity may not be always a sole function of promoted lateral root (LR) production, but also of the integrated responses of nodal root (NR) development. In this study, we aimed to evaluate the effects of mild drought intensities on the development of the NR and LR, and their contribution to the entire RSD. We used six genotypes including KDML105 (indica, lowland adapted), a high lateral rooting ability genotype. The plants were grown up to heading or maturity stage for two years under soil with limited soil depth (20 cm) assuming the presence of the hardpan and at different moisture gradients generated by the line source sprinkler system. The effects of drought intensities generally differed between the development of NR and LR. In both years, all genotypes showed highest LR development under mild drought stress intensities. However, in some genotypes including KDML105, NR development was maintained in a limited soil moisture range only, which was narrower and wetter than that in which LR plasticity was expressed. Furthermore, the entire RSD was maintained only when both the NR and LR were simultaneously promoted or maintained. These results suggest that the NR have less plasticity than the LR in response to drought and the contribution of the plasticity in LR development to the entire RSD is dependent on both the soil moisture and nodal rooting ability.

Water scarcity and quality degradation represent real threats to economic, social, and environmental development of arid and semi-arid regions. Drip irrigation associated to Deficit Irrigation (DI) has been investigated as a water saving technique. Yet its environmental impacts on soil and groundwater need to be gone into in depth especially when using brackish irrigation water. Soil water content and salinity were monitored in a fully drip irrigated potato plot with brackish water (4.45 dSm&minus, 1) in semi-arid Tunisia. The HYDRUS-1D model was used to investigate the effects of different irrigation regimes (deficit irrigation (T1R, 70% ETc), full irrigation (T2R, 100% ETc), and farmer&rsquo, s schedule (T3R, 237% ETc) on root water uptake, root zone salinity, and solute return flows to groundwater. The simulated values of soil water content (&theta, ) and electrical conductivity of soil solution (ECsw) were in good agreement with the observation values, as indicated by mean RMSE values (&le, 0.008 m3&middot, m&minus, 3, and &le, 0.28 dSm&minus, 1 for soil water content and ECsw respectively). The results of the different simulation treatments showed that relative yield accounted for 54%, 70%, and 85.5% of the potential maximal value when both water and solute stress were considered for deficit, full. and farmer&rsquo, s irrigation, respectively. Root zone salinity was the lowest and root water uptake was the same with and without solute stress for the treatment corresponding to the farmer&rsquo, s irrigation schedule (273% ETc). Solute return flows reaching the groundwater were the highest for T3R after two subsequent rainfall seasons. Beyond the water efficiency of DI with brackish water, long term studies need to focus on its impact on soil and groundwater salinization risks under changing climate conditions.

Climatic projections indicate that, Belgium might face a significant change in rainfall regime with up to 25% less precipitations in Summer by the end of the 21st century. This would lead to water stresses for plants. Grasslands, that make for a large part of Belgian agricultural system would be amongst the most affected by these dryer conditions and would be likely to see their yields decrease. To tackle the issue, UCLouvain launched the ForDrought project in 2012, whose overal goal is to improve the performance of forage crops to cope with climate change. The specific objectives of this study were to observe the responses of root systems of grassland plants when facing a water deficit and to assess the effect of plant species association. The species studied were Dactylis glomerata and Medicago sativa. Large pits were dug in front of experimental agricutural parcels to carry out differents measurements on the root system. The root numbers in each pit were counted, soil samples containing roots were taken and the roots were analyzed genetically to determine the specie to which they belonged. In addition, the parcels were harvested and yield calculated four times over the course of one year. The results showed that a water deficit has an impact on the number of roots of both species. The genetic analyses showed, quite surprisingly, that Dactylis was not detected when it was alongside alfalfa. Finally, they showed that the yield is negatively impacted by water deficit while the association of species does not produce. (1) Taggart J.M., Cahill J.F., McNickle G.G., Hall J.C., (2011), Molecular identification of roots from a grassland community using size differences in fluorescently labelled PCR amplicons of three cpDNA regions: Molecular diagnostics and DNA taxonomy, Molecular Ecology Resources, 11,1, 185-195 (2) Robbins N.E., Dinneny J.R., (2015), The divining root: moisture-driven responses of roots at the micro- and macro-scale, Journal of Experimental Botany, 66, 8, 2145-2154

