Causes and consequences of pronounced variation in the isotope composition of plant xylem water

Abstract

Abstract. Stable isotopologues of water are widely used to derive relative root water uptake (RWU) profiles and average RWU depth in lignified plants. Uniform isotope composition of plant xylem water (δxyl) along the stem length of woody plants is a central assumption of the isotope tracing approach which has never been properly evaluated. Here we evaluate whether strong variation in δxyl within woody plants exists using empirical field observations from French Guiana, northwestern China, and Germany. In addition, supported by a mechanistic plant hydraulic model, we test hypotheses on how variation in δxyl can develop through the effects of diurnal variation in RWU, sap flux density, diffusion, and various other soil and plant parameters on the δxyl of woody plants. The hydrogen and oxygen isotope composition of plant xylem water shows strong temporal (i.e., sub-daily) and spatial (i.e., along the stem) variation ranging up to 25.2 ‰ and 6.8 ‰ for δ2H and δ18O, respectively, greatly exceeding the measurement error range in all evaluated datasets. Model explorations predict that significant δxyl variation could arise from diurnal RWU fluctuations and vertical soil water heterogeneity. Moreover, significant differences in δxyl emerge between individuals that differ only in sap flux densities or are monitored at different times or heights. This work shows a complex pattern of δxyl transport in the soil–root–xylem system which can be related to the dynamics of RWU by plants. These dynamics complicate the assessment of RWU when using stable water isotopologues but also open new opportunities to study drought responses to environmental drivers. We propose including the monitoring of sap flow and soil matric potential for more robust estimates of average RWU depth and expansion of attainable insights in plant drought strategies and responses.

A preliminary isotope‐based evapotranspiration partitioning approach for tropical Costa Rica

“…One main assumption of isotope‐based ET separation is the so‐called isotopic steady state, whereby the T (isotopically) represents an unfractionated sample of the water source (Gibson & Edwards, 2002). However, it has been shown with in situ measurements that this assumption is not always true (Deurwaerder et al, 2020; Dubbert et al, 2013; Poca et al, 2019) and nonsteady state occurs over short periods of time. However, when integrating time periods longer than a day, the mean diurnal isotope signal of T was found to match fairly well with estimations under the steady‐state conditions (Dubbert et al, 2013; Rothfuss et al, 2010).…”

On the use of leaf water to determine plant water source: A proof of concept

“…But even early on in this area of study, the limitations of the procedures used for determining the source of water used by plants were becoming apparent (Brunel, Walker, & Kennett-Smith, 1995). While many strides have been made in recent years on quantifying the subsurface soil water pools used by plants (Bowling, Schulze, & Hall, 2017;Brooks, Barnard, Coulombe, & McDonnell, 2010;Evaristo, Jasechko, & McDonnell, 2015), one continuing vexing issue is potential fractionation linked to the transpiration process itself (Barbeta et al, 2019;De Deurwaerder et al, 2020;Ellsworth & Sternberg, 2015;Martín-Gómez, Serrano, & Ferrio, 2017;von Freyberg, Allen, Grossiord, & Dawson, 2020). Incomplete knowledge about fractionation throughout the transpiration process is perhaps the thing that most affects our ability to trace plant water source.…”

Comment on bg-2021-35

“…Knighton et al (2020) citing the study of Gaines et al (2016), claimed they had "estimated time lags between RWU and transpiration", while they actually had estimated time lags between tracer injection to the stem base and transpiration. Similarly, de Deurwaerder et al (2020) propose a modelling framework that erroneously equates δ RWU to δ xyl at the stem base, leading them to the prediction of unlikely clear δ xyl signals further up the stem. The FPLD between root tips and stem base (h F1 in Fig.…”