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

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 (δ H offset) is a common phenomenon and how it varies among different climates and plant functional types remains unclear. 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. Both temperature and relative humidity affected δ H offsets. In cool and moist environments, temperature mainly affected δ H offsets negatively due to its role in physiological activity. In warm and dry environments, relative humidity mainly affected δ H offsets, likely by impacting plant leaf stomatal conductance. These δ H offsets also showed substantial linkages with leaf water O enrichment, an indicator of transpiration and evaporative demand. Further studies focusing on the ecophysiological and biochemical understanding of plant δ H dynamics under specific environments are essential for understanding regional ecohydrological processes and for conducting paleoclimate reconstructions.