Water dynamics in the soil-plant continuum: which features regulate the uptake?

How, why, and what water flows through the soil-plant continuum are quite complex questions that are not yet well understood quantitatively. Soil and plant-induced heterogeneity, soil evaporation, and root water uptake are some of the main controlling factors of water flow dynamics in the soil-plant continuum. Coupling these processes is thus of quite importance to advance our understanding of subsurface mixing and soil-plant interaction and, especially, water sources used by trees. In this study, we combine hydrological and stable water isotopes (2H and 18O) field data in an integrated flow and transport model to investigate which water sources are used up by trees under different wetness conditions.We conducted a field experiment on two sets of three Scots pine trees (Pinus sylvestris) in a forested plot within the Vallcebre research catchments (NE Spain). The experiment was carried out from May to September 2022. We monitored throughfall, sap flow, and dial stem diameter variation, as well as soil water potential and soil water content (in vertical profiles down to 70cm) at high temporal (5min) resolution. Furthermore, we sampled weekly water from the different water pools (throughfall, soil water (bulk and mobile), groundwater, and xylem water (twigs)) for isotopic analysis. The analysis of these data helped in clarifying the interaction between the different water pools and the effect of soil water potential and soil water content dynamics on the isotopic signals in the soil-plant continuum.To further analyze the field data, we developed a numerical model using R-SWMS to simulate the flow in the vadose zone by solving Richards equation coupled with root water uptake, soil evaporation, and isotopic fractionation. To achieve this, we created a 3-D heterogeneous soil matrix that contains a root system. Field data (soil water retention and conductivity curves, initial water content, environmental conditions) from this and previous studies conducted in the catchment were used as the input data. The root system and its hydraulic properties were determined from theoretical values from literature. The isotopic fractionation during evaporation was modelled using the Craig-Gordon model. The model was used to estimate root water uptake distribution, soil water potential, soil water content, and isotopic composition distribution. 

Four different soil types including red, alluvial, calcareous, and black soils along with rice cultivated on them were collected from various parts of India and analyzed for potassium dynamics in the soil plant continuum. Soil potassium (K) dynamics were studied under submerged and non-submerged conditions, and potassium content was analyzed in rice roots, shoots, and grains, along with other soil properties. Red (S1: 5.9) and alluvial (S5: 5.16) soils were moderately acidic, while black (S8: 8.01) and calcareous (S7: 8.1) soils were alkaline. Black soil (S8) had the highest cation exchange capacity (CEC: 31.25 cmol (p+)/kg) and clay content (41.2%), while alluvial soil had the most organic carbon (S5: 1.74%). Submerged conditions enhanced potassium availability, with red soil showing the highest levels of water-soluble K (WsK), exchangeable K (ExK), and non-exchangeable K (NEK), particularly Step-K and constant rate K (CR-K) forms. Rice potassium content was highest in grains, followed by shoots and roots, with red soil containing the most available potassium. A strong correlation was found between soil potassium forms and rice plant potassium uptake. Sensitivity analysis indicated that WsK and ExK from non-submerged soil to be the most favorable forms for potassium uptake, especially in the rice roots and grains. Machine learning models, particularly Random Forest, accurately predicted potassium availability and uptake, highlighting their potential in optimizing soil fertility and advancing precision agriculture for better crop yields and soil health.

Soil water availability is a critical factor in determining how plants regulate their water relations, with drying soils imposing hydraulic constraints that affect root water uptake and stomatal behavior. As soils dry, their hydraulic conductivity is reduced, limiting water movement to the roots and ultimately impacting the flow of water within the soil-plant continuum. When root water uptake exceeds the flow rate allowed by the bulk soil, transpiration cannot be sustained for long. In theory, the critical point when root water uptake is no longer matched by soil water flow should be concomitant with a local depletion of water in the rhizosphere. However, such local depletion has never been observed.In this study, we used a time-series neutron radiography performed at the ICON beamline of the Paul Scherrer Institute (Villigen PSI, Switzerland) to visualize and quantify root water uptake and soil water distribution in maize samples. Seedlings were grown under controlled conditions in rhizoboxes filled with sandy and loamy soils for two weeks, followed by a period of progressive drying. High-resolution imaging revealed a clear shift in water uptake patterns as the soil dried: initially, water was extracted predominantly from the bulk soil, but under drier conditions, uptake increasingly shifted to the rhizosphere. As soil drying progressed, the rate of water uptake from the rhizosphere became insufficient to meet the transpiration demand. The critical point when water uptake shifted from the bulk to the rhizosphere soil occurred at less negative water potentials in sandy soils (-4 to -5 kPa) than in loamy soils (-100 to -300 kPa), reflecting the differences in hydraulic properties between the two soil types.These results show that under drought conditions, the rhizosphere serves as a primary water source for plants but cannot fully sustain transpiration over time, ultimately leading to stomatal closure and reduced water loss. By providing direct experimental evidence of how soil hydraulic limitations and rhizosphere water dynamics shape plant responses, this study provides new experimental evidence on the key role of rhizosphere water dynamics in regulating plant water use.

