The determining factors of hydrogen isotope offsets between plants and their source waters

center dot A fundamental assumption when using hydrogen and oxygen stable isotopes to understand ecohydrological processes is that no isotope fractionation occurs during plant water uptake/transport/redistribution. A growing body of evidence has indicated that hydrogen isotope fractionation occurs in certain environments or for certain plant species. However, whether the plant water source hydrogen isotope offset (delta H-2 offset) is a common phenomenon and how it varies among different climates and plant functional types remains unclear.center dot Here, we demonstrated the presence of positive, negative, and zero offsets based on extensive observations of 12 plant species of 635 paired stable isotopic compositions along a strong climate gradient within an inland river basin.center dot Both temperature and relative humidity affected delta H-2 offsets. In cool and moist environments, temperature mainly affected delta H-2 offsets negatively due to its role in physiological activity. In warm and dry environments, relative humidity mainly affected delta H-2 offsets, likely by impacting plant leaf stomatal conductance. These delta H-2 offsets also showed substantial linkages with leaf water O-18 enrichment, an indicator of transpiration and evaporative demand.center dot Further studies focusing on the ecophysiological and biochemical understanding of plant delta H-2 dynamics under specific environments are essential for understanding regional ecohydrological processes and for conducting paleoclimate reconstructions.