Microorganisms are key to our understanding of early life on land, especially in terms of below-ground processes. In the fossil record, exceptionally preserved silicified systems are the best sources to document the diversity of Fungi and microorganisms and the roles that they played, including their interactions with plants. Continued advances in technology allow us to document these in unprecedent detail at sites like the Rhynie cherts (Scotland, UK), dating to 407 Ma, and the Grand Croix chert (Massif Central, France), dating to ca 307 ̶ 303 Ma. Techniques we used include Confocal Laser Scanning Microscopy (CLSM) and Laser-Induced Breakdown Spectroscopy (LIBS)-based imaging, among others.

In the Rhynie cherts, plant-fungal associations and glomeromycotan spores have been observed as well as other diverse microorganisms colonizing the substrate. In modern vascular plants, endomycorrhizas are typically associated with roots, but most of the vascular plants at Rhynie were rootless, and endomycorrhizas developed in aerial axes. This is probably the plesiomorphic state for land plants. Data from Grand Croix demonstrate that by the end of the Carboniferous, endomycorrhizas had become associated with the root systems of trees. The evolution of the endomycorrhizal symbioses during the Paleozoic, from early plants to trees, is associated with important changes in the nature of the symbiosis, the structure of the soil, and changes in level of carbon dioxide gas in the atmosphere. Using a combination of techniques to decipher the nature of the organisms and their interactions is as an area of developing interest, particularly in the context of recent work on modern relatives.

How to cite: Strullu-Derrien, C., Spencer, A. R. T., Mitchell, R., and Kenrick, P.: Advanced techniques for the study of plant interactions with Fungi and other microorganisms in early environments, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2042, https://doi.org/10.5194/egusphere-egu23-2042, 2023.

Chemical gradients around roots are formed by water uptake and selective uptake of elements and thereby triggered radial transport processes. Gradients on different root segments are expected to vary in magnitude, e.g. root age determines duration of root-soil-contact and thus the dimension of depletion or accumulation zones. Current knowledge with respect to chemical rhizosphere gradients is primarily based on linearized (compartmentalized) or pseudo-linearized (rhizobox) systems, which do not represent the radial geometry of transport to and from roots. Within the DFG-funded Priority Program 2089 we developed a new targeted sampling on undisturbed samples containing different root segments to overcome these shortcomings.

In order to evaluate the temporal change of root system architecture, we apply X-ray computed tomography (X-ray CT) and advanced tools of image analysis and registration, as the direct observation of roots in a 3D system is hindered by the non-transparency of soil.

This allows a targeted sampling of specific root ages/types by extracting intact subsamples (ø 1.6 cm) from larger pots (ø 7 cm), in which the plants were grown. To investigate the influence of soil texture and root age on the formation of chemical gradients, this new subsampling protocol was first tested in a pot experiment with two Zea mays L. genotypes  (the wild-type (WT) and the corresponding mutant defective in root hair elongation (rth3)) grown for three weeks in two different textures (sand vs. loam). Resin embedded subsamples containing either segments of the primary root or young roots were imaged with micro X-ray fluorescence (μXRF) to evaluate element distributions as a function of distance to the root surfaces. First results show a higher precipitation of calcium and sulfur in the vicinity of the primary root than in the vicinity of young roots indicating an age effect. Magnitude and extend of the gradient differs between sand and loam.

How to cite: Lippold, E., Schlüter, S., Kilian, R., Braatz, E., Mikutta, R., and Vetterlein, D.: Spatio-temporal patterns of chemical gradients around roots investigated with µXRF and X-ray CT, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15339, https://doi.org/10.5194/egusphere-egu23-15339, 2023.

Mechanical stress induced by soil compaction affects drastically root system architecture and the morphology of individual root segments. Typical effects of mechanical stress on the root responses include decreased root elongation, increased radial thickening and sloughing of cap cells. Many studies have established a link between the mechanical stress encountered by roots (generated in triaxial cells or by restricting root growth with a barrier) and the gaseous plant hormone ethylene; the data suggests that ethylene acts as an endogenous root growth regulator. To the best of our knowledge however, none of the studies on the ethylene and mechanical stress feedback mechanisms measured ethylene concentrations under realistic soil conditions, i.e., in the soil gas phase and over a long period of time. In this study, we aimed at filling this knowledge gap and set up an experiment which allowed to measure ethylene concentration directly in the soil gas phase with passive diffusive samplers and for maize plants subjected to different levels of soil compaction over a period of 21 days in repacked soil columns in the laboratory. With the help of X-ray computed tomography, we investigated the spatiotemporal patterns of ethylene concentrations in the vicinity of roots, in order to assess which type of roots act as a major source of ethylene in the soil. In accordance with the literature, soil compaction induced a significant increase in ethylene concentration in the soil gas phase, which impacted root growth by reducing significantly the growth of fine roots and increasing the share of thicker roots. A visual inspection of the X-ray CT images at different time points of gas sampling showed that high concentrations of ethylene (i.e., above the third quartile of the distribution) were not strictly ascribable to the abundance or type of roots in the vicinity of a probe. Yet, the highest concentrations of ethylene were recorded on the occasions where roots were present close to a probe. The sampling depths and time of sampling had no or very little effect on the measured ethylene concentrations. Our results suggest that ethylene was diffusing rather homogeneously in the soil columns and that microbial activity was also responsible for a good fraction of the ethylene production. Future experiments are planned to assess the contribution of microbes to the total ethylene production.

