Summary: We compiled hydrogen and oxygen stable isotope compositions (δ²H and δ¹⁸O) of leaf water from multiple biomes to examine variations with environmental drivers. Leaf water δ²H was more closely correlated with δ²H of xylem water or atmospheric vapour, whereas leaf water δ¹⁸O was more closely correlated with air relative humidity. This resulted from the larger proportional range for δ²H of meteoric waters relative to the extent of leaf water evaporative enrichment compared with δ¹⁸O. We next expressed leaf water as isotopic enrichment above xylem water (Δ²H and Δ¹⁸O) to remove the impact of xylem water isotopic variation. For Δ²H, leaf water still correlated with atmospheric vapour, whereas Δ¹⁸O showed no such correlation. This was explained by covariance between air relative humidity and the Δ¹⁸O of atmospheric vapour. This is consistent with a previously observed diurnal correlation between air relative humidity and the deuterium excess of atmospheric vapour across a range of ecosystems. We conclude that ²H and ¹⁸O in leaf water do indeed reflect the balance of environmental drivers differently; our results have implications for understanding isotopic effects associated with water cycling in terrestrial ecosystems and for inferring environmental change from isotopic biomarkers that act as proxies for leaf water.

Summary: 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 (δ²H 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 δ²H offsets. In cool and moist environments, temperature mainly affected δ²H offsets negatively due to its role in physiological activity. In warm and dry environments, relative humidity mainly affected δ²H offsets, likely by impacting plant leaf stomatal conductance. These δ²H offsets also showed substantial linkages with leaf water ¹⁸O enrichment, an indicator of transpiration and evaporative demand. Further studies focusing on the ecophysiological and biochemical understanding of plant δ²H dynamics under specific environments are essential for understanding regional ecohydrological processes and for conducting paleoclimate reconstructions.