Revealing a significant isotopic offset between plant water and its sources using a global meta-analysis
Abstract. Isotope-based approaches to study plant water sources rely on the assumption that root water uptake and within-plant water transport are non-fractionating processes. However, a growing number of studies have reported offsets between plant and source water stable isotope composition, for a wide range of ecosystems. These isotopic offsets can result in the erroneous attribution of source water used by plants and potential overestimations of groundwater uptake by the vegetation. We
... ted a global meta-analysis to quantify the magnitude of these plant-source water isotopic offsets and explore whether their variability could be explained by either biotic or abiotic factors. Our database compiled 112 studies, spanning arctic to tropical biomes that reported the dual water isotope composition (δ2H and δ18O) of plant (stem) and source water, including soil water. We calculated 2H offsets in two ways: a line conditioned excess (LC-excess) that describes the 2H deviation from the local meteoric water line, and a soil water line conditioned excess (SW-excess), that describes the deviation from the soil water line, for each sampling campaign within each study. We tested for the effects of climate (air temperature and soil water content), soil class and plant traits (growth form, leaf habit, wood density and parenchyma fraction and mycorrhizal habit) on LC-excess and SW-excess. Globally, stem water was more depleted in 2H than soil water (SW-excess < 0) by 3.02 ± 0.65 ‰. In 95 % of the cases where SW-excess was negative, LC-excess was negative, indicating that the uptake of water from mobile pools was unlikely to explain the observed soil-plant water isotopic offsets. SW-excess was more negative in cold and wet sites, whereas it was more positive in warm sites. Soil class and plant traits did not have any significant effect on SW-excess. The climatic effects on SW-excess suggest that methodological artefacts are unlikely to be the sole cause of observed isotopic offsets. Instead, our results support the idea that these offsets are caused by isotopic heterogeneity within plant stems whose relative importance will depend on soil and plant water status and evaporative demand. Our results would imply that plant-source water isotopic offsets may lead to inaccuracies when using the isotopic composition of bulk stem water as a proxy to infer plant water sources.