Investigating the root plasticity response of Centaurea jacea to soil water availability changes from stable isotopic analysis

Root water uptake (RWU) is a key ecohydrological process for which a physically-based understanding has been developed in the past decades.

Isotopic analysis of soil and sap xylem water allows for a quantification of RWU profiles relative to plant transpiration at the whole-plant scale and provides independent means for constraining these models.

However, due to methodological constraints, mainly having to do with destructive sampling and subsequent extraction of water under vacuum, significant knowledge gaps remain about the plastic response of a whole plant root system to a rapidly changing environment.

We designed a laboratory experimental setup consisting of a plant chamber coupled to a 60-cm long acrylic soil column equipped with gas-permeable tubing at eight different depths for the non-destructive monitoring of the stable isotopic compositions in plant transpiration of an herbaceous species ( Centaurea jacea ) and of water across the soil profile, respectively. In addition, soil water content was monitored at five different depths in the soil column. Isotopically labeled water was brought to the plant alternatively from the top and the bottom of the soil column to simulate a rain event and capillary rise of groundwater. Finally, root length density (RLD) was measured destructively at the end of the experimental period.

Fast shifts in the isotopic composition of both soil and transpiration water could be observed with the setup and translated into dynamic and pronounced shifts of RWU profile by means of a statistical Bayesian multi-source mixing model.

The incorporation of plant physiological and soil physical information into statistical modelling improved the model output. A simple exercise of water balance closure underlined the non-unique relationship between RWU profile on the one hand, and water content and RLD profiles on the other, illustrating the continuous adaptation of the plant RWU as a function of its root hydraulic architecture and soil water availability during the experiment.