An analytical root depth distribution

Root growth is critical for crops to use soil water under water-limited conditions. A field study was conducted to investigate the effect of available soil water on root and shoot growth, and root water uptake in winter wheat (Triticum aestivum L.) under deficit irrigation in a semi-arid environment. Treatments consisted of rainfed, deficit irrigation at different developmental stages, and adequate irrigation. The rainfed plots had the lowest shoot dry weight because available soil water decreased rapidly from booting to late grain filling. For the deficit-irrigation treatments, crops that received irrigation at jointing and booting had higher shoot dry weight than those that received irrigation at anthesis and middle grain filling. Rapid root growth occurred in both rainfed and irrigated crops from floral initiation to anthesis, and maximum rooting depth occurred by booting. Root length density and dry weight decreased after anthesis. From floral initiation to booting, root length density and growth rate were higher in rainfed than in irrigated crops. However, root length density and growth rate were lower in rainfed than in irrigated crops from booting to anthesis. As a result, the difference in root length density between rainfed and irrigated treatments was small during grain filling. The root growth and water use below 1.4 m were limited by a caliche (45% CaCO3) layer at about 1.4 m profile. The mean water uptake rate decreased as available soil water decreased. During grain filling, root water uptake was higher from the irrigated crops than from the rainfed. Irrigation from jointing to anthesis increased seasonal evapotranspiration, grain yield, harvest index and water-use efficiency based on yield (WUE), but did not affect water-use efficiency based on aboveground biomass. There was no significant difference in WUE among irrigation treatments except one-irrigation at middle grain filling. Due to a relatively deep root system in rainfed crops, the higher grain yield and WUE in irrigated crops compared to rainfed crops was not a result of rooting depth or root length density, but increased harvest index, and higher water uptake rate during grain filling.

Plant roots connect belowground moisture with aboveground vegetation functionalities, making root plasticity critical for drought resilience. This study employs an enhanced land surface ecohydrological model, Duke Coupled Hydrology Model with Vegetation and Dynamic Roots (DCHM-VDR), to investigate how root dynamics affect vegetation adaptation under a range of climate conditions. DCHM-VDR features a moisture-driven root parameterization that simulates dynamic root profiles and hydraulic redistribution (HR). Applied to a semiarid woodland with groundwater-dependent mesquites and shallow-rooted shrubs, model results compare well against AmeriFlux tower data and capture observed soil moisture patterns tied to root water uptake, including mesquite’s dimorphic root profile and shifts in water source and the direction of HR. Accounting for HR lowers overall water use efficiency (WUE) by more than 50% in the dry season due to release of deep root water uptake to moisten dry soil layers. Dynamic root profiles reduce water demand by avoiding dry soil patches and utilizing moist layers with increased WUE, especially for shrubs. Mesquites with a dimorphic root profile show a 3-fold annual transpiration (Tr) increase from 183 mm to 629 mm/year and reduced HR/Tr from 34% to 6%, along with a small reduction of around 10% for shrubs, reflecting the importance of groundwater sourcing and the codependence of shrubs on mesquite for water stress resilience. Future climate scenarios were examined using the Thermodynamic Global Warming downscaled data. Mesquites and shrubs respond positively to wetter winters, albeit with opposite water use strategies in the drier growing seasons due to different rooting depths and HR modulation of soil moisture. Mesquites increase transpiration by adjusting root fraction, water uptake, and HR closer to the saturated zone, while shrubs reduce transpiration and increase WUE by 17% compared to a decrease of 13% in mesquite WUE under the driest scenario. The intertwined water use strategies of mesquites and shrubs expressed by the root water uptake dynamics determine ecosystem response aboveground under more extreme climate conditions, highlighting the importance of modeling root architecture dynamics and co-adaptive ecohydrological processes in predicting ecosystem responses to climate change.

