Published in:Biogeosciences discussions, (2016). doi:10.5194/bg-2016-410
Language:
English
DOI:
https://doi.org/10.5194/bg-2016-410
ISSN:
1810-6285;
1810-6277
Origination:
Footnote:
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Description:
Plant root water uptake (RWU) and release (i.e., hydraulic redistribution – HR, and its particular case hydraulic lift – HL) have been documented for the past five decades from water stable isotopic analysis. By comparing the (hydrogen or oxygen) stable isotopic composition 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) authors could determine the relative contributions of these water sources to RWU. Other authors have confirmed the existence of HR and HL from the isotopic analysis of the plant xylem water following a labelling pulse. 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 (e.g., their inability to locate or quantify HR) while it underlines the performance of one Bayesian mixing model, but only when the number of considered water sources in the soil is the highest to closely reflect the vertical distribution of the soil water isotopic composition. 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, future challenges in studying RWU with stable isotopic analysis are evocated with focus on new isotopic monitoring methods and sampling strategies, and on the implementation of isotope transport in physically based RWU models.