Drought Sensitivity of the Carbon Isotope Composition of Leaf Dark-Respired CO2 in C3 (Leymus chinensis) and C4 (Chloris virgata and Hemarthria altissima) Grasses in Northeast China

Whether photosynthetic pathway differences exist in the amplitude of nighttime variations in the carbon isotope composition of leaf dark-respired CO2 (delta C-13(l)) and respiratory apparent isotope fractionation relative to biomass (Delta(R),(biomass)) in response to drought stress is unclear. These differences, if present, would be important for the partitioning of C-3-C-4 mixed ecosystem C fluxes. We measured delta C-13(l), the delta C-13 of biomass and of potential respiratory substrates and leaf gas exchange in one C-3 (Leymus chinensis) and two C-4 (Chloris virgata and Hemarthria altissima) grasses during a manipulated drought period. For all studied grasses, delta C-13(l) decreased from 21:00 to 03:00 h. The magnitude of the nighttime shift in delta C-13(l) decreased with increasing drought stress. The delta C-13(l) values were correlated with the delta C-13(l) of respiratory substrates, whereas the magnitude of the nighttime shift in delta C-13(l) strongly depended on the daytime carbon assimilation rate and the range of nighttime variations in the respiratory substrate content. The Delta(R),(biomass) in the C-3 and C-4 grasses varied in opposite directions with the intensification of the drought stress. The contribution of C-4 plant-associated carbon flux is likely to be overestimated if carbon isotope signatures are used for the partitioning of ecosystem carbon exchange and the delta C-13(l) of biomass is used as a substitute for leaf dark-respired CO2. The detected drought sensitivities in delta C-13(l) and differences in respiratory apparent isotope fractionation between C-3 and C-4 grasses have marked implications for isotope partitioning studies at the ecosystem level.