Diel variations in the carbon isotope composition of respired CO2 and associated carbon sources: a review of dynamics and mechanisms

Recent advances have improved our methodological approaches and theoretical understanding of post-photosynthetic carbon isotope fractionation processes. Nevertheless we still lack a clear picture of the origin of short-term variability in delta C-13 of respired CO2 (delta C-13(res)) and organic carbon fractions on a diel basis. Closing this knowledge gap is essential for the application of stable isotope approaches for partitioning ecosystem respiration, tracing carbon flow through plants and ecosystems and disentangling key physiological processes in carbon metabolism of plants. In this review we examine the short-term dynamics in delta C-13(res) and putative substrate pools at the plant, soil and ecosystem scales and discuss mechanisms, which might drive diel delta C-13(res) dynamics at each scale. Maximum reported variation in diel delta C-13(res) is 4.0, 5.4 and 14.8 parts per thousand in trunks, roots and leaves of different species and 12.5 and 8.1 parts per thousand at the soil and ecosystem scale in different biomes. Temporal variation in post-photosynthetic isotope fractionation related to changes in carbon allocation to different metabolic pathways is the most plausible mechanistic explanation for observed diel dynamics in delta C-13(res). In addition, mixing of component fluxes with different temporal dynamics and isotopic compositions add to the delta C-13(res) variation on the soil and ecosystem level. Understanding short-term variations in delta C-13(res) is particularly important for ecosystem studies, since delta C-13(res) contains information on the fate of respiratory substrates, and may, therefore, provide a non-intrusive way to identify changes in carbon allocation patterns.