Drought Sensitivity of the Carbon Isotope Composition of Leaf Dark-Respired CO in C () and C ( and ) Grasses in Northeast China.

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