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

Abstract

The hydrogen isotope fractionation between leaf wax compounds and source water, the apparent fractionation (εapp), necessary for the reconstruction of hydrogen isotopic composition (δD) of precipitation, is mainly assessed through field and transect studies. The current εapp 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 εapp more accurately and also quantify εapp variability within a species and among different species. Among-species εapp 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 εapp 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 εapp. The fractionation of  δD between n-C29 alkane and n-C30 alkanoic acid (ε29/30) and between n-C31 alkane and n-C32 alkanoic acid (ε31/32) 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 εapp of deciduous and evergreen species; changes in the relative proportion of this vegetation in a community might not affect its εapp value.