Disentangling temporal and population variability in plant root water uptake from stable isotopic analysis: when rooting depth matters in labeling studies

Valentin Couvreur (1) , Youri Rothfuss (2) , Félicien Meunier (3) , Thierry Bariac (4) , Philippe Biron (4, 5, 6) , Jean-Louis Durand (7) , Patricia Richard (4) , Mathieu Javaux (1, 2)

1 ELI - Earth and Life Institute [Louvain-La-Neuve]
2 IBG - Institute of Bio- and Geosciences [Jülich]
3 CAVElab - Computational & Applied Vegetation Ecology
4 iEES Paris - Institut d'écologie et des sciences de l'environnement de Paris
5 SU - Sorbonne Université
6 iEES Paris - Institut d'écologie et des sciences de l'environnement de Paris
7 P3F - Unité de Recherche Pluridisciplinaire Prairies et Plantes Fourragères

Isotopic labeling techniques have the potential to minimize the uncertainty of plant root water uptake (RWU) profiles estimated using multisource (statistical) modeling by artificially enhancing the soil water isotopic gradient. On the other end of the modeling continuum, physical models can account for hydrodynamic constraints to RWU if simultaneous soil and plant water status data are available. In this study, a population of tall fescue (Festuca arundinacea cv. Soni) was grown in amacro-rhizotron and monitored for a 34 h long period following the oxygen stable isotopic (O-18) labeling of deep soil water. Aboveground variables included tiller and leaf water oxygen isotopic compositions (delta(tiller) and delta(leaf), respectively) as well as leaf water potential (psi(leaf)), relative humidity, and transpiration rate. Belowground profiles of root length density (RLD), soil water content, and isotopic composition were also sampled. While there were strong correlations between hydraulic variables as well as between isotopic variables, the experimental results underlined the partial disconnect between the temporal dynamics of hydraulic and isotopic variables. In order to dissect the problem, we reproduced both types of observations with a one-dimensional physical model of water flow in the soil-plant domain for 60 different realistic RLD profiles. While simulated psi(leaf) followed clear temporal variations with small differences across plants, as if they were "onboard the same roller coaster", simulated Stiller values within the plant population were rather heterogeneous ("swarm-like") with relatively little temporal variation and a strong sensitivity to rooting depth. Thus, the physical model explained the discrepancy between isotopic and hydraulic observations: the variability captured by delta(tiller) reflected the spatial heterogeneity in the rooting depth in the soil region influenced by the labeling and may not correlate with the temporal dynamics of psi(leaf). In other varied words, psi(leaf) varied in time with transpiration rate, while delta(tiller) across plants with rooting depth. For comparison purposes, a Bayesian statistical model was also used to simulate RWU. While it predicted relatively similar cumulative RWU profiles, the physical model could differentiate the spatial from the temporal dynamics of the isotopic composition. An important difference between the two types of RWU models was the ability of the physical model to simulate the occurrence of hydraulic lift in order to explain concomitant increases in the soil water content and the isotopic composition observed overnight above the soil labeling region.

Domaines: Sciences agricoles

Valentin Couvreur, Youri Rothfuss, Félicien Meunier, Thierry Bariac, Philippe Biron, et al.. Disentangling temporal and population variability in plant root water uptake from stable isotopic analysis: when rooting depth matters in labeling studies. Hydrology and Earth System Sciences, 2020, 24 (6), pp.3057-3075. ⟨10.5194/hess-24-3057-2020⟩. ⟨hal-03123926⟩