Hydrogen isotope fractionation between leaf wax compounds and source water in tropical angiosperms

The hydrogen isotope fractionation between leaf wax compounds and source water, the apparent fractionation (ε_app), necessary for the reconstruction of hydrogen isotopic composition (δD) of precipitation, is mainly assessed through field and transect studies. The current ε_app dataset, however, exhibit a bias toward mid-latitude regions of the Northern Hemisphere. Here we report the results of an outdoor experiment wherein four evergreen and three deciduous species were grown with water of known δD value (-1.8‰) in a tropical semi-arid monsoon region. This allowed us to estimate ε_app more accurately and also quantify ε_app variability within a species and among different species. Among-species ε_app values varied by -119 ± 23‰ (for n-alkane of chain length n-C₃₁) and -126 ± 27‰   (for n-alkanoic acid of chain length n-C₃₀). The similarity of the among-species variability in ε_app reported here and that observed in field and transect studies suggested the species-effect, rather than uncertainty in δD of source water, control the uncertainty in community-averaged ε_app. The fractionation of  δD between n-C₂₉ alkane and n-C₃₀ alkanoic acid (ε_29/30) and between n-C₃₁ alkane and n-C₃₂ alkanoic acid (ε_31/32) were 7 ± 25‰ and 6 ± 15‰, respectively, suggesting minimal fractionation of hydrogen isotopes during decarboxylation. Further, as we did not observe a systematic difference between the ε_app of deciduous and evergreen species; changes in the relative proportion of this vegetation in a community might not affect its ε_app value.