The root density and depth patterns of four boreal forest age sequences were analyzed for successional trends. Rooting depths increased with age on sandy substrates which supported aspen (Populustremuloides Michx.) and jack pine (Pinusbanksiana Lamb.) communities. Rooting depth did not change in an aspen series on fine-textured substrates or in a black spruce (Piceamariana (Mill.) BSP) series growing on organic substrates. Plant communities growing on mineral soils showed a decrease in near-surface root densities and understory vascular plant cover with increasing age. Maximum rooting was deepest on sandy substrates and shallowest on organic deposits. Roots in all stands were concentrated near the ground surface. In most cases 50% of all roots were located within 15 cm of the forest floor. Root densities in this zone ranged from 11 000 to 30 000 roots m−2 of vertical surface. Densities were approximately 4300 roots m−2 for the overall rooting zone.

Core Ideas A 3D soil–root model was used to investigate root–biopore interactions. Known effects of biopores on root growth, i.e., increased root length and depth were reproduced. Despite reducing root–soil contact, biopores led to increased water uptake in dry periods. Biopores had a larger impact on water uptake for more compact and less conductive soils. Roots are known to use biopores as preferential growth pathways to overcome hard soil layers and access subsoil water resources. This study evaluates root–biopore interactions at the root‐system scale under different soil physical and environmental conditions using a mechanistic simulation model and extensive experimental field data. In a field experiment, spring wheat ( Triticum aestivum L.) was grown on silt loam with a large biopore density. X‐ray computed tomography scans of soil columns from the field site were used to provide a realistic biopore network as input for the three‐dimensional numerical R‐SWMS model, which was then applied to simulate root architecture as well as water flow in the root–biopore–soil continuum. The model was calibrated against observed root length densities in both the bulk soil and biopores by optimizing root growth model input parameters. By implementing known interactions between root growth and soil penetration resistance into our model, we could simulate root systems whose response to biopores in the soil corresponded well to experimental observations described in the literature, such as increased total root length and increased rooting depth. For all considered soil physical (soil texture and bulk density) and environmental conditions (years of varying dryness), we found biopores to substantially mitigate transpiration deficits in times of drought by allowing roots to take up water from wetter and deeper soil layers. This was even the case when assuming reduced root water uptake in biopores due to limited root–soil contact. The beneficial impact of biopores on root water uptake was larger for more compact and less conductive soils.

Water infiltration into vegetated soils is affected by interactions between soil properties and plant activity. Water uptake by plant roots depends on the soil hydraulic properties and infiltration rate. In turn, roots are important water movers and induce nonuniform water content distributions with consequent impact on the infiltration rate. Our goal was to use neutron radiography to investigate root water uptake during rapid infiltration events and subsequent water redistribution. Neutron radiography is an imaging technique capable of measuring with high spatial and temporal resolution root density, soil structure, and water distributions in the vicinity of roots. We used time‐series neutron radiography to image infiltration events in 0.17‐ by 0.15‐ by 0.013‐m boxes filled with a sand mixture and that contained either lupine ( Lupinus L.) or maize ( Zea mays L.) plants. We measured four replications at two stages of plant growth. Neutron radiography can detect small variations in water content with high accuracy and high spatial and temporal resolution. Due to their high water content, roots were easily visible in the radiograms before infiltration. Following calibration, we quantified the water content in the sample during and after each infiltration event. Effects of soil heterogeneities such as thin soil layers were visible, as was water uptake by roots. We observed lower water contents along the main root of lupine during the early stage of infiltration, and slower and more diffuse uptake during the later stages of water redistribution, when the wetting front had propagated downward and the water content in the soil decreased. The observed root water uptake was not uniform but rather was localized in certain regions. For maize we observed a loss of water mainly in the upper part of the root system, closer to the stem rather than at root tips.

Effects of climate and terrestrial storage on temporal variability of actual evapotranspiration

The chemical compounds that plant roots release into the soil, referred to as rhizodeposits, affect soil hydraulic properties like the surface tension and viscosity of soil water, and the contact angle between menisci and the pore surface. What remains less clear is how these effects manifest when considering soil water infiltration and retention, and the consequent impact on the availability of water for uptake by roots. A novel model for soil water transport was developed that incorporates the influences of rhizodeposits on soil hydraulics and root water uptake. The finite element method was used to generate model simulations and investigate how the presence of rhizodeposits of wheat ( Triticum aestivum L.) affected water uptake of three differently aged root systems under precipitation regimes that varied in total precipitation and delivery pattern. The overriding effect on soil hydraulics of wheat rhizodeposits was to facilitate infiltration. A trade‐off was identified in how this could improve water availability to the root system by reducing evaporation but also worsen it by increasing deep percolation. When total precipitation was shared over several rainfall events, the effect of rhizodeposits on root water uptake was positive. However, when higher levels of total precipitation were delivered in a single event, the presence of rhizodeposits reduced the water uptake of shallower root systems. This suggests that the influence of rhizodeposits on soil hydraulics and root water uptake is nuanced and will require careful consideration when developing crops for improved water use efficiency and stress resilience.

