Pulse labeling of dissolved (13) C-carbonate into tree xylem: developing a new method to determine the fate of recently fixed photosynthate.

Stable carbon isotopes are often employed as tracers in plant and soil systems to study the fate and transformations of carbon as is it assimilated by the forest canopies and then translocated into the soil matrix and soil microorganisms. This experiment tested a new method of (13) C-labeling. We dissolved (13) C-carbonate into 12 mL of water and injected it into the xylem of a 6-cm diameter tree. The isotopic composition of foliage, stem CO(2) , and phloem contents were measured before the experiment and up to two weeks after the pulse label. Isotopic enrichments of 6.1‰ and 7.7‰ were observed in stem CO(2) and phloem contents, respectively. No enrichment in bulk foliage was observed. The pulse came through the phloem five days after the label was injected, consistent with expectations based on transport rates through the tree. The application of this xylem pulse-labeling method may provide new insights into labile carbon sequestration in trees, perhaps even in much larger trees. Furthermore, the method could be applied under experimental treatments that would elucidate the mechanisms controlling the fate and transformation of recently fixed photosynthate in forests.