Final Report Summary - COSIWAX (Compound Specific Hydrogen Isotope Analyses of Leaf Wax n-Alkanes as a Novel Tool to Assess Plant and Ecosystem Water Relations Across new Spatial and Temporal Scales) Leaf waxes such as n-alkanes or fatty acids are long-chained lipids that are vital components of plant cuticles. What makes leaf waxes unique is that their stable hydrogen isotope composition (d2H) contains information on precipitation and plant water relations. In addition, leaf waxes are abundant in leaves, soils, sediments and even the atmosphere and can persist with their d2H values over millions of years. With this exceptional combination of properties, leaf waxes and their d2H values have been celebrated as the much-needed ecohydrological proxy that provides information on the hydrological cycle and plant water relations across spatial and temporal scales that range from leaves to biomes and from weeks to millions of years. Despite the enormous potential that leaf waxes have as ecohydrological proxy for a range of different research areas, the exact type of hydrological information that is recorded in the d2H values of leaf waxes was still unclear. In particular, it was unclear, if the signal recorded in the d2H values of leaf waxes is a mere hydrological signal reflecting the amount or origin of precipitation or a plant-shaped signal indicating plant water relations such as evapotranspiration.COSIWAX performed the experimental work that was necessary to resolve the key mechanisms that determine the d2H values leaf waxes and to identify as such the exact hydrological signal that is recorded in the d2H values leaf waxes. COSIWAX was divided in four individual but tightly connected work packages (WPs): Experiments in WP1 tested effects of (i) leaf water, (ii) timing of wax biosynthesis (iii) biosynthetic hydrogen isotope fractionation (eBIO), and (iv) canopy heterogeneity on the d2H values of leaf waxes. In WP2 a mechanistic model for the prediction and interpretation of d2H values in leaf waxes was developed. In WP3 the model was temporally and spatially validated with (i) d2H values of archived plant samples from the unique century-old Rothamstead experiment and with spatial data that were collected across Europe. In WP4 the new model was successfully applied to three case studies (i) interpretation of the Rothamsted time series, (ii) interpretation of d2H values of leaf waxes in aerosols, (iii) and a lake sediment in Maui.The most important finding of COSIWAX is that biological influences are of high relevance for shaping leaf wax d2H values. COSIWAX was able to quantify these influences and incorporate the resulting parameters into a mechanistic H isotope models for plant organic compounds. With these findings, COSIWAX significantly contributed to resolve what determines the d2H values of leaf waxes. In turn this allows now a much-improved interpretation of foliar wax d2H values in ecological, environmental or paleo-hydrological studies significantly enhances the accuracy with which these important and widely applied signals can be interpreted.