Root length in the images analyzed by Rootfly (5) was converted from pixels to millimeters by that PPMM value. Before calibration, the PPMM value was 397.8, and after calibration it was 314.9. Therefore, all our published root length values that were obtained using Rootfly need to be corrected according to the new PPMM. Therefore, a factor of 0.8 was used to correct RLD and total root length under a horizontal unit soil surface in 2. The corrections are conducted in corresponding places in the two published papers below. “A digital camera with a visible frame of 13.5 mm (vertical) by 18 mm (horizontal) was used to capture the root images from both the left and right sides of the rhizotubes” should be replaced by “A digital camera with a visible frame of 16.5 mm (vertical) by 23.5 mm (horizontal) was used to capture the root images from both the left and right sides of the rhizotubes.” The phrase “roots were counted in 120 13.5- by 18-mm images” should be replaced by “roots were counted in 120 16.5- by 23.5-mm images.” “The SEM is around 0.06 counts cm−2 in the upper facility …” should be replaced by “The SEM is around 0.04 counts cm−2 in the upper facility …”. “the SEM was larger and reached up to 0.12 counts cm−2 at the 60- and 80-cm depth” should be replaced by “the SEM was larger and reached up to 0.07 counts cm−2 at the 60- and 80-cm depth.” “around 0.75 counts cm−2 in the sheltered and rainfed plots and 1.1 counts cm−2 in the irrigated plot” should be replaced by “around 0.45 counts cm−2 in the sheltered and rainfed plots and 0.66 counts cm−2 in the irrigated plot.” The values of root density in Fig. 8, 10 (left), and 11 should be rescaled by 0.6, whereas the values in Fig. 9 should be rescaled by 0.62 (0.36) according to the definition of a variogram. “The images with a size of 13.5 by 18 mm were analyzed by Rootfly” should be replaced by “The images with a size of 16.5 by 23.5 mm were analyzed by Rootfly.” Total root length under a unit horizontal surface was obtained based on RLD values and the given rooting depth. Thus, the total root length has to be corrected according to the corrected RLD′ values. Therefore, “the calculated total root length per plant in Week 8 when roots were first observed at 80 cm was 47.20 and 5.77 m from observed lengths and observed impacts, respectively” should be replaced by “the calculated total root length per plant in Week 8 when roots were first observed at 80 cm was 37.76 and 4.62 m from observed lengths and observed impacts, respectively”, and “the root hydraulic conductance per unit root length was 1.23 × 10−8 and 1.01 × 10−7 cm h−1 for Week 8” should be replaced by “the root hydraulic conductance per unit root length was 1.54 × 10−8 and 1.26 × 10−7 cm h−1 for Week 8.” Because normalized RLD was used in both the Feddes and Couvreur models, there was no effect on the inverse modeling and the simulation of root water uptake.

Dual-purpose crops for grazing and grain production can be highly profitable, provided grazing does not cause significant loss of grain yield. In many plants, defoliation causes a transient reduction in the allocation of resources to stem and root growth and remobilisation of soluble resources to re-establish leaf area rapidly. In Australia, the usual autumn and winter period of defoliation for grazed crops, May–July, coincides with a phase of near-linear root depth penetration in ungrazed crops, and the crop recovery period after grazing occurs during stem elongation, when grain number and yield potential are determined. However, few studies have investigated the potential impact of crop defoliation through grazing on root growth of wheat in the field. We investigated the effect of defoliation by grazing or shoot removal on the root growth of wheat crops in four field experiments in south-eastern Australia in which the timing, frequency and intensity of defoliation varied. Despite significant impacts of defoliation on aboveground biomass (50–90% reduction) and grain yield (10–43% reduction) in all experiments, we found little evidence of effects on the rate of root penetration or final rooting depth. A notable exception was observed in one experiment when defoliation commenced very early (four-leaf stage, Zadoks growth stage Z14) in a repeatedly defoliated crop, reducing rooting depth from 1.65 to 1.35 m. The only other measured impact on roots was in an early-sown winter wheat crop grazed by sheep for 3 months (6 June–3 September), in which root length density was reduced by ~50% in surface layers above 1.0 m depth, but there was no impact on maximum root depth or root length density at 1.0–2.0 m depth. Our results suggest that grazing has little impact on the rooting depth of wheat unless it occurs very early and repeatedly, when plants are allocating significant resources to establish the primary roots. However, there may be some reduction in the density of roots in surface layers during recovery after long-term grazing, presumably associated with reduced proliferation of the nodal root system. We conclude that most significant yield penalties due to grazing relate to impacts on the assimilation of aboveground resources, rather than to reduced water or nutrient acquisition by roots.