How to cite: Phalempin, M., Lippold, E., Brauweiler, F., Apelt, B., Würsig, H., Tarkka, M., Schlüter, S., and Vetterlein, D.: Maize roots under mechanical stress induce increased ethylene concentration in the soil gas phase, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12448, https://doi.org/10.5194/egusphere-egu23-12448, 2023.

Various rhizosphere traits have been explored as plant adaptations to modulate the soil-root interface to acquire resources and to enhance plant water status under stress conditions. Mucilage exudation has been suggested to enable water uptake during soil drying. This hypothesis was tested using artificial root analogy due to technical limitations and the lack of suitable plant materials. Here, we tested whether mucilage exudation facilitates water uptake in intact cowpeas (Vigna unguiculata L.) plants growing in loamy soil during drying. We used a root pressure chamber system to measure the gradients in water potential at the root surface as well as the relationship between transpiration rate and leaf xylem water potential in two genotypes with contrasting mucilage production. Higher mucilage exudation attenuated the drop in matric potential at the root surface. In contrast, the gradients in water potential were much steeper in cowpea with less mucilage production. The attenuation of matric potential at the root surface resulted in a linear relationship between transpiration rate and leaf xylem water potential. We conclude that mucilage exudation maintains the hydraulic continuity between roots and soil and decelerates water potential dissipation near the root surface during soil drying. Our findings provide the first in vivo evidence on the role of mucilage on root water uptake.

How to cite: Abdalla, M. and Ahmed, M.: Mucilage enables water uptake in cowpeas (Vigna unguiculata L.) under soil water deficit, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4892, https://doi.org/10.5194/egusphere-egu23-4892, 2023.

Intercropping management strategy involves the use of legumes alongside a commercial crop to achieve various benefits, such as improved soil nutrient circulation, water retention, and pest control. However, research has shown that there is a lack of understanding of the long-term benefits of legumes within cropping systems and their specific interactions with the soil.

In our experimental setup, we conducted four treatments using soil columns that were 24 cm in diameter and 1 m in length. We grew two winter rapeseeds as a monocrop, an intercrop of one rapeseed and one faba-bean, and two faba-beans as a monocrop in each soil column. We killed the faba-bean during the winter frost and left it as green mulch in the rapeseed intercrop. The final treatment was bare soil control columns. We measured soil nitrogen, the biomass, and nitrogen content of living plants and plant litter in September, October, November, and June. Additionally, we studied the decomposition of rapeseed and faba-bean residues in the soil in a laboratory incubation experiment to measure carbon and nitrogen mineralization.

The majority of mineral nitrogen leaching occurred during late autumn at the beginning of the growing season (September to October) in all treatments. The soil mineral nitrogen contents over the growing season for the rapeseed-faba-bean intercrop system were similar to the rapeseed monocrop system, but lower than in the faba-bean monocrop system. The nitrogen balance in the columns for each treatment revealed that bare soil lost the most nitrogen over time due to leaching and lack of plants to uptake mineral nitrogen and immobilize it as biomass. The second one which lost the most nitrogen by leaching is The faba-bean monocrop. In this treatment, the plant did not use soil nitrogen in the lower portions of the soil column and this portion of the nitrogen in the soil may have been lost by leaching. The nitrogen was better conserved in the soil column and in the plants in the treatments with rapeseed intercrop and monocrop. Soil nitrogen was removed by the plant more efficiently leading to less leaching.

The nitrogen content and biomass of one rapeseed plant in the intercrop was nearly double the one rapeseed plant in monocrop. Indeed, the total biomass and nitrogen content of the two rapeseeds in monocrop was equivalent to the single rapeseed in the intercrop. Conversely, rapeseed mulch had less nitrogen in intercrops than in the monocrop system.

Lastly, the incubation of crop residues initially immobilized soil mineral nitrogen. The faba-bean mulch started releasing more mineral nitrogen than the bare soil after day 70. The release of mineral nitrogen of rapeseed and rapeseed-faba-bean mulch mixture exceeded the nitrogen of bare soil after day 90.

Overall, it is clear that intercropping with legumes can have positive effects on soil nitrogen and plant growth, but more research is needed to fully understand the long-term benefits and interactions between legumes, the soil, and commercial crops.

How to cite: Al Naemi, M., Garnier, P., Jullien, A., and Richard-Molard, C.: Effects of Winter Rapeseed - Faba-bean intercrop and litter mulch on soil Nitrogen , EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14362, https://doi.org/10.5194/egusphere-egu23-14362, 2023.

In a future CO₂ rich world, nitrogen (N) limitation is projected to decrease the CO₂ fertilization effect limiting the ability of temperate forests to mitigate climate change. There are limited direct measurements of roots showing if mature trees are able to increase their N uptake in a future climate. Additionally, it is not currently understood if roots of mature trees can change their N form preference under elevated CO₂ to maintain or enhance their N uptake. These are all gaps in knowledge identified as adding uncertainty to modelling efforts assessing ecosystem response to climate change. We quantified the rate of N uptake of living mature oak tree roots (Quercus robur) and their preference of N forms. On two occasions (July and November 2022) we carefully excavated live oak roots of three mature trees in each of the six experimental Free Air Carbon Enrichment (FACE) arrays (three ambient and three +150 ppm CO2) at the BIFoR FACE facility located in Staffordshire (United Kingdom). The live roots were cleaned and pre-incubated in acid washed sand and a nutrient solution for 24 hours to establish an acclimatized baseline condition following excavation. The roots were then exposed to a mix of inorganic and organic N forms where only one form was labelled in each treatment (¹⁵N-nitrate, ¹⁵N-ammonium, a mix of 20 ¹⁵N labelled amino acids and an unlabelled control) to elucidate N preferences and rate of uptake during a two-hour incubation period. By analysing the root tissue for ¹⁵N our findings will investigate the preferences and uptake rates of N by mature trees under elevated CO₂. We hope to shed light on these mechanisms mediating N uptake of mature trees to explain how mature forest stands respond to climate change in a temperate climate.    

