Emerging topics in stable isotope ecology: are there isotope effects in plant respiration?

The ability to distinguish between ‘light’ and ‘heavy’ forms of carbon dioxide in the atmosphere has proven to be a useful tool in the study of plant ecology and the role of terrestrial ecosystems in the global carbon cycle. Plant processes imprint unique isotopic signatures on atmospheric CO 2 such that measurements of the ratio of heavy to light carbon in CO 2 can provide a means of detecting plant physiological processes integrated over whole canopies, ecosystems, or even regions. To interpret these measurements, a mechanistic understanding of the processes that affect the proportion of 12 CO 2 vs 13 CO 2 in the atmosphere is required, such as the well-known model of ‘discrimination’ against 13 CO 2 in photosynthesis caused by preferential diffusion and enzymatic reaction with 12 CO 2 (Farquhar et al ., 1982). In this issue, Hymus et al . (pp. 377– 384) show that short-term variations in isotopes of respiratory CO 2 under natural conditions may be as large as variations in photosynthesis, and may contain additional information about plant metabolic pathways that have not been fully explored.