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 (δ13CR) 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 δ13CR. 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 δ13CR and the respiratory apparent fractionation, we measured the δ13CR, the leaf gas exchange, and the δ13C 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 C3 and C4 plants, trees, grasses, and herbs. A significant nocturnal shift in δ13CR was detected in 20 of the studied species, with the magnitude of the shift ranging from 1‰ to 5.8‰. The magnitude of the nighttime variation in δ13CR was strongly correlated with the daytime cumulative carbon assimilation, which suggests that variation in δ13CR 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 δ13CR between the C3 and C4 plants, as well as among the woody plants, herbs and graminoids. Leaf respired CO2 was enriched in 13C 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 δ13CR. 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 δ13C of bulk leaf material as a proxy for the δ13C of leaf-respired CO2.