An increasing number of scientists and disciplines around the world are benefitting from the application of stable water isotope techniques (2H and 18O) –, especially in plant ecology and ecohydrology. Since water uptake and transport by plants do not change the isotopic composition, stable water isotope analyses can improve our understanding of plant water source acquisition. If samples of different plant water sources are extracted and analyzed for their water isotopic signature, it is so far be assumed to assess the origin of water used by plants (groundwater, deep and shallow soil water or surface water). For this reason, several water extraction methods have been developed (1).

A critical issue is that we know little about which soil water extraction method captures which soil water pool (mobile to tightly bounded water). Attempting to unravel plant available soil water pools, Brooks et al. (2) introduced a new soil water concept assuming that two independent (decoupled) soil water pools exist in the soil (the two water world hypothesis). Accordingly, one represents mobile soil water expressed in the stream and the other consists of tightly bound water retained in the soil and used by plants. This hypothesis thus proposes that plants prefer water that is not easiest, energetically, to obtain and that vegetation and streams return different pools of water to the hydrosphere (McDonnell, 3).

In this study, we would like to test this hypothesis by isotopically determining the plant available soil water pools by mimicking the pressure plants apply for water uptake. For this purpose, recently developed polymer tensiometers would be used (4). The determined pressure would accordingly be applied for soil water extractions for isotope analyses.

Against this background, the potential candidate would:

• Setup a greenhouse experiment
• Sample different plant and soil materials for isotope analyses
• Conduct water extraction experiments (1) with different soil types saturated to various moisture contents
• Determine isotopic differences between different pressure levels
• Analyze and interpret the isotopic data

The student/s should be interested in conducting lab work and be familiar with statistical software, such as R or Python.

This project is in collaboration with Utrecht University and Wageningen University. The greenhouse experiment will be setup at Utrecht University and the lab work will be conducted at University of Freiburg.

• Natalie Orlowski, University of Freiburg, Germany, Tel. +49761 203 9283, natalie.orlowski@hydrology.uni-freiburg.de
• Hugo de Boer, Utrecht University, Netherlands, Tel. +31 30 253 6951, H.J.deBoer@uu.nl
• Martine van der Ploeg, Wageningen University, Netherlands, Tel. +31317483714, martine.vanderploeg@wur.nl

Englisch

1. Orlowski, N., Pratt, D. L. & McDonnell, J. J. Intercomparison of soil pore water extraction methods for stable isotope analysis. Hydrol. Process. 30, 3434–3449 (2016).
2. Brooks, J., R., Barnard, H., R., Coulombe, R. & McDonnell, J., J. Ecohydrologic separation of water between trees and streams in a Mediterranean climate. Nature Geosci 3, 100–104 (2010).
3. McDonnell, Jeffrey J. 2014. ‘The Two Water Worlds Hypothesis: Ecohydrological Separation of Water between Streams and Trees?’ Wiley Interdisciplinary Reviews: Water 1 (4): 323–29. doi:10.1002/wat2.1027.
4. Ploeg, M. J. van der, H. P. A. Gooren, G. Bakker, C. W. Hoogendam, C. Huiskes, L. K. Koopal, H. Kruidhof, and G. H. de Rooij. 2010. ‘Polymer Tensiometers with Ceramic Cones: Direct Observations of Matric Pressures in Drying Soils’. Hydrol. Earth Syst. Sci. 14 (10): 1787–99. doi:10.5194/hess-14-1787-2010. - Nummerierter Listenpunkt