We determined the short-term direct and long-term indirect effects of CO(2) on apparent dark respiration (CO(2) efflux in the dark) in ponderosa pine (Pinus ponderosa Dougl. ex Laws.) seedlings grown in 35 or 70 Pa CO(2) partial pressure for 163 days in naturally lit, controlled-environment chambers. Two soil N treatments (7 and 107 ppm total N, low-N and high-N treatments, respectively) were imposed by watering half the plants every 2 weeks with 15/15/18 fertilizer (N,P,K) and the other half with demineralized water. Direct effects of ambient CO(2) partial pressure on apparent dark respiration were measured during short-term manipulations (from minutes to hours) of the CO(2) environment surrounding the aboveground portion of individual seedlings. Short-term increases in the ambient CO(2) partial pressure consistently resulted in significant decreases in CO(2) efflux of seedling in all treatments. Efflux of CO(2) decreased by 3 to 13% when measurement CO(2) partial pressure was increased from 35 to 70 Pa, and by 8 to 46% over the entire measurement range from 0 to 100 Pa. No significant interactions between the indirect effects of growth CO(2) partial pressure and the direct effects of the measurement CO(2) partial pressure were found. Seedlings grown in the high-N treatment were significantly less sensitive to short-term changes in CO(2) partial pressures than seedlings grown in the low-N treatment. Apparent respiration tended to decrease in seedlings grown in elevated CO(2), but the decrease was not significant. Nitrogen had a large effect on CO(2) efflux, increasing apparent respiration more than twofold on both a leaf area and a leaf or shoot mass basis. Both the direct and indirect effects of elevated CO(2) were correlated with changes in the C/N ratio. A model of cumulative CO(2) efflux for a 160-day period demonstrated that, despite a 49% increase in total plant biomass, seedlings grown in the high-N + high-CO(2) treatment lost only 2% more carbon than seedlings grown in the high-N + low-CO(2) treatment, suggesting increased carbon use efficiency in plants grown in elevated CO(2). We conclude that small changes in instantaneous CO(2) efflux, such as those observed in ponderosa pine seedlings, could scale to large changes in carbon sequestration.