n-Alkane biosynthetic hydrogen isotope fractionation is not constant throughout the growing season in the riparian tree Salix viminalis

Compound-specific delta H-2 values of leaf wax n-alkanes have emerged as a potentially powerful paleohydrological proxy. Research suggests terrestrial plant n-alkane delta H-2 values are strongly correlated with meteoric water delta H-2 values, and may provide information on temperature, relative humidity, evaporation, and precipitation. This is based upon several assumptions, including that biosynthetic fractionation of n-alkanes during synthesis is constant within a single species. Here we present a multi-isotope study of the n-alkanes of riparian Salix viminalis growing in Norwich, UK. We measured n-alkane delta H-2, leaf water delta H-2, xylem water delta H-2, and bulk foliar delta C-13 and evaluated the variability of n-alkane delta H-2 values and net biosynthetic fractionation (epsilon(lw-wax)) over a whole growing season. S. viminalis n-alkane delta H-2 values decreased by 40 parts per thousand between the start of the growing season in April and the time when they stabilized in July. Variation in leaf and xylem water delta H-2 did not explain this variability. e(lw-wax) varied from -116 parts per thousand during leaf expansion in April to -156 parts per thousand during the stable phase. This suggests that differential biosynthetic fractionation was responsible for the strong seasonal trends in S. viminalis n-alkane delta H-2 values. We suggest that variability in e(lw-wax) is driven by seasonal differences in the carbohydrate source and thus the NADPH used in n-alkane biosynthesis, with stored carbohydrates utilized during spring and recent occurring growing season assimilates used later in the season. This is further supported by bulk foliar delta C-13 values, which are C-13-enriched during the period of leaf flush, relative to the end of the growing season. Our results challenge the assumption that biosynthetic fractionation is constant for a given species, and suggest that H-2-enriched stored assimilates are an important source for n-alkane biosynthesis early in the growing season. These findings have implications for the interpretation of sedimentary n-alkanes and call for a careful design of calibration studies using contemporary samples. (C) 2015 Elsevier Ltd. All rights reserved.