How to cite: Pihlblad, J., Hamilton, R. L., Rumeau, M., Sayer, E. J., Hartley, I. P., and Ullah, S.: Root soil Nitrogen acquisition by mature Oak trees exposed to elevated CO2: Nitrogen preference and uptake rate under a future climate, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15808, https://doi.org/10.5194/egusphere-egu23-15808, 2023.

As safety mechanism during droughts, plants close their stomata to reduce transpiration losses and prevent excessively negative water potentials. Although the coordination between stomatal closure, xylem vulnerability and leaf traits has been extensively investigated, the role of soil hydraulic limitation remains elusive. Here, we test the hypothesis that stomatal closure is triggered by the loss of soil hydraulic conductivity in the root zone. As the soil hydraulic conductivity is a function of soil texture, we further hypothesize that stomatal closure, and more precisely the relation between stomatal conductance and leaf water potential, are soil texture dependent. An alternative hypothesis is that the shoot to root ratio adapts to the specific soil conditions, and it is lower in soils with low hydraulic conductivity. We compared three field sites in an inner alpine valley in Switzerland (yearly rainfall of 600 mm) with the same oak tree species (Quercus pubescens) and varying soil textures. We used the model of Carminati and Javaux (2020), which predicts that the decline in transpiration rate is controlled by the soil hydraulic conductivity, and, consequently, it is more abrupt in 1) coarse textured soils compared to fine textured soils and 2) in plants with high shoot to root ratio.

To test these working hypotheses, stem water status and soil matric potential were measured at various depths of three oak sites. The soil matric potentials were then linked to the hydraulic conductivity and soil water content using a new pedotransfer function (PTF) for forest soils, which overcomes the limits of existing PTFs that were trained for arable soils. A simple water balance model based on changes in water content (deduced from PTF and measured potentials) was used to calculate transpiration rates and was compared with sap flow measurements conducted on two trees per site. Leaf water potentials were estimated from dendrometers after calibration with pressure chamber measurements of leaves. The sap flow measurements correlate well with estimated transpiration rates (R²=0.7).

Comparisons between sites show a similar decrease in stomatal conductance at all three sites during drought, regardless of soil texture. Rather, our data suggest the trees hydraulically adapt to the local soil texture by adjusting their leaf area index and root length density to the local water demand and supply. In coarse-textured soils, oak had a low leaf area index, reducing their water demand, and high fine root length density, increasing their supply. In conclusion, our study suggests a hydraulic adaptation of trees to their local soil texture by adapting their “shoot to root ratio” as quantified here by leaf area index and root density.

How to cite: Schoch, J., Walthert, L., Lehmann, P., Unverricht, P., and Carminati, A.: The Effects of Soil Texture on Transpiration Losses During Droughts: A Field Study of Three Oak Forests in an Inner Alpine Valley of Switzerland, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7635, https://doi.org/10.5194/egusphere-egu23-7635, 2023.

Individual plants vary in their ability to respond to environmental changes. The plastic response of a plant enhances its ability to avoid environmental constraints, and hence supports growth, reproduction, and evolutionary and agricultural success.

Major progress in the analysis of above- and belowground processes on individual plants has been made by the application of non-invasive imaging methods including Magnetic Resonance Imaging (MRI) and Positron Emission Tomography (PET).

MRI allows for repetitive measurements of roots growing in soil and facilitates quantification of root system architecture traits in 3D. PET, on the other hand, opens a door to analyze dynamic physiological processes in plants such as long-distance carbon transport in a repeatable manner. Combining MRI with PET enables monitoring of short livedCarbon tracer (¹¹C) allocation along the transport paths (i.e. roots visualized by MRI) into active sink structures.

To analyse the link between root-internal C allocation patterns and C metabolism in the rhizosphere, we are combining ¹¹CO₂ with stable ¹³CO₂ labelling of plants. Isotope ratio mass spectrometry (IRMS) analyses of rhizosphere soil is applied to link root-internal C allocation patterns with distribution of ¹³C in the rhizosphere soil. The metabolically active rhizosphere organisms are subsequently identified based on DNA ¹³C stable isotope probing.

In our presentation we will highlight our approaches for gathering quantitative data from both image-based technologies in combination with destructive analysis that provides insights into the functioning and dynamics of C transport processes in the plant-soil system.

How to cite: Koller, R., Huber, G., Pflugfelder, D., van Dusschoten, D., Hinz, C., Schultes, S., Chlubek, A., Knief, C., and Metzner, R.: Monitoring spatial and temporal carbon dynamics in the plant soil system by co-registration of Magnetic Resonance Imaging and Positron Emission Tomography for image guided sampling, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13308, https://doi.org/10.5194/egusphere-egu23-13308, 2023.