Vegetation can potentially have a strong influence on the water budget and hydrological processes in vegetated ecosystems due to canopy rainfall interception and the partitioning of rainfall into throughfall and stemflow, as well as root water uptake from the vadose zone or groundwater table. This study aims to explore the potential effects of vegetation on soil moisture dynamics and groundwater recharge in a subtropical coastal area of eastern Australia. This area is characterized by highly permeable sands, intense summer rainfall events and three typical vegetation covers (exotic pine plantation, native woodland and grassland). First of all, the spatial variability of both throughfall and stemflow at the soil surface was investigated in a 12-year-old managed pine plantation over one year on Bribie Island using tipping-bucket rain gauges. Rainfall loss by canopy interception and subsequent evaporation from this pine plantation and a native banksia woodland were also quantified and compared using field measurements and two analytical models of rainfall interception. In addition, the potential hydrological impacts of changes in vegetation cover in this shallow sandy groundwater system (depth to water table l 2 m) was evaluated by estimating groundwater recharge and discharge by evapotranspiration (ETg) under the three contrasting vegetation covers over a 2-year period using the water table fluctuation method and the White method, respectively. To further monitor the actual water percolation processes in deep sand dune profiles (depth to water table g 10 m), spatial patterns and seasonal dynamics of root-zone soil moisture were quantified under three contrasting vegetation covers on North Stradbroke Island by combining two geophysical techniques: surface electric resistivity tomography (surface ERT) and spatial time domain reflectometry (spatial TDR). Based on the field investigations of rainfall and root distributions at the under-canopy and inter-canopy zones, spatial distributions of vadose zone soil moisture and deep drainage in this subtropical coastal forest overlying deep sand dunes were finally simulated using HYDRUS models. On the Bribie Island sites, the highest throughfall was found on the east side of the tree trunks (~85% of gross rainfall) and the lowest in the midway between tree rows (~68% of gross rainfall) in the pine plantation. These spatial patterns persist for around 84% of recorded rainfall events. This is explained by canopy interception of the inclined rainfall resulting from the prevailing easterly wind direction throughout the experiment. Annual rainfall interception loss in the banksia woodland was lower (~16% of gross rainfall) than that in the pine plantation (~23% of gross rainfall) due to the lower canopy storage capacity and higher aerodynamic resistance of the banksia woodland. The RGAM and WiMo models predicted the interception losses from these forest stands reasonably well. The average annual gross recharge was largest at the sparse grassland site, followed by the exotic pine plantation and then native banksia woodland. Lower recharge values at forested sites are most likely resulted from higher rainfall interception losses and shallower water table depths. The pine plantation extracted more groundwater through ETg than the banksia woodland, whereas sparse grassland was found not consuming groundwater. In the open pine forest on North Stradbroke Island, the joint use of surface ERT and spatial TDR methods allowed spatially monitoring of root-zone moisture dynamics of the forest soils and the detection of typical features of rainfall interception, root water uptake and preferential infiltration of stemflow. Both surface ERT measurements and HYDRUS modelling identified higher soil moisture and deep drainage at the inter-canopy area relative to those under the canopy due to lower rainfall interception loss and higher root water uptake. The HYDRUS modelling experiments indicated deep drainage was underestimated by 130 mm to 162 mm (9.7%n12.0% drop compared to baseline scenario) as a consequence of uniform representation of spatial root systems in one- or two-dimensional HYDRUS models.The results of this study confirmed the vegetation in these coastal systems has a significant impact on the spatial distribution of rainfall at the soil surface and root water uptake, changing water infiltration and evapotranspiration patterns. Recharge in these shallow sandy aquifers is governed by seasonal rainfall but restricted in the wet season by wet antecedent soil moisture when the water table is approaching the soil surface, i.e., potential recharge is rejected. Groundwater use by vegetation is largely driven by potential ET but also limited by the depth to water table. The establishment of commercial pine plantations in these areas of native vegetation may reduce deep drainage and ultimately groundwater recharge, especially during extensive dry seasons, due to higher interception losses and groundwater uptake. In recharge modelling, our HYDRUS simulations show that translating the hydrological effect of the two-dimensional tree structure (rainfall redistribution and heterogeneous roots) to a one-dimensional lumped vertical conceptualization needs to be undertaken with caution.

