Carbon Isotope Composition of Nighttime Leaf-Respired CO2 in the Agricultural-Pastoral Zone of the Songnen Plain, Northeast China

Variations in the carbon isotope signature of leaf dark-respired CO2 (delta C-13(R)) within a single night is a widely observed phenomenon. However, it is unclear whether there are plant functional type differences with regard to the amplitude of the nighttime variation in delta C-13(R). These differences, if present, would be important for interpreting the short-term variations in the stable carbon signature of ecosystem respiration and the partitioning of carbon fluxes. To assess the plant functional type differences relating to the magnitude of the nighttime variation in delta C-13(R) and the respiratory apparent fractionation, we measured the delta C-13(R), the leaf gas exchange, and the delta C-13 of the respiratory substrates of 22 species present in the agricultural-pastoral zone of the Songnen Plain, northeast China. The species studied were grouped into C-3 and C-4 plants, trees, grasses, and herbs. A significant nocturnal shift in delta C-13(R) was detected in 20 of the studied species, with the magnitude of the shift ranging from 1 parts per thousand to 5.8 parts per thousand. The magnitude of the nighttime variation in delta C-13(R) was strongly correlated with the daytime cumulative carbon assimilation, which suggests that variation in delta C-13(R) were influenced, to some extent, by changes in the contribution of malate decarboxylation to total respiratory CO2 flux. There were no differences in the magnitude of the nighttime variation in delta C-13(R) between the C-3 and C-4 plants, as well as among the woody plants, herbs and graminoids. Leaf respired CO2 was enriched in C-13 compared to biomass, soluble carbohydrates and lipids; however the magnitude of enrichment differed between 8 pm and 4 am, which were mainly caused by the changes in delta C-13(R). We also detected the plant functional type differences in respiratory apparent fractionation relative to biomass at 4 am, which suggests that caution should be exercised when using the delta C-13 of bulk leaf material as a proxy for the delta C-13 of leaf-respired CO2.