Concentrations and δ²H values of cuticular n-alkanes vary significantly among plant organs, species and habitats in grasses from an alpine and a temperate European grassland.

Abstract

n-Alkanes are long-chained hydrocarbons contained in the cuticle of terrestrial plants. Their hydrogen isotope ratios (δ(2)H) have been used as a proxy for environmental and plant ecophysiological processes. Calibration studies designed to resolve the mechanisms that determine the δ(2)H values of n-alkanes have exclusively focused on n-alkanes derived from leaves. It is, however, unclear in which quantities n-alkanes are also produced by other plant organs such as roots or inflorescences, or whether different plant organs produce distinct n-alkane δ(2)H values. To resolve these open questions, we sampled leaves, sheaths, stems, inflorescences and roots from a total of 15 species of European C3 grasses in an alpine and a temperate grassland in Switzerland. Our data show slightly increased n-alkane concentrations and n-alkane δ(2)H values in the alpine compared to the temperate grassland. More importantly, inflorescences had typically much higher n-alkane concentrations than other organs while roots had very low n-alkane concentrations. Most interestingly, the δ(2)H values of the carbon autonomous plant organs leaves, sheaths and stems were in general depleted compared to the overall mean δ(2)H value of a species, while non-carbon autonomous organs such as roots and inflorescences show δ(2)H values that are higher compared to the overall mean δ(2)H value of a species. We attribute organ-specific δ(2)H values to differences in the H-NADPH biosynthetic origin in different plant organs as a function of their carbon relationships. Finally, we employed simple mass balance calculations to show that leaves are in fact the main source of n-alkanes in the sediment. As such, studies assessing the environmental and physiological drivers of n-alkanes that focus on leaves produce relationships that can be employed to interpret the δ(2)H values of n-alkanes derived from sediments. This is despite the significant differences that we found among the δ(2)H values in the different plant organs. Our study brings new insights into the natural variability of n-alkane δ(2)H values and has implications for the interpretation of n-alkane δ(2)H values in ecological and paleohydrological research.