In exploring