The lack of water resource in these past decades encourages the implementation of the precision agriculture system towards the sustainability in palm oil plantation. Therefore, it requires a specific information about the palm oil performance regarding the water balance system that affect the water consumption through the plant root water uptake. However, the prediction of root water uptake distribution is still a challenge. Another method to investigate the soil water dynamics under the plant root system is through the numerical simulations that are widely use to assess the soil water flow of the plant. In alignment with the idea of promoting the sustainable palm oil plantation, the investigation of root water uptake and water content under oil palm tree is highly demanding. As an introduction, through this study, it is find of interest to simulate the root water uptake and water content pattern of oil palm tree using the 2D simulation soil-water flow. The study was performed by applying the 2D simulation soil-water flow model to 17th year old oil palm tree located in Siak, Riau with the loam soil type. The climate data was used as primary data to predict the rate of evapotranspiration. The soil properties and root dimension and distribution of oil palm was taken by the literature study. The simulation over 30 days illustrated the root water uptake distribution, water content change, pressure head and flow velocity. The most intensive root water uptake occurred in the upper root zone of oil palm tree as an impact of the higher root density. The significant root water uptake in the upper root zone lead to the decreasing of water content and increasing of pressure head in the soil. Consequently, there was a change of water flow direction from the wet area in the downward and sideward do dry root zone as the water supply to the oil palm tree.

We hypothesized that if aquatic plants were competing for nutrients then the spatial distribution and sizes of roots should change when competitors were present. Stukenia pectinatus, Potamogeton nodosus, and Hydrilla verticillata (monoecious type) were grown in either monocultures or mixtures in polyvinylchloride containers. Four 5 cm diameter minirhizotrons were mounted horizontally across the containers so that they were 2.5, 12.5, 22.5, or 32.5 cm below the surface of the sediment, and we inserted a specialized video camera in each minirhizotron tube and recorded root images at 21 locations in each tube. For two of three cases, adding plants of another species changed the root depth distribution and increased the volume of soil occupied by roots compared to monocultures. Additionally, smaller diameter roots were more abundant when competitors were present. This implies that the plants adjusted to the presence of competitors by producing more roots capable of nutrient uptake. These results imply that competition for sediment-based nutrients occurred in these systems.

<p>Mitigating plant water stress while reducing irrigation is one of the biggest challenges that sustainable agricultural practices aim to tackle. The rhizosphere is the main unknown component controlling the water distribution in the soil, but non-destructive observations of root physiology are often lacking due to methodological limitations.  </p><p>Numerous studies relate the use of electrical resistivity tomography (ERT) or stem-based methods to measure soil water content changes associated with root water uptake (RWU) in the rhizosphere area. Nevertheless, geoelectrical data are correlated with many rhizosphere parameters and their interpretation needs to be supported by physics-based models of root hydrology.</p><p>Here, we use a Data Assimilation (DA) framework to combine geoelectrical data with a hydrological model (Mary et al. 2021). DA offers the possibility to estimate model parameters governing RWU, such as in the well-known Feddes approach while introducing data observations to update them. </p><p>In a synthetic experiment mimicking a top-down infiltration in a rhizotron containing a wine plant (Vitis Vinifera), we simulated different scenarios of ERT data assimilation with the CATHY surface-subsurface hydrological model. The rooting depth associated with the Feddes parameters are perturbed to generate the ensemble states. At each observation time, model states and root depth parameters are updated using the Ensemble Kalman Filter (ENKF). </p><p>Expected results aim to demonstrate (i) what is the best ENKF scheme to integrate ERT measurements with the hydrological model and (ii) how much the uncertainties on the Feddes parameters can be reduced with the assimilation of ERT data. In a future work, the best approach identified will be applied to real soil and plant observation datasets.</p><p><em>Mary, B., Peruzzo, L., Iván, V., Facca, E., Manoli, G., Putti, M., et al. (2021). Combining Models of Root-Zone Hydrology and Geoelectrical Measurements: Recent Advances and Future Prospects. Front. Water 3, 767910. doi:10.3389/frwa.2021.767910.</em></p>

