Ideas and perspectives: Tracing terrestrial ecosystem water fluxes using hydrogen and oxygen stable isotopes – challenges and opportunities from an interdisciplinary perspective

Abstract

In this commentary, we summarize and build upon discussions that emerged during the workshop "Isotopebased studies of water partitioning and plant-soil interactions in forested and agricultural environments" held in San Casciano in Val di Pesa, Italy, in September 2017. Quantifying and understanding how water cycles through the Earth's critical zone is important to provide society and policymakers with the scientific background to manage water resources sustainably, especially considering the ever-increasing worldwide concern about water scarcity. Stable isotopes of hydrogen and oxygen in water have proven to be a powerful tool for tracking water fluxes in the critical zone. However, both mechanistic complexities (e.g. mixing and fractionation processes, heterogeneity of natural systems) and methodological issues (e.g. lack of standard protocols to sample specific compartments, such as soil water and xylem water) limit the application of stable water isotopes in critical-zone science. In this commentary, we examine some Published by Copernicus Publications on behalf of the European Geosciences Union. 6400 D. Penna et al.: Tracing terrestrial ecosystem water fluxes using stable isotopes of the opportunities and critical challenges of isotope-based ecohydrological applications and outline new perspectives focused on interdisciplinary research opportunities for this important tool in water and environmental science.

The Demographics of Water: A Review of Water Ages in the Critical Zone

“…One pitfall for the application of water stable isotopes in ecohydrological and unsaturated zone studies is the lack of standard protocols for soil (and plant) water extraction for isotope analysis (Orlowski, Breuer, et al, ; Orlowski, Winkler, et al, ; Penna et al, ). Several laboratory‐ and field‐based water extraction methods for isotope analysis have been developed (see review by Sprenger, Herbstritt, & Weiler, ).…”

Depth distribution of soil water sourced by plants at the global scale: A new direct inference approach

“…The stable isotopes of hydrogen and oxygen ( 2 H and 18 O) are effective tools to determine the proportions of water sources to plant transpiration. Due to their conservative nature through soils and their occurrence in the water molecule, stable isotopes are increasingly used for tracing water fluxes in ecohydrological and other interdisciplinary studies (Penna et al, ; Scandellari & Penna, ). The quantification of the main water sources for plant transpiration on the basis of isotopic tracers is typically carried out through a graphical inference method (Brunel, Walker, & Kennett‐Smith, ), two end‐member mixing models (e.g., Thorburn & Walker, ), or statistically‐based multisource mixing models (e.g., Schwendenmann, Pendall, Sanchez‐Bragado, Kunert, & Hölscher, ).…”

Understanding Catchment‐Scale Forest Root Water Uptake Strategies Across the Continental United States Through Inverse Ecohydrological Modeling

“…Despite the broad application of stable isotopic data to estimate tree RWU strategies, several aspects of these methodologies may be limiting. First, end member mixing analysis (e.g., Evaristo & McDonnell, ; Knighton, Conneely, et al, ) necessitates assumptions about the temporal invariance of samples (De Deurwaerder et al, ; Penna et al, ; Rothfuss & Javaux, ) and requires that we neglect time lags induced by tree‐water storage (Evaristo et al, ). Further, it remains practically difficult to collect and analyze a sufficient number of water isotopic samples to characterize the heterogeneity of soil (e.g., Oerter & Bowen, ) and tree‐stored waters (e.g., Knighton, Conneely, et al, ; Knighton, Souter‐Kline, et al, ).…”