Testing plant use of mobile vs immobile soil water sources using stable isotope experiments

Summary We tested for isotope exchange between bound (immobile) and mobile soil water, and whether there is isotope fractionation during plant water uptake. These are critical assumptions to the formulation of the ‘two water worlds’ hypothesis based on isotope profiles of soil water. In two different soil types, soil‐bound water in two sets of 19‐l pots, each with a 2‐yr‐old avocado plant (Persea americana), were identically labeled with tap water. After which, one set received isotopically enriched water whereas the other set received tap water as the mobile phase water. After a dry down period, we analyzed plant stem water as a proxy for soil‐bound water as well as total soil water by cryogenic distillation. Seventy‐five to 95% of the bound water isotopically exchanged with the mobile water phase. In addition, plants discriminated against 18O and 2H during water uptake, and this discrimination is a function of the soil water loss and soil type. The present experiment shows that the assumptions for the ‘two water worlds’ hypothesis are not supported. We propose a novel explanation for the discrepancy between isotope ratios of the soil water profile and other water compartments in the hydrological cycle.