Plant roots exposed to water scarcity respond by modulating root functional traits, such as deep and prolific root growth, to maximize resource acquisition. Such adjustments, which include the alterations of root morphology and anatomy to optimize water uptake and/or maximize root survival, may also increase the carbon demand for soil exploration. But this altered carbon allocation to foster belowground resource acquisition limits aboveground plant development. In the present study we investigated the relationship between root physiology, root trait plasticity and whole plant growth under drought stress. We quantified shoot and root traits of nine contrasting spring barley cultivars grown in soil-filled rhizoboxes under well-watered (4 weeks, 55% of field capacity) or drought conditions (2 weeks, 55% of field capacity + 2 weeks without water). Time-lapse imaging was applied to quantify root and shoot growth rates, and combined with measurements of root distribution, morphology, anatomy as well as mycorrhizal colonization. Aboveground traits had a strong and uniform response to drought compared to belowground traits. Root traits’ plasticity were variable and differed among cultivars. The differences between cultivars were particularly pronounced for the proportion of root length and root biomass in deep soil layers as well as changes in root morphological and anatomical traits. We suggest that cultivars characterized by an increase in root conduits, a greater hierarchical structure and a reduction of specific root length and area may be good candidates to promote hydraulic lift while lowering carbon cost for root growth. Increased root length and depth, root density and specific root length in drought condition are different cultivars’ strategies that may promote soil exploration and optimize water uptake. This is directly linked to the interplay of above and belowground carbon investment, with some cultivars yielding both a high shoot biomass and enhanced resource acquisition. Based on our findings, testing new agronomic strategies to mobilize the diversity of cultivars could be key to enhance drought resistance and resilience of barley cropping systems.

How to cite: Germon, A., Colombi, T., Müller-Stöver, D., Keller, T., and Müller, C.: How to invest carbon under drought, different strategies in early root system development among barley cultivars, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12222, https://doi.org/10.5194/egusphere-egu23-12222, 2023.

Their flexible root growth provides plants with a strong ability to adapt and develop resilience to droughts and climate change. But this adaptability is badly included in crop- and climate models. Most of them rely on a simplified representation of root growth, independent of soil moisture availability. To model plant development in changing environments, we need to include the survival strategies of plants, but data of subsurface processes and interactions, needed for model set-up and validation, are scarce.

Here we investigated soil moisture driven root growth. To this end we installed subsurface drip lines and small soil moisture sensors (0.2 L measurement volume) inside rhizoboxes (length x width x height, 45 x 7.5 x 45cm). The development of the vertical soil moisture and root growth profiles are tracked with a high spatial and temporal resolution.

The results confirm that root growth is predominantly driven by vertical soil moisture distribution, while influencing soil moisture at the same time. Besides support for the functional relationship between the soil moisture and the root density growth rate, the experiments also suggest that vertical root growth stops when the soil moisture at the root tip drops below a threshold value. We show that even a parsimonious one-dimensional water balance model, driven by the measured water input and output fluxes, can be convincingly improved by implementing root growth driven by soil moisture availability. The model performance suggests that soil moisture is a key parameter determining root growth.

How to cite: Maan, C., ten Veldhuis, M.-C., and van de Wiel, B.: Dynamic root growth in response to depth-varying soil moisture availability: a rhizobox study, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17337, https://doi.org/10.5194/egusphere-egu23-17337, 2023.

Transpiration response of plants during both soil drying and increasing vapor pressure deficit (VPD) have been thoroughly studied separately. However, the interactive effects of both on soil-plant hydraulics remain largely unknown. In this study, we tested the combined effects of soil and atmospheric drying on soil-plant hydraulics of sorghum.

Sorghum plants were grown in sandy soil under well-watered conditions with a daily VPD increment, increasing in five steps from 0.5 to 3.7 kPa. After 30 days, the soil was dried over five days. We measured transpiration rate (E), soil water content (θ), soil and leaf water potential (ψsoil, ψleaf) both under wet soil and during soil drying. A soil-plant hydraulic model was used to reproduce the data and provide further insight to disentangle soil and atmospheric effects.

Both soil drying and VPD affected the relation between transpiration rate and leaf water potential. In wet soil conditions, the E (ψleaf-x) relation was linear even at high VPD. During soil drying, this relation was linear in relatively low VPD conditions (0.5 – 2.5 kPa) but exhibited a non-linear relation under relatively high VPD (2.5 – 3.7 kPa). This response was also reflected in a breakpoint of soil-plant conductance at around 2.5 kPa VPD, resulting in a decrease in transpiration. 

We conclude that decreasing soil water status has a stronger impact on soil-plant conductance and water uptake than increasing VPD. Furthermore, the modeling revealed the importance of understanding how soil parameters are changed by the presence of plants, especially during soil drying. We suggest that for a holistic understanding of plant response to drought, more emphasis would need to be given to the interactions between VPD and soil drying, as the effects of VPD become increasingly important with soil drying.

How to cite: Sauer, A. M., Abdalla, M., Wankmüller, F. J. P., and Ahmed, M. A.: Coupled effects of soil and atmospheric drying on soil-plant hydraulics of sorghum, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9846, https://doi.org/10.5194/egusphere-egu23-9846, 2023.

