Rooted in potential: advances in estimating spatiotemporal root water uptake in situ

Abstract

Rickard et al. utilized soil moisture sensors in a managed perennial ryegrass and annually-cultivated wheat field to explore compensatory root water uptake patterns across soil depths upon drying and rewetting. Using a water balance framework combined with soil and root data – including soil water release curves, root length density, and root growth – they developed models to estimate root water potential and radial root permeability at different soil depths over time. By measuring diurnal water flux across soil sections with moisture sensors, they quantified root water uptake rates and relative contributions from topsoil and subsoil layers. Additionally, the authors calculated changes in effective root water potential and radial root water permeability in relation to root water uptake and the vertical distribution of root segments throughout the soil profile.

Their calculations revealed that both plant systems preferentially extracted water from the topsoil (< 15 cm), with distinct shifts in uptake occurring in response to precipitation events and reduced topsoil moisture during drier periods. Their results suggest that topsoil water content acted as a mechanistic cue for adjusting water uptake patterns, prompting plants to optimize subsoil water use during dry periods and rapidly switch back to topsoil uptake after rewetting by modulating their root hydraulic conductivity. Further analysis using fitted functions showed a negative correlation between root water uptake and soil matric potential after midseason rainfall, reinforcing the concept that plants dynamically adjust their uptake strategies in response to changing water availability. Interestingly, accounting for root growth had little effect on calculated radial root permeability across both plant systems, suggesting that permeability adjustments, rather than structural root changes, were a significant factor in regulating water uptake. Most importantly, the mass balance approach proposed by Rickard et al. provides a simple, scalable method to estimate daily root water uptake and track how root hydraulics change in response to fluctuations in water availability.

Root water uptake is fundamental to plant growth and a key driver of ecosystem water balance. However, uncertainties remain about how plants modulate root hydraulics and rooting strategies to meet their water demands. While plant-specific characteristics – such as the general physiology, allometry, and resource acquisition strategies of a species – govern water uptake, a combination of bioclimatic factors (e.g. solar radiation and vapor pressure deficit) and biophysical properties (e.g. soil texture and porosity) regulate ecosystem water availability (Stocker et al., 2023). Previous research has found that plant water uptake is closely tied to soil water balance, with roots preferentially extracting water from shallow layers to minimize evaporative losses, but water uptake may also shift toward deeper layers during dry periods (Schenk, 2008; Feldman et al., 2023).

Plant root system characteristics that regulate water and nutrient uptake (e.g. rooting depth distribution, radial root permeability, and root symbioses) are often linked to environmental characteristics such as moisture variability, although these patterns vary on a global scale (Maurel & Nacry, 2020; Glass et al., 2023). Investigating the specific root traits and functions that contribute to differential water uptake patterns remains an active area of research with many unresolved questions (Maurel & Nacry, 2020; Boursiac et al., 2022). As suggested by Rickard et al., one characteristic that may play a significant role in regulating plant water uptake patterns is radial root hydraulic conductivity, or a root segment's ability to conduct water radially across the semipermeable surface of a root given a pressure gradient. Plants are likely able to control their hydraulic conductivity to adjust to a range of environmental conditions in order to avoid cavitation of water-conducting tissues, although the above- and belowground coordination and overall plant plasticity may vary across species and environments (Weigelt et al., 2021; Müllers et al., 2023). Rickard et al. propose differential control of radial root hydraulic conductivity as the mechanism that enables plants in their experimental plots to alter water uptake patterns to adapt to changes in moisture conditions.

The method implemented by Rickard et al. demonstrates a potential advancement in our understanding of how plants regulate water uptake to meet resource demands. By complementing widely used techniques, this approach builds upon our knowledge of plant hydraulic strategies. Accurately measuring plant water uptake during variable resource conditions is critical for assessing vegetation viability over space and time, both for managed and for natural systems, particularly in the context of water stress and drought. While various approaches exist to estimate root water uptake, each has strengths and limitations.

Water uptake has been effectively and commonly estimated for individual plants using natural abundance and tracer isotopic techniques to track the flow of water from sources to plants (Nippert & Knapp, 2007; Liu et al., 2020; Bachofen et al., 2024). However, natural abundance methods do not work in all systems (but labeled tracers are often utilized to account for this), and isotopic signatures are sensitive to even small changes in local hydrology or to fractionation that could occur naturally or artificially (von Freyberg et al., 2020). Furthermore, these techniques are relatively complex, labor-intensive, and costly (see box 1 of Bachofen et al., 2024 for an overview of plant water uptake methods). As a result, isotopic techniques are more commonly used to track uptake at a single or few time points, and are therefore limited in their spatial coverage.

At broader spatial scales (e.g. the ecosystem level), mass balance approaches using remote sensing-derived water balance estimations of evapotranspiration and rooting-zone water storage are being increasingly used (Stocker et al., 2023). These methods often leverage tools such as satellites and aerial vehicles to approximate root-zone soil moisture content and plant water uptake temporally and occasionally at depth across the most active (shallower) soil layers (Feldman et al., 2023; Kasim et al., 2025). While they can be effective, these approaches are costly, struggle to capture water uptake patterns from deeper soil, and are limited in their temporal resolution and spatial scale. At finer resolutions, mass balance approaches have been developed at the plot and individual plant scales with measurements such as soil water potentials across depths, stem sap flow, root sap flow, rooting depth distribution, and predawn foliar water potential (Bachofen et al., 2024; Noory et al., 2025). However, these methods are more intensive, as they require depth-resolved estimates or assumptions of root and soil properties and an understanding of the overall ecosystem water budget. The method provided by Rickard et al. is comparable to water balance approaches, in that it relies on changes in soil moisture and transpiration to estimate water uptake rates. Nevertheless, the method allows for much finer resolution spatiotemporal coverage with limited instrumentation.

The simple mass balance approach utilized by Rickard et al. has several distinct advantages. By combining characteristics of larger scale water balance models in a controlled experimental field site, their method effectively bridges the gap between broader spatial estimations and experimental precision. Additionally, the ability to calculate dynamic changes in water uptake across soil depths and at finer temporal scales could further enhance our understanding of spatiotemporal water uptake dynamics. With a methodology that is both simple and highly scalable, Rickard et al.'s contributions provide several exciting implications for the future of water uptake research, particularly for high-resolution implementation. A meaningful next step would be to cross-validate their mass balance calculations to assess large-scale applicability, especially across both managed and natural ecosystems with varying plant communities. Since this experiment was conducted in managed, monotypic plots, applying this technique in more natural systems, such as forests with greater species and structural diversity, would likely present additional challenges. A comparative study integrating stable isotopic techniques or other direct water uptake measurements, alongside Rickard et al.'s estimations, could help establish a highly scalable, low-cost, and low-effort approach to estimating radial root hydraulic conductivity and water uptake across depths. Furthermore, incorporating species-specific rooting strategies and moisture-related functions could provide deeper insights into the mechanisms driving water uptake variations across space, time, and species. By leveraging fundamental principles of the plant–soil water balance, Rickard et al.'s approach offers a promising framework for scaling spatiotemporal root water uptake estimations. As global hydrological intensification leads to longer periods of water stress and more extreme precipitation events, advancements in these methods will be critical for understanding plant water use and ecosystem resilience.

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