In-situ measurements of dissolved gases in tree xylem sap as tracers for plant physiology

Common hydrogeological methods make use of natural gases as tracers to better understand the spatial and temporal evolution of groundwater flow, to constrain water residence time, and to reconstruct environmental conditions at recharge [1-3]. Noble gases can be used as complement of the stable water isotope tracers for understanding complex hydrological systems [4,5,6].

We adapted these methods to in-situ measurements of gases in tree xylem sap to better understand the plant-mediated water and gas flux between the hydrosphere, the biosphere, and the atmosphere.

Using a “miniRuedi” portable mass-spectrometer [7] and tailored semi-permeable membrane probes, the partial pressures of He, Ar, Kr, N₂, O₂, CO₂, and CH₄ were continuously monitored in-situ in the soil, the tree, and the atmosphere. Diurnal variations of CO₂ and O₂ were observed that reflected the tree physiological activities [8].

Since transpiration by plants is a major component of the hydrological cycle, such measurement techniques offer new opportunities to better understand plant water and CO₂ dynamics, within the soil-plant-atmosphere continuum.

[1] Kipfer et al. (2002), Reviews in Mineralogy and Geochemistry, 47, 615–700; [2] Brennwald et al. (2013), Advances in Isotope Geochemistry – The Noble Gases as Geochemical Tracers, 123-153; [3] Brennwald et al. (2022), Frontiers in Water, 4, 107-115; [4] Althaus et al. (2009), Journal of Hydrology, 370, 64-72. [5] Schilling et al. (2019), Reviews of Geophysics, 57, 146-182. [6] Xu et al. (2017). Hydrogeology Journal, 25(7), 2015–2029; [7] Brennwald et al. (2016), ES&T, 50, 13455-1346; [8] Marion et al. (2024), Tree Physiology, tpae062.