PT  - JOURNAL ARTICLE
AU  - Hannes De Deurwaerder
AU  - Marco D. Visser
AU  - Matteo Detto
AU  - Pascal Boeckx
AU  - Félicien Meunier
AU  - Liangju Zhao
AU  - Lixin Wang
AU  - Hans Verbeeck
TI  - Diurnal variation in xylem water isotopic signature biases depth of root-water uptake estimates
AID  - 10.1101/712554
DP  - 2019 Jan 01
TA  - bioRxiv
PG  - 712554
4099  - http://biorxiv.org/content/early/2019/07/24/712554.short
4100  - http://biorxiv.org/content/early/2019/07/24/712554.full
AB  - Stable water isotopes are a powerful and widely used tool to derive the depth of root water uptake (RWU) in lignified plants. Uniform xylem water isotopic signature (i-H2O-xyl) along the length of a lignified plant is a central assumption, which has never been properly evaluated.Here we studied the effects of diurnal variation in RWU, sap flow velocity and various other soil and plant parameters on i-H2O-xyl signature within a plant using a mechanistic plant hydraulic model.Our model predicts significant variation in i-H2O-xyl along the full length of an individual plant arising from diurnal RWU fluctuations and vertical soil water heterogeneity. Moreover, significant differences in i-H2O-xyl emerge between individuals with different sap flow velocities. We corroborated our model predictions with field observations from French Guiana and northwestern China. Modelled i-H2O-xyl varied considerably along stem length ranging up to 18.3‰ in δ2H and 2.2‰ in δ18O, largely exceeding the range of measurement error.Our results show clear violation of the fundamental assumption of uniform i-H2O-xyl and occurrence of significant biases when using stable isotopes to assess RWU. As a solution, we propose to include monitoring of sap flow and soil water potential for more robust RWU depth estimates.