Apparent fractionation of hydrogen isotope from precipitation to leaf wax n-alkanes from natural environments and manipulation experiments

Abstract

Knowledge of hydrogen isotopic fractionation (ε) of plant leaf waxes is the foundation for applying hydrogen isotope values (δ2H) in environmental reconstructions. In this work, we systematically investigated plant ε values (εalk/precipitation, εalk/soil water, εalk/leaf water and εalk/lake water, representing the isotopic fractionation between plant n-alkane δ2H and precipitation δ2H, soil water δ2H, leaf water δ2H and lake water δ2H) from the natural environments and manipulation experiments. The results show that the εalk/precipitation values of terrestrial plants have large variations (from −190 ‰ to −20 ‰) and become more negative with increasing aridity index. This phenomenon is possibly caused by the δ2H changes in source water (from precipitation to soil water and then to leaf water) during plant leaf wax synthesis under various evapotranspiration conditions in different climatic zones. The rainfall manipulation experiments show that leaf water δ2H values are generally higher than soil water δ2H values, and the latter are higher than precipitation δ2H values. This finding further demonstrates that the evapotranspiration effect on source water δ2H affects the quantification of the leaf wax apparent ε values (εalk/leaf water < εalk/soil water < εalk/precipitation). The εalk/lake water values of submerged plants display a smaller range (−153 ± 5 ‰) than the εalk/precipitation values of terrestrial plants, which is close to the terrestrial εalk/precipitation values in humid areas. Therefore, the biosynthetic ε value of terrestrial plant leaf waxes is relatively constant (ca. –153 ± 5 ‰), and the observed variable apparent εalk/precipitation values are possibly caused by the varied degree of evapotranspiration effect on the water that plants used in different climatic conditions. This effect should be considered when applying δ2H values of leaf waxes to trace environmental changes.