Ecohydrological studies aiming to understand patterns in root water uptake by trees based on plant and soil water isotope data are often confined to one or a few nearby locations. In this study, we took advantage of recently established pan-European hydrogen (δ2H) and oxygen (δ18O) isotope datasets (10.16904/envidat.542) to assess root water uptake depth for beech and spruce trees across Europe. For a subset of sites, δ17O data were available as well.Our analysis revealed consistent isotopic enrichment in xylem water of spruce trees compared to beech trees across all mixed-species sites (N=13), suggesting that spruce predominantly used shallower soil water regardless of environmental conditions. Additionally, we observed isotopic enrichment in stem xylem water from spring to summer at most beech and spruce sites (N=32), suggesting both species relied on isotopically enriched summer precipitation. Interestingly, for a subset of sites (N=8), there was an inverse pattern, with isotopic depletion in summer, implying shifts to deeper soil water sources or uptake of shallow soil water that was isotopically depleted in summer compared to spring conditions.To further explore these findings, we will visually and statistically examine them using isotope data from the soil (10–90 cm depth). We will analyze the role of climate (using gridded data), alongside site-, soil-, and tree-specific metadata to better understand the factors influencing the variation in root water uptake at the continental scale. Additionally, we will explore the potential of oxygen-17 excess to provide further insights into root water uptake dynamics.

Abstract. This paper presents a time-lapse application of electrical methods (electrical resistivity tomography, ERT; and mise-à-la-masse, MALM) for monitoring plant roots and their activity (root water uptake) during a controlled infiltration experiment. The use of non-invasive geophysical monitoring is of increasing interest as these techniques provide time-lapse imaging of processes that otherwise can only be measured at few specific spatial locations. The experiment here described was conducted in a vineyard in Bordeaux (France) and was focused on the behaviour of two neighbouring grapevines. The joint application of ERT and MALM has several advantages. While ERT in time-lapse mode is sensitive to changes in soil electrical resistivity and thus to the factors controlling it (mainly soil water content, in this context), MALM uses DC current injected into a tree stem to image where the plant root system is in effective electrical contact with the soil at locations that are likely to be the same where root water uptake (RWU) takes place. Thus, ERT and MALM provide complementary information about the root structure and activity. The experiment shows that the region of likely electrical current sources produced by MALM does not change significantly during the infiltration time in spite of the strong changes of electrical resistivity caused by changes in soil water content. Ultimately, the interpretation of the current source distribution strengthened the hypothesis of using current as a proxy for root detection. This fact, together with the evidence that current injection in the soil and in the stem produces totally different voltage patterns, corroborates the idea that this application of MALM highlights the active root density in the soil. When considering the electrical resistivity changes (as measured by ERT) inside the stationary volume of active roots delineated by MALM, the overall tendency is towards a resistivity increase during irrigation time, which can be linked to a decrease in soil water content caused by root water uptake. On the contrary, when considering the soil volume outside the MALM-derived root water uptake region, the electrical resistivity tends to decrease as an effect of soil water content increase caused by the infiltration. The use of a simplified infiltration model confirms at least qualitatively this behaviour. The monitoring results are particularly promising, and the method can be applied to a variety of scales including the laboratory scale where direct evidence of root structure and root water uptake can help corroborate the approach. Once fully validated, the joint use of MALM and ERT can be used as a valuable tool to study the activity of roots under a wide variety of field conditions.