Reviews and syntheses: Isotopic approaches to quantify root water uptake: a review and comparison of methods

Youri Rothfuss,Mathieu Javaux

doi:10.5194/bg-14-2199-2017

Abstract

Abstract. Plant root water uptake (RWU) has been documented for the past five decades from water stable isotopic analysis. By comparing the (hydrogen or oxygen) stable isotopic compositions of plant xylem water to those of potential contributive water sources (e.g., water from different soil layers, groundwater, water from recent precipitation or from a nearby stream), studies were able to determine the relative contributions of these water sources to RWU. In this paper, the different methods used for locating/quantifying relative contributions of water sources to RWU (i.e., graphical inference, statistical (e.g., Bayesian) multi-source linear mixing models) are reviewed with emphasis on their respective advantages and drawbacks. The graphical and statistical methods are tested against a physically based analytical RWU model during a series of virtual experiments differing in the depth of the groundwater table, the soil surface water status, and the plant transpiration rate value. The benchmarking of these methods illustrates the limitations of the graphical and statistical methods while it underlines the performance of one Bayesian mixing model. The simplest two-end-member mixing model is also successfully tested when all possible sources in the soil can be identified to define the two end-members and compute their isotopic compositions. Finally, the authors call for a development of approaches coupling physically based RWU models with controlled condition experimental setups.

Highlights

Understanding how the distribution of soil water and root hydraulic architecture impact root water uptake (RWU) location and magnitude is important for better managing plant irrigation, developing new plant genotypes more tolerant to drought or tackling ecological questions in water-limited ecosystems, such as the competition for soil water by different plants (Javaux et al, 2013).RWU – defined as the amount of water abstracted by a root system from soil over a certain period of time – is principally driven by transpiration flux taking place in the leaves
The flux of water depends on soil water availability, i.e., the ability of the soil to provide water at the plant imposed rate (Couvreur et al, 2014): a highly conductive root segment will not be able to extract water from a dry soil. This is the difference of water potential between the root and the soil which drives RWU, and its magnitude is controlled by the radial hydraulic resistances in the rhizosphere, at the soil–root interface and in the root system (Steudle and Peterson, 1998)
The only exception was a soil with a deep groundwater table and a dry surface, where this dry layer limited RWU (DeDr_hT)

Summary

Introduction

Understanding how the distribution of soil water and root hydraulic architecture impact root water uptake (RWU) location and magnitude is important for better managing plant irrigation, developing new plant genotypes more tolerant to drought or tackling ecological questions in water-limited ecosystems, such as the competition for soil water by different plants (Javaux et al, 2013).RWU – defined as the amount of water abstracted by a root system from soil over a certain period of time – is principally driven by transpiration flux taking place in the leaves. Its magnitude depends on the atmospheric evaporative demand and stomatal opening The latter depends amongst others on leaf water status and stress hormonal signals from the roots transported to the leaves (e.g., Huber et al, 2015; Tardieu and Davies, 1993). The flux of water depends on soil water availability, i.e., the ability of the soil to provide water at the plant imposed rate (Couvreur et al, 2014): a highly conductive root segment will not be able to extract water from a dry soil This is the difference of water potential between the root and the soil which drives RWU, and its magnitude is controlled by the radial hydraulic resistances in the rhizosphere, at the soil–root interface and in the root system (Steudle and Peterson, 1998).

Methods

Results

Conclusion