Plant water use during drought depends on atmospheric demand and soil water supply. Typically, transpiration response to drought is evaluated in two types of experiments: either exposure to a stepwise increase in vapor pressure deficit (VPD), or exposure to soil drying over the course of weeks. Surprisingly however, the extent of similarities and differences of the underlying mechanisms remains poorly documented. This hampers progress towards breeding for well-adapted crops targeting environments with high VPD, high risk of soil moisture deficit, or both. We present an extensive review of the two experimental approaches and use a soil-plant hydraulic model to simulate transpiration responses to both environmental drivers. Existing experimental results lead to contradicting results regarding the role of the plant hydraulic conductance for the transpiration response to atmospheric drying vs. to soil drying: a high plant hydraulic conductance triggers an earlier transpiration decline (i.e. in wetter soil conditions) during soil drying; but enables plants to sustain transpiration at high VPD. A hydraulic framework hypothesizing that transpiration responds to a decline in soil-plant conductance helps to explain the contradiction. At high VPD, water potential gradients mainly develop within the plant, and thus it is the plant hydraulic conductance that limits the water flow during atmospheric drying. During soil drying, the gradients develop in the soil, and thus the soil hydraulic conductivity controls the flow. The plant hydraulic conductance is expected to impact the plant’s sensitivity to the development of water potential gradients around the roots that occurs during soil drying. Thus, stomatal closure and hence transpiration response is related to a drop in hydraulic conductivities in both scenarios but the relevant hydraulic traits differ between the two environmental changes in a predictable way. Such a finding could better guide breeding efforts targeting adaptation to specific drought regimes.

How to cite: Köhler, T., Wankmüller, F. J. P., Sadok, W., and Carminati, A.: Transpiration response to atmospheric drying vs. to soil drying: underlying physical and physiological mechanisms and related plant traits, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1150, https://doi.org/10.5194/egusphere-egu23-1150, 2023.

Typically, root water uptake (RWU or U_tot) is said to be driven by transpiration (Tr). It is however more accurate to state that transpiration causes a reduction in leaf water content that reduces the leaf water potential such that a water potential gradient builds up between leafs and soil water, such that water can be extracted from the soil. For herbaceous plants, the amount of water that is hereby lost is typically assumed to be negligible so the plant can be treated as a resistive system. In how far this is true is open to discussion as quantifying shoot water changes is not easily feasible, especially when the soil-root system is drying out. A balance cannot observe water moving between the soil and the shoot and shoots have empty spaces such that 3D cameras provide an incomplete picture. Shoot weight determination requires that the amount of soil water is independently assessed to discriminate between the two pools of water. This can be achieved when a balance is combined with a Soil Water Profiler (SWaP) on the same soil-plant system. The precision of the SWaP is comparable to that of an expensive balance (<10mg for a 6kg system).

Here we performed experiments with the SWaP – balance combination under modulated light with progressive soil dehydration for sunflower and faba bean (N=4). Our data shows that transpiration precedes U_tot by about 5 to 10 mins under wet conditions (pF~2) and U_tot can exceed Tr by up to 20%. Gradually, with decreasing soil water content we find that U_tot becomes smaller than Tr and at the same time the delay between Tr and U_tot increases. For pF>3.5 most of the transpired water stems from the shoot, not from root water uptake, indicating that Tr is a poor proxy for RWU for pot experiments where soil is drying at a rate of ~5% per day at well watered conditions. This is very important for calculations of root conductance during drying scenarios. We found significant differences between sunflower sensitivity to soil drying as compared to faba beans that are somewhat more sensitive. We also present data that shows that the delay between Tr and local water uptake is rather dependent on depth and not so much dependent on local pF, which is typically lower for shallow sections of the pot. This may potentially be explained by loss of root water when Tr increases with light, analogous to shoot water losses.

The combination of the SWaP and gravimetric methods opens up a new way of looking at root water uptake as driven by transpiration and shoot water loss dynamics as it provides hitherto inaccessible information about these processes.

How to cite: van Dusschoten, D., Pflugfelder, D., and Kochs, J.: Root water uptake in relation to plant transpiration, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12018, https://doi.org/10.5194/egusphere-egu23-12018, 2023.

The relation between plant transpiration rate (E) and leaf water potential (LWP) is a function of both soil and plant hydraulics and can be affected by local rhizosphere processes. Measuring these very localized processes remains a huge challenge, while observing their impact on the E-LWP relationship is easy. Therefore, the underlying mechanisms of how these processes impact root water uptake (RWU) and whether it is soil texture specific remain unknown. In this study we used a 3-D detailed functional-structural root-soil model to investigate how root and rhizosphere hydraulics control the E-LWP relationship for two maize genotypes (with and without root hairs) grown in two soil types (loam and sand) during soil drying. We assumed that the rhizosphere hydraulic resistance can be taken into account via two processes: (1) a drop in soil water potential between the bulk soil and the soil-root interface and (2) a partial soil-root contact. The simulations revealed that the key process controlling the uptake was soil-dependent. In loam, a drop in soil water potential between the bulk soil and the soil-root interface affected the uptake and RWU started to be limited below soil water potential of -610 hPa. In sand, however, the poor soil-root contact was the main constraint, and the rhizosphere conductance limited RWU at much higher soil water potential (around -90 hPa). In contrast to effective models, our explicit three-dimensional simulations provide exact location and the main driver (root or rhizosphere) of the water RWU distribution patterns as well as the quantification of the active root surface ratio for RWU.