Plant transpiration and root water uptake are dependent on multiple traits that interact with site soil characteristics and environmental factors such as radiation, atmospheric temperature, relative humidity, and soil-moisture content. Models of root architecture and functions are increasingly employed to simulate root-soil interactions. Root water uptake is thereby affected by the root hydraulic architecture, the soil moisture conditions, soil hydraulic properties and the transpiration demand as controlled by atmospheric conditions. Stomatal conductance plays a vital role in regulating transpiration in plants. Isohydric plants follow the strategy to close their stomata in order to maintain the leaf water potential at a constant level, while anisohydric plants leave their stomata open when leaf water potentials fall due to drought stress. Modeling the stomatal regulation effectively will result in a more reliable model that will regulate the excessive loss of water. We performed the simulations of plant water uptake and transpiration on a DUNE-based simulator DuMux. It simulates the flow and transport processes in porous media that can represent the porous medium (including soil) with embedded hierarchical biological networks (in this case, root system). We implemented hydraulic and chemical stomatal control of root water uptake in DuMux following the current approach where stomatal control is regulated by simulated water potential and/or chemical signal concentration. We then performed a mesh convergence study in order to verify the accuracy of our results. Finally, we conducted the simulations for different scenarios to study the effect of hydraulic and chemical regulation on root system performance under drought stress. Future work will include incorporating further modeling capabilities into DuMux which will facilitate the consideration of further soil and rhizosphere processes in the simulations, such as nutrients, root exudes or soil microorganisms.

In the recent years, many computational tools, such as image analysis, data management, process-based simulation and upscaling tools, were developed to help quantify and understand water flow in the soil-root system, at multiple scales (tissue, organ, plant and population). Several of these tools work together or, at least, are compatible. However, for the un-informed researcher, they might seem disconnected, forming a unclear and disorganised succession of tools.In this article, we present how different pieces of work can be further developed by connecting them to analyse soil-root-water relations in a comprehensive and structured network. This “explicit network of soil-root computational tools” informs the reader about existing tools and help them understand how their data (past and future) might fit within the network. We also demonstrate the novel possibilities of scale-consistent parameterizations made possible by the network with a set of case studies from the literature. Finally, we discuss existing gaps in the network and how we can move forward to fill them.HighlightsMany computational tools exist to quantify water flow in the soil-root system. These tools can be arranged in a comprehensive network that can be leveraged to better interpret experimental data.

The root system is the plant’s organ that has the important function of taking up water and plant nutrients from the soil. Root system functioning depends on its multiscale structure: the size and structure of cells in the conductive tissues, the development and spatial organization of tissues, and the network of root segments that make up the root system. Breeders select for these structural properties of roots and root systems to develop crop varieties with high water and nutrient efficiencies that give a high or stable yield under non-controllable adverse environmental conditions(droughts, nutrient poor soils, soil salinity, …). But, a direct relation between the functioning of a root system and its structure is not at hand. Functional-structural root system models that simulate the relations between root architectural and hydraulic properties, and the spatio-temporal patterns of water and solute fluxes in the root zone can therefore play an important role in this selection process. However, it must be first demonstrated that these models accurately reproduce the processes they intend to describe but this is a challenging task for two reasons. First, the root system is the so-called ‘hidden halve’ of the plant so that its structure and the flow and transport processes in the soil towards individual roots are hard to observe. Secondly, the properties that control water flow in root segments and that are required as input parameters in structural-functional root models vary with age and root order and quantitative information about these relations is scarce. In this paper, we demonstrate how this deadlock could be broken by combining co-registered root structure and tracer distributions obtained from magnetic resonance imaging, a functional-structural root system model, and inverse modeling. The main features in the tracer patterns were well reproduced by the model using root hydraulic parameters that were obtained using inverse modeling and that were found to correspond with available information about these parameters in the literature. The simulation results further demonstrated that water uptake location and intensity cannot be directly derived from neither observations of tracer accumulation nor water depletion. This proves that functionalstructural root system model simulations, combined with observations, are required to translate the observed variables (tracer accumulation and water depletion) into information about local processes and root system properties.

