Multi-isotope labelling of organic matter by diffusion of 2H/18O-H2O vapour and 13C-CO2 into the leaves and its distribution within the plant

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 (CO2)-C-13 and depleted in (H2O)-H-2-O-18. After 1 week, the water-soluble leaf OM ((delta C-13= 1346 162 parts per thousand) and the leaf water were strongly labelled (delta O-18= -63 +/- 8, (delta H-2= 156 +/- 15 %o). The leaf water isotopic composition was between the atmospheric and stem water, indicating a considerable back-diffusion of vapour into the leaves (58-69%) in the opposite direction to the net transpiration flow. The atomic ratios of the labels recovered (O-18/C-13, H-2/C-13) were 2-4 times higher in leaves than in the stems and roots. This could be an indication of the synthesis of more condensed compounds in roots and stems (e.g. lignin vs. cellulose) or might be the result of 0 and H exchange and fractionation processes during phloem transport and biosynthesis. We demonstrate that the three major OM elements (C, 0, H) can be labelled and traced simultaneously within the plant. This approach could be of interdisciplinary interest in the fields of plant physiology, palaeoclimatic reconstruction or soil science.