How to cite: Axelle, K., Cai, G., Meunier, F., Ahmed, M. A., and Javaux, M.: Explicit 3D modelling of the rhizosphere processes at plant scale demonstrates the impact of soil texture on root water uptake, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15906, https://doi.org/10.5194/egusphere-egu23-15906, 2023.

The current acceleration of climate change in Europe makes it essential to assess spatially the impact of drought and heat waves on forest disturbances risk (mortality, wildfire risk, etc…). Recent studies have shown that hydraulic failure is a key driver of forest disturbances. Hydraulic failure can be modelled with state-of-the-art plant hydraulic models that are driven by climate data and different traits including (i) hydraulic traits (such as xylem cavitation resistance and stomatal regulation), (ii) leaf area index and (iii) total soil water capacity. Among these traits soil water capacity is highly sensitive, but is poorly available at large scale.
In this study we used the process based plant hydraulic model SUREAU (Cochard et al., 2021; Ruffault et al 2022) to estimate hydraulic failure risk for forest at the European scale for the last 3 decades. To initialize the model we used spatialized  climate (ERA5), LAI data (from Copernicus remote sensing) and land cover (ESA CCI). Species hydraulic traits for major European species were extracted from global databases. In order to initialize the total soil water capacity at European scale and compensate the lack of soil water data, we developed an algorithm of model inversion based on ecohydrological assumption. The ecohydrological assumption is that forest adjust their total available water capacity through rooting depth, for a given climate, traits combination and Leaf area index to maintain a low embolism rate under normal conditions (excluding extreme drought). Our simulation approach simulations allowed to spatialized forest vulnerability to drought and to map total soil water capacity under forest stands.

How to cite: Druel, A., Martins, N., Cochard, H., DeCaceres, M., Delzon, S., Mencuccini, M., Torres-Ruiz, J., and Ruffault, J.: European forest vulnerability to hydraulic failure: an ecohydrological approach, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17068, https://doi.org/10.5194/egusphere-egu23-17068, 2023.

Sufficient water supply to the roots is needed to sustain transpiration demand.  However, detailed measurements of water fluxes into the roots and the gradients in water potential driving these fluxes are rare, particularly in the field, mainly due to the difficulty in accessing roots and performing measurements on them. As a result, field monitoring of tree water use often neglects the root system, and the water fluxes from the soil to the shoot of trees remain a frontier in soil plant water relations.  This measurement gap and lack of understanding of water fluxes in roots makes it difficult to properly determine what drives root water uptake and how it might vary depending on rooting depth, soil matric potential and transpiration rate in the field.

During dry summer 2022, we equipped beech and spruce trees with sap flow sensors and dendrometers on the stem and on roots accessing different soil depths. This allowed us to monitor water fluxes and potentials in the different plant parts, from the integrated fluxes in the stem to the water uptake of single roots. We conducted the measurements at the “Waldlabor” ecoydrological monitoring forest in Zurich, where we comprehensively monitor the soil-plant-atmosphere continuum of beech and spruce with dendrometers, sap flow sensors and frequently measured stem & leaf water potential as well as stomatal conductance. To uncover the roots with minimal disturbance we removed the soil with a special air pressurizer and vacuum pump that allowed soil removal without damaging the roots, installed sensors at roots accessing different soil depths and afterwards covered the roots with soil again. Soil matric potential was measured at 10, 20, 40 and 80 cm depth in proximity to selected roots. At the canopy level, we measured stomatal conductance and leaf water potential throughout the summer.

Here, we demonstrate how the collected data help to understand to which extent trees diversify their water uptake depending on water availability at different soil depths. Because of the scarce precipitation and the limiting soil water availability during the summer, pre-dawn leaf water potential, stomatal conductance as well as sap flow decreased, indicating a reduction in transpiration. Beech trees reduced their stomatal conductance more dramatically than spruce trees, thereby using soil water more quickly. The comparison of sap flow in the roots and the integrated signal measured along the stem reveals differences between roots, with roots accessing deeper soil upholding higher sap flow velocities than roots accessing shallow soil in both species.

The results allow to assess the interplay between aboveground tree hydraulics and water status and belowground water uptake for beech and spruce under drying conditions.

How to cite: Martinetti, S., Floriancic, M., Carminati, A., and Molnar, P.: Monitoring root water uptake for understanding tree water use dynamics during dry periods, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13045, https://doi.org/10.5194/egusphere-egu23-13045, 2023.

Climate change impacts on forests cannot be understood without representing the hydraulic functioning of forests. In this work we present SoVegI (Soil-Vegetation Interaction model), a numerically efficient, process-based model of the soil-plant-atmosphere continuum, developed to represent groundwater-forest interactions under drought conditions for broadleaf and deciduous forests of Europe.

The model includes (1) a single layer sun/shade model of mass and energy fluxes at the canopy scale, (2) a stomatal conductance model depending, among other things, on leaf water potential describing the direct link with soil water availability, (3) a process-based soil-root-xylem hydraulic transport scheme assuming the hydraulic transport to be analogous with water transport in a porous media, and (4) a root water uptake model representing the direct coupling between the soil and the vegetation.

