Isotopic composition of plant water sources

Javaux, Mathieu [UCL] Rothfuss, Youri [Agrosphere, Forschungszentrum of Juelich, Germany] Vanderborght, Jan [Agrosphere, Forschungszentrum of Juelich, Germany] Vereecken, Harry [Agrosphere, Forschungszentrum of Juelich, Germany] Brüggemann, Nicolas [Agrosphere, Forschungszentrum of Juelich, Germany]

In their study, Evaristo et al.1 collected an extensive data set on the basis of which they statistically determined the isotopic compositions of the plant water source (δ18 O intersect and δ2 H intersect, called respectively δ18 O intercept and δ2 H intercept in their paper) as the x and y coordinates in (δ18 O, δ2 H) space of the intersection between the local meteoric water line (LMWL) and the plant xylem water ‘evaporation line’ (EL) for a range of climates and vegetation types. Evaristo et al.1 showed that for 80% of their referenced sampling sites, the mean value of the groundwater hydrogen isotopic composition (δ2 H GW) was statistically different from δ2 H intersect, supporting their hypothesis that the precipitation sources for groundwater recharge are different from the precipitation sources for plant water uptake. However, we consider that Evaristo et al.1 made a mistake in their equation (2), such that their analysis of rainfall segregation between plant transpiration and groundwater is not valid. This result does however not bring into question the paper’s conclusion1 of hydrological separation formulated on the basis of the precipitation offset. There is a Reply to this Brief Communication Arising by Evaristo, J., Jasechko, S. & McDonnell, J.J. Nature 536, http://dx.doi.org/10.1038/nature18947 (2016).