X Water Worlds and how to investigate them: A review and future perspective on <i>in situ</i> measurements of water stable isotopes in soils and plants

Abstract

Abstract. The number of ecohydrological studies involving water stable isotopes has been increasing steadily due to technological (i.e. field deployable laser spectroscopy and cheaper instruments) and methodological (i.e. tracer approaches or improvements in root water uptake models) advances in recent years. This enables researchers from a broad scientific background to incorporate water isotope-based methods into their studies. Several isotope effects are currently not fully understood, but essential when investigating root water uptake depths of vegetation and disentangle isotope processes at the soil-vegetation-atmosphere continuum. In particular different viewpoints exist on i) extraction methods for soil and plant water and methodological artefacts potentially introduced by them; ii) the pools of water measured with those methods and iii) spatiotemporal issues related with water stable isotope research. In situ methods have been proposed as an innovative and necessary way to address these issues and are required in order to disentangle isotope effects and take them into account when studying root water uptake depths of plants and for studying soil-plant-atmosphere interaction based on water stable isotopes. Herein, we review the current status of in situ measurements of water stable isotopes in soils and plants, point out current issues and highlight potential for future research. Moreover, we put a strong focus and incorporate practical aspects into this review. Finally, we propose an integrated methodology for measuring both soil and plant water isotopes when carrying out studies at the soil-vegetation-atmosphere interface. For all in situ methods, extreme care needs to be taken particularly during set-up in order to obtain reliable data. In situ methods for soils are well established. For transpiration, reliable methods also exist but are not common in ecohydrological field studies due to the required effort. Little attention has been payed to in situ xylem water isotope measurements. Research needs to focus on improving and further developing those methods. There is a need for a consistent and combined (soils and plants) methodology for ecohydrological studies. Such systems should be designed and adapted to the environment to be studied. We further conclude that many studies currently might not rely on in situ methods extensively because of the technical difficulty. Hence, future research needs to aim on developing a simplified approach that provides a reasonable trade-off between practicability and precision/accuracy.