Experimental evidence suggests dominance of species effect on the variability in hydrogen isotope fractionation between leaf wax compounds and source water

Abstract

Quantifying the hydrogen isotope fractionation between leaf wax compounds and source water (εapp) is a prerequisite for hydrogen isotopic composition (δD) based paleo-hydrological studies. However, characterization of the εapp values, mostly done in field-based studies, are predominantly carried out in northern mid-latitude regions, as compared to that in the tropics. Further, the εapp values estimated in field-based studies are often associated with inherent uncertainties which could stem from (i) incorrect source water δD values, (ii) a species-effect, and (iii) varying climatic conditions (as in transect studies). Hence, to characterize the εapp values in tropics and to decouple the factors affecting the variability of εapp, we conducted an outdoor experiment wherein four evergreen and three deciduous angiosperm trees were grown under similar climatic conditions for 85 days with water of known δD value (−2‰). The εapp values in the studied species were −119 ± 23‰ (n = 14) for n-alkanes and −126 ± 27‰ (n = 12) for n-alkanoic acids of chain lengths C31 and C30, respectively. We observed inter-species variabilities in εapp values that are consistent with previous field and transect studies. As the plants were grown under similar climatic conditions and irrigated with water of the same δD value, the variability in εapp values observed here suggested that the species-specific hydrogen isotopic fractionation likely has a dominant control over the uncertainty in the community-averaged εapp values. Further, the εapp values of deciduous and evergreen species showed no systematic differences, suggesting that changes in the relative proportion of these taxa may not affect the community-averaged εapp and the reconstructed δD values of paleo-precipitation in angiosperm tree dominated catchments.