<p>Sustaining world food production under a changing climate and a growing population demands for higher optimization of agricultural resources including water. This requires an accurate understanding and prediction of root water uptake from soils, which depends on several root traits. The role of root hairs in root water uptake is still under debate, with experimental data that both prove and reject the hypothesis that root hairs can facilitate root water uptake, especially under drought conditions. Our objective was to investigate the effect of root hairs in maize at the field scale. A wildtype maize variety (with root hairs) and a hairless mutant were grown in two substrates (loam and sand) at a field site near Halle, Germany (Vetterlein et al., 2020, JPLN). Transpiration, leaf water potential, soil water content and potentials were monitored during 2019 and 2020. Root length density and leaf area were measured at four different plant development stages. A version of Hydrus 1D coupled with Couvreur’s macroscopic root water uptake model (Couvreur et al., 2012, HESS) was parameterized and used to further investigate soil-water relations in this field experiment. In both years, plants emptied the available water in the profile by July, and relied on rain and irrigation afterwards. Non-significant differences in cumulative water losses from the soil, estimated from soil water content measurements, were observed among the four treatments in both years. These results are in agreement with simulated water losses, which also showed small differences in cumulative transpiration among treatments. Mutant plants developed significantly smaller shoots while transpiring similar water volumes as wildtype plants, indicating lower water use efficiency. While there was no visible effect of the genotype in the soil-water relations, a clear effect of the soil type was observed. Simulated collar water potentials and field observations of rolled leaves indicated water stress occurred first in the loam compared to the sand treatments. Plants grew faster in the loam, leading to earlier onset of water stress. Even though plants in the loam produced less roots than in the sand, the onset of stress was not caused by the smaller root system since simulations presuming a larger root system did not predict a later onset of stress. Similarly, a simulation run using a smaller root system in the sandy soil did not predict a significantly earlier onset of stress. Finally, although our model simulations considered only differences in root density among treatments and did not consider different root or rhizosphere properties of the different soils and genotypes, it simulated the observed water dynamics well. Water depletion in the loamy soil was simulated earlier than it was measured. We hypothesize that this is caused by changing root hydraulic properties when roots develop and mature, and suggest that young roots do not start taking up water immediately. Nevertheless, the data quantity and quality obtained in this field experiment exposes the difficulties and challenges we face to monitor water potentials and fluxes in the soil-plant continuum in annual grasses at the field scale.</p>

Water and sediment temperature dynamics in shallow tidal environments: The role of the heat flux at the sediment-water interface

arises in simulating various processes in mechanics of continuous medium. Among them there are the process of heat (mass) transfer in mediums with thermal conductivity coefficient (diffusion) which powerwise depends on the temperature (concentration), and the process of filtering of polytropic gas into porous medium. In the last case (1.1) is called the Leibenson equation, and for m = 1 the Boussinesq equation. The same equation arises in dynamics of ground water. We shall consider (1.1) on the semiaxis x > 0 with the initial and boundary conditions u(x, 0) = cx (x > 0), u(0, t) = 0 (t > 0). (1.2)

The lack of understanding the space-time dynamics of water and matter transport in the soil-plant continuum of estuarine ecosystems remains an impediment to accurate prediction to support the establishment of appropriate strategies for pollution control and environmental protection. In this paper, a three dimensional model of water and substance flow in the soil plant system is set up based on cohesion-tension theory. Water transport in soil and tree is conceived as a continuous hydraulic process, which is driven by canopy transpiration. State variables of the model are water potential and contaminant concentrations in the soil, roots, xylem, core and canopy. The model equations are obtained by application of Richards equations with van Genuchten-Mualem approaches for hydraulic conductivity and water retention curves. The water transport equations are coupled to the contaminant transport equations via the Darcy velocity and the dispersion tensor. Exchanges between compartments are mediated by a diffusion model on the boundary for transport across membranes. Water evaporation from leaf mesophyll cells is taken into account by a transpiration sub model, which is driven by environmental variables such as air water potential, wind speed, radiation and temperature. The governing equations consist of a system of coupled nonlinear partial differential equations with reaction terms, which were implemented into the finite element tool COMSOL MULTIPHYSICS based on the Petrov-Galerkin scheme. First results show that the model is capable of reproducing typical spatial concentration patterns of metals in young mangrove plants.