- A major contributor to the global carbon cycle is plant respiration. Elevated atmospheric CO2 con-centrations may either accelerate or decelerate plant respiration for reasons that have been uncertain. We recently established that elevated CO2 during the daytime decreases plant mitochondrial respiration in the light and protein concentration because CO2 slows the daytime conversion of NO3- (nitrate) into protein. This derives in part from the inhibitory effect of CO2 on photorespiration and the dependence of shoot NO3- assimilation on photorespiration. Elevated CO2 also inhibits the translocation of NO2- (nitrite) into the chloroplast, a response that influences shoot NO3- assimilation both day and night. Here, we exposed Arabidopsis and wheat plants to daytime or nighttime elevated CO2 and supplied them with NO3- or NH4+ (ammonium) as a sole N (nitrogen) source. Six independent measures-plant biomass, shoot NO3-, shoot organic N, 15N isotope fractionation, 15NO3- assimilation, and the ratio of shoot CO2 evolution to O2 consumption-indicated that elevated CO2 at night slowed NO3- assimilation and thus decreased dark respiration in the plants reliant on NO3-. These results provide a straightforward explanation for the diverse responses of plants to elevated CO2 at night and suggest that soil nitrogen source will have an increasing influence on the capacity of plants to mitigate human greenhouse gas emissions. Copyright © 2015, Plant Physiology.