How does root water uptake affect the isotopic composition of soil water?
Identifying the subsurface water sources that trees use is essential to predicting how they will respond to climate change. Stable water isotopes (δ18O, δ2H) are commonly used as conservative tracers, tracking the diverse subsurface water sources that trees may use. However, interpreting these isotope signatures is challenging because isotopic fractionation processes, both in the subsurface and in trees, are poorly understood. Although isotopic fractionation in soils without trees is well described in the literature, recent studies indicate that trees themselves may also alter the isotopic composition of their source water. Membranes in tree roots preferentially transmit light isotopes, thus making the surrounding soil water isotopically heavier. Due to a scarcity of studies that directly monitor water uptake and fractionation effects, we currently lack an adequate understanding of how trees affect the isotopic composition of the subsurface water that they use. This knowledge gap complicates efforts to understand tree water sources from isotopes in tree xylem water. In this project, I will apply a newly-developed vapor-sampling system that enables high-precision, quasi-continuous measurements of stable water isotopes in soil water. I will collaborate with the developer of this system to rigorously benchmark test it, and further refine it for unattended operation. I will then use this system in laboratory experiments to test whether tree water uptake isotopically fractionates soil water and/or xylem water. Tree seedlings will be exposed to changing soil moisture conditions and the isotopic evolution in the soils and trees will be monitored.
References: Oerter, E. J., Perelet, A., Pardyjak, E., and Bowen, G.: Membrane inlet laser spectroscopy to measure H and O stable isotope compositions of soil and sediment pore water with high sample throughput, Rapid Commun Mass Sp, 31, 75-84, 2017. Oerter, E. J., and Bowen, G.: In situ monitoring of H and O stable isotopes in soil water reveals ecohydrologic dynamics in managed soil systems, Ecohydrology, 10, 10.1002/eco.1841, 2017.