Many important functions of the soil and the land surface are related to processes that take place in the root zone at the interface between the soil and the vegetation. Jan Hopmans has made significant contributions that provided new insights in these processes and that inspired new research. Of special notice is his pioneering work on imaging root systems and on soil-root process simulations that resolve 3D root architectures. In this presentation, we present insights that we derived from imaging and simulation of soil-root processes. We developed the R_SWMS and CROOTBox codes that link features of 3D root systems to their functions with respect to water and nutrient uptake. 3D representations of root systems in simulation models can effectively valorize information obtained with novel measurements techniques which allow functional structural imaging and monitoring of root processes. These models allow for functional phenotyping of root systems. Finally, these models were used to couple soil conditions (water content and salinity) to crop and vegetation stresses and to derive mechanistic representations of these relations in larger scale soil, crop and land surface models. The importance of the root system for the interaction of the crop or vegetation with its environment calls for a better mechanistic understanding of the development and plasticity of root systems. Currently, we are developing CPLANTBOX, a functional-structural soil-plant model which simulates the distribution of assimilates and plant growth in function of environmental conditions by coupling carbon assimilates and water fluxes in 3D whole-plant architectures.

Electrical anisotropy is a typical feature observed in geology and can be used to characterize sub-surface heterogeneity (Al-Hazaimay et al., 2016). In soil-root medium, the effective electrical anisotropy factor (EAF) in electrical conductivity (EC) is due to root processes such as root water uptake, root architectural evolution, exudation and solute uptake. Under homogeneous soil conditions, (i.e. well irrigated scenario and no water uptake pattern in the soil) and with root water uptake (Javaux et al., 2008), we hypothesize that EAF of soil-root medium could contain information on root architecture (geometrical indices) and thus can be used for phenotyping root systems. To support our hypothesis, we performed 2D finite-element modeling of these properties in terms of EC distribution for a series of simulated root systems, with explicit representation of the root architecture in the finite-element mesh. We then find the correlation between geometry of root network and EAF and assess under what condition will there exist high correlation and hence EAF could provide information on root architecture. Root architecture was explicitly resolved in the finite element mesh along with root water uptake (Javaux et al., 2008). To parameterize the model, we measured complex conductivity of Maize root segments at different frequencies (IP spectra). The roots elements in the finite element mesh had frequency dependent electrical properties, including the resolution of cortex and stele. Effective electrical anisotropy and geometrical anisotropy are examined for different synthetic root architecture generated in Crootbox software. We then relate the modeled effective electrical anisotropy to the geometrical anisotropy of root structure. To formulate geometrical anisotropy, we discuss different approaches. We also demonstrate high correlation between geometrical anisotropy of root networks and simulated electrical anisotropy of effective conductivity of soil-root continuum. The understanding of relation between geometry of root network, electrical conductivity contrast between soil-root and effective electrical property of soil-root continuum is very important to monitor rooted zone via electrical methods and our modeling results helps to give insight.

Climatic projections indicate that Europe should face a significant change in seasonal rainfall distribution, with up to 25% less precipitations in Summer by the end of the 21st century.. The yield of grasslands, that make for a large part of Belgian agricultural system, would be amongst the most affected by these new scenarios of water deficit. The ovseral goal of the ForDrought project, launched at UCLouvain in 2012, is to improve the performance of forage crops to cope with climate change. The specific objectives of this study were to observe the responses of root systems of grassland plants exposed to a water deficit and to assess the value of plant species association. The species studied were Dactylis glomerata and Medicago sativa. Large pits were dug on the border of experimental plots of pure and mixed composition, exposed or not to a one-month long summer drought, to carry out different root system observations. The number of roots in each pit were counted following the trench profile method. In addition, roots were extracted from vertical and horizontal soil cores and root DNA was analysed to determine the specific root distribution of each species. The parcels were harvested and yield measured four times over the course of one year. Finally, the soil profile and the depth of the ploughpan for each plot were also determined. The results showed that a water deficit had an impact on the root density of both species. However, this impact differed between species: Medicago had more roots when exposed to the deficit while Dactylis tended to produce fewer roots. The genetic analysis turned out to be biased against Dactylis which could not be detected when it was mixed with Medicago, despite many protocol adjustments were implemented to circumvent the problem. The yield was negatively impacted by water deficit, for all plots. The association of species did not produce any clear effect on the yield. However, the soil observation showed that the ploughpan was abnormally thick in all of plots, which may have limited the expression of the species specificities and of the differences between pure and mixed stands.