The novelty of the model is to present a fast and efficient numerical implementation of the hydraulic transport process within the whole soil-plant-atmosphere continuum that allows coupling with large spatial models. The porous media analogy results in a set of three coupled nonlinear partial differential equations similar to Richards’ equation for porous media in soil. The system is solved using a finite volume, time-implicit approach and an advanced iterative scheme is used to treat the non-linearity of the system.

This new numerical model was successfully tested at the tree and the forest scales at two sites in northern France after being calibrated against sap flow measurements and eddy covariance data, respectively. At the tree scale, the model was able to reproduce the mid-day partial stomatal closure showing the availability of the model to catch the dynamic feedback between the atmospheric and soil water conditions. The model was also capable of reproducing the water storage pool drainage during day time and the night time replenishment, opening interesting perspectives to investigate forests’ risks to hydraulic failure. At the forest scale, the model was able to reproduce the transpiration response to the 2003 soil drought and heat wave in northern Europe with limited computational efforts. These preliminary steps open interesting research perspectives where SoVegI will be coupled with a physically-based integrated hydrologic model to assess the impact of extreme events on groundwater-forest relations.

How to cite: Corvi, O., Weill, S., Belfort, B., Ackerer, P., Bonal, D., and Cuntz, M.: SoVegI: a new and efficient model coupling photosynthesis and hydraulic transport within the soil-plant continuum, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-983, https://doi.org/10.5194/egusphere-egu23-983, 2023.

Root water uptake is a central component in the modulation of water transport in the soil-plant-atmosphere continuum. The mechanistic description of this process, based on root hydraulics, is needed for improving predictions of water fluxes at plant, field or regional scales, and for increasing our understanding of the environmental conditions and vegetation properties affecting it. Functional-structural models can be used for this purpose, but they depend on the availability of accurate data on root hydraulic properties for their parametrization. 

Here, we present an open access root hydraulic properties database obtained from an extensive literature review of more than 200 studies published between 1973–2022. This includes measurements of the radial conductivity and the axial conductance of root segments and individual roots, as well as of the resulting conductance of the whole root system for multiple species, plant functional types (PFT’s) and experimental treatments. To our knowledge, this is the most extensive root hydraulic properties database that has been compiled.

The database shows a very large range of variation in reported root hydraulic properties, which cannot be explained by systematic differences among PFT’s or species, alone, but rather by factors such as root system age, experimental treatments or the driving force used for measurement (hydrostatic or osmotic). Based on these observations, we used the functional-structural model CPlantBox to explore the relationship between root system age and whole root system conductance in more detail, using crop species as an example. For this, both the data needed for model parametrization (hydraulic properties of root segments) and validation (root system conductance) were extracted from the database. The results indicate a decrease in the total conductance per unit root surface area at later stages of development, which could be associated with a larger proportion of less conductive old root tissues.

This analysis exemplifies the importance of the root hydraulic database in two fronts: (1) it serves as a link between experimental data and functional-structural models; and (2) it facilitates the mechanistic description of the factors affecting root hydraulic properties across species and under contrasting environmental conditions.

How to cite: Baca Cabrera, J., Vanderborght, J., and Lobet, G.: A root hydraulic properties database: the link between experimental data and functional-structural models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10161, https://doi.org/10.5194/egusphere-egu23-10161, 2023.

Soil-root hydraulic resistance variation and stomatal regulation are two critical hydrophysiological responses of plants to drought stress; however, few studies have been developed to quantify their interactions. To fill this gap, we developed a soil-plant hydraulic model (SR-HRV) that attempts to characterize the effects of stomatal regulation and three universal soil-root hydraulic resistance variations, i.e., root aquaporins promotion (AQU), apoplastic path damage (APD), and root-soil contact loosening (CONTACT). The sensitive parameters of the SR-HRV model were analyzed and optimized based on a field experiment with sunflower plants (Helianthus annuus L.). Several simulation scenarios were designed to clarify the individual and interactive effects of soil-root hydraulic resistance variations for plants with different stomatal sensitivities. Results show that the sensitivity of simulated stomatal conductance and soil water content response to stomatal regulation parameters, especially to abscisic acid-related parameters, are more active than to soil-root hydraulic resistance variation parameters. But as the soil dries, the sensitivities to APD and CONTACT parameters are rapidly increased. The simulation demonstrates that AQP alleviates the leaf water potential drop-down and maintains relatively high root water absorption of the plant when it is in mild drought conditions, while CONTACT and APD respectively restrict the water flux and drought signal responses with continuous soil dehydration. Moreover, the AQP effects are more pronounced but the effects of APD and CONTACT would be restricted for plants with higher stomatal sensitivity to drought signals. These simulation results imply the diverse response strategies of plants to drought, the collaborations between stomatal regulation and soil-root hydraulic resistance variations should be considered in soil-plant water transport modeling.

How to cite: Lei, G., Zeng, W., Ao, C., Dong, L., Liu, S., and Ren, Z.: Relating Soil-Root Hydraulic Resistance Variation to Stomatal Regulation in Soil-Plant Water Transport Modeling, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4637, https://doi.org/10.5194/egusphere-egu23-4637, 2023.

