An explanation for the isotopic offset between soil and stem water in a temperate tree species

• A growing number of field studies report isotopic offsets between stem water and its potential sources that prevent the unambiguous identification of plant water origin using water isotopes. We explored the causes of this isotopic offset by conducting a controlled experiment on the temperate tree species Fagus sylvatica. • We measured δ²H and δ18O of soil and stem water from potted saplings growing on three soil substrates and subjected to two watering regimes. • Regardless of substrate, soil and stem water δ²H were similar only near permanent wilting point. Under moister conditions, stem water δ²H was 11 ± 3‰ more negative than soil water δ²H, coherent with field studies. Under drier conditions, stem water δ²H became progressively more enriched than soil water δ²H. Although stem water δ18O broadly reflected that of soil water, soil–stem δ²H and δ18O differences were correlated (r = 0.76) and increased with transpiration rates indicated by proxies. • Soil–stem isotopic offsets are more likely to be caused by water isotope heterogeneities within the soil pore and stem tissues, which would be masked under drier conditions as a result of evaporative enrichment, than by fractionation under root water uptake. Our results challenge our current understanding of isotopic signals in the soil–plant continuum.