In agricultural practices the huge demand on fresh water for irrigation, together with water scarcity, encourages the reuse of wastewater as a water resource. Integrated management of water resources by considering the efficient use of wastewater could result in fresh water saving. A field experiment was conducted to compare the effects of two irrigation water sources, brackish secondary treated wastewater and surface canal fresh water, on maize crop. During the irrigation cycle, soil water content distribution was estimated by means of time-lapse mode electrical resistivity tomography (ERT), soil electrical resistivity being highly sensitive to soil moisture and water salinity. The effects of the two water sources on the spatial-temporal distribution of ERT-derived soil moisture values were assessed, and different roots’ behaviors observed. Results show a decreased root water uptake with brackish irrigation water with respect to fresh water. This result implies an increase in water savings due to reduced crop water requirement, which has significant implications for economic and environmental management.

As water often limits crop production, a more complete understanding of plant water capture and transport is necessary. Here, we developed MECHA, a mathematical model that computes the flow of water across the root at the scale of walls, membranes, and plasmodesmata of individual cells, and used it to test hypotheses related to root water transport in maize (Zea mays). The model uses detailed root anatomical descriptions and a minimal set of experimental cell properties, including the conductivity of plasma membranes, cell walls, and plasmodesmata, which yield quantitative and scale-consistent estimations of water pathways and root radial hydraulic conductivity (kr). MECHA revealed that the mainstream hydraulic theories derived independently at the cell and root segment scales are compatible only if osmotic potentials within the apoplastic domains are uniform. The results suggested that the convection-diffusion of apoplastic solutes explained most of the offset between estimated kr in pressure clamp and osmotic experiments, while the contribution of water-filled intercellular spaces was limited. Furthermore, sensitivity analyses quantified the relative impact of cortex and endodermis cell conductivity of plasma membranes on root kr and suggested that only the latter contributed substantially to kr due to the composite nature of water flow across roots. The explicit root hydraulic anatomy framework brings insights into contradictory interpretations of experiments from the literature and suggests experiments to efficiently address questions pertaining to root water relations. Its scale consistency opens avenues for cross-scale communication in the world of root hydraulics.

Various issues related to oil palm production, such as biodiversity, drought, water scarcity, and water and soil resource exploitation, have become major challenges for environmental sustainability. The water footprint method indicates that the quantity of water used by plants to produce one biomass product could become a parameter to assess the environmental sustainability for a plantation. The objective of this study is to calculate the water footprint of oil palm on a temporal scale based on root water uptake with a specific climate condition under different crop age and soil type conditions, as a means to assess environmental sustainability. The research was conducted in Pundu village, Central Kalimantan, Indonesia. The methodology adopted in carrying out this study consisted of monitoring soil moisture, rainfall, and the water table, and estimating reference evapotranspiration (ETo), root water uptake, and the oil palm water footprint. Based on the study, it was shown that the oil palm water usage in the observation area varies with different crop ages and soil types from 3.07&ndash, 3.73 mm/day, with the highest contribution of oil palm water usage was in the first root zone which correlates to the root density distribution. The total water footprint values obtained were between 0.56 and 1.14 m3/kg for various plant ages and soil types. This study also found that the source of green water from rainfall on the upper oil palm root zone delivers the highest contribution to oil palm root water uptake than the blue water from groundwater on the bottom layer root zone.