Isotope labelling is a powerful tool to study elemental cycling within terrestrial ecosystems. Here we describe a new multi-isotope technique to label organic matter (OM). We exposed poplars (Populus deltoides x nigra) for 14 days to an atmosphere enriched in ¹³CO and depleted in ²H¹O. After one week, the water-soluble leaf OM (¹³C = 1346 ± 162‰) and the leaf water were strongly labelled (¹O = −63± 8‰, ²H = −156 ± 15‰). The leaf water isotopic composition was between the atmospheric and stem

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... er, indicating a considerable diffusion of vapour into the leaves (58-69%). The atomic ratios of the labels recovered (¹O/¹³C, ²H/¹³C) were 2-4 times higher in leaves than in the stems and roots. This either indicates the synthesis of more condensed compounds (lignin vs. cellulose) in roots and stems, or be the result of O and H exchange and fractionation processes during transport and biosynthesis. We demonstrate that the three major OM elements (C, O, H) can be labelled and traced simultaneously within the plant. This approach could be of interdisciplinary interest for the fields of plant physiology, paleoclimatic reconstruction or soil science.