To describe plant transpiration in drying soil, several models use ‘α-stress functions’, which represent the ratio of the maximal possible water uptake when the plant reaches the wilting point to the transpiration demand or potential transpiration, as a function of the soil water potential. Water potentials vary within the root zone, and the plant ‘senses’ with its root system an average root zone water potential and redistributes the uptake from drier to wetter zones in the root zone. This redistribution or root water uptake compensation is accounted for using an average stress index, ω, which is a weighted average of the local stress indices α at different depths in the root zone, and a critical stress index ω_c (Jarvis, 2011; Simunek & Hopmans, 2009). When ω > ω_c, root water uptake is equal to the potential root water uptake or the energy limited potential transpiration. The α-ω approach refers to a mechanistic description of water fluxes in the soil-root system but remains semi-empirical missing a direct link with soil and, especially, root hydraulic properties. In this contribution, we derive the α-ω approach starting from a mechanistic description of water flow in a hydraulic root architecture assuming that resistance to flow in the soil towards the soil-root interface can be neglected. In a second step, we include the non-linear soil resistance.

For relatively wet soil conditions and neglecting the soil resistance, root water uptake functions can be cast in a form that is identical to the α-ω approach that was derived by Jarvis (2011), but for opposite conditions, i.e., Jarvis neglected the root resistance compared to soil resistance. Following Jarvis, the α-function should be interpreted as the ratio of the maximal possible uptake by the root system for a certain soil water potential to the maximal possible uptake by the system when the soil is fully saturated, which differs from its common interpretation. This means that the α-function is just a linear function that ranges from zero when the soil water potential is equal to the wilting point to 1 when the soil water potential is zero and that it is independent of the transpiration rate. Another outcome is that the critical stress level ω_c is inverse proportional to the hydraulic conductance of the root system and is not a constant but a variable parameter that is proportional to the transpiration rate. For dry soil conditions, when soil resistance is important, we find that α and ω are non-linear functions of the soil water potential. Using α and ω functions that are derived from soil and root hydraulic properties, the uptake distributions can be calculated directly from the soil water potentials without solving a non-linear equation with iterations to derive water potentials in the plant. But, this approach is based on a simplification, which requires further testing.

Jarvis, N. J. (2011). Hydrology and Earth System Sciences, 15(11), 3431-3446. doi:10.5194/hess-15-3431-2011

Simunek, J., & Hopmans, J. W. (2009). Ecological Modelling, 220(4), 505-521. doi:10.1016/j.ecolmodel.2008.11.004

How to cite: Vanderborght, J., Schnepf, A., Leitner, D., Couvreur, V., and Javaux, M.: A mechanistic derivation of 'alpha-omega' root water uptake models., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3484, https://doi.org/10.5194/egusphere-egu23-3484, 2023.

Schnepf et al., (2020) defined benchmark scenarios for root growth models, soil water flow models, root water flow models, and for water flow in the coupled soil-root system. All benchmarks and corresponding reference solutions were published in the form of Jupyter Notebooks on the GitHub repository https://github.com/RSAbenchmarks/collaborative-comparison. Several groups of functional-structural model developers have joined this benchmarking activity and provided the results of their individual implementations of the different scenarios.

The focus of this contribution is on water uptake from a drying soil by a static root architecture. The numerical solutions of the different participating simulators as compared to the provided reference solution.

The participating simulators are CPlantBox, DuMu^x, R-SWMS, OpenSimRoot and SRI. They have in common that they simulate water flow in the 3D soil domain, water flow inside the root system that is represented as a mathematical tree graph, and the coupling between the two domains in form of a volumetric sink term that describes the transfer of water between the two domains. The simulators differ in the numerical schemes used for solving the water flow equations in roots and soil domains, as well as in the way the sink term is formulated, in particular in the way the possibly increased rhizosphere resistance to water flow is accounted for. 

The results to the water flow in soil benchmarks show how the different simulators perform against the analytical solution to a problem of infiltration into an initially dry soil, as well as a problem of evaporation from initially moist soil. All of the simulators could accurately predict the infiltration front in different soil types as well as the actual evaporation curves.

The coupled problem of root water uptake by a static root architecture from an initially already dry soil posed a bigger challenge to the different simulators and revealed some diversity between the different solutions. The Benchmark with an initially rather dry soil defined a potential transpiration that immediately induced water stress of the plant. The simulators had to simulate the consequent rhizosphere drying and associated increase in rhizosphere resistance. All of the soil simulators smoothed the gradients in the rhizosphere at the soil grid size such that root water uptake was significantly overestimated unless the rhizosphere resistance was explicitly accounted for in the root water uptake model. As a result, all simulators came close to the reference solution (that itself is a numerical solution, see Schnepf et al. 2020 for details). 

In this study, we showed that all simulators are generally able to solve the benchmark problems but minor differences occur amongst the simulators when simulating different soil types. Benchmarking led to model improvements and helped interpret model results in a more informed way. The availability of “reference solutions“ made modellers aware of the range of validity of their numerical solution and encouraged them to improve either their numerical solution or to introduce new processes Future efforts may aim to extend the benchmarks from water flow to further processes, such as solute transport or rhizodeposition.

Schnepf et al., 2020, Front. Plant Sci. 11

How to cite: Schnepf, A., Black, C. K., Couvreur, V., Delory, B. M., Doussan, C., Heymans, A., Javaux, M., Khare, D., Koch, A., Koch, T., Kuppe, C. W., Landl, M., Leitner, D., Lobet, G., Meunier, F., Postma, J., Schäfer, E., Selzner, T., Vanderborght, J., and Vereecken, H.: Benchmarking of Functional-Structural Root Architecture Models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4425, https://doi.org/10.5194/egusphere-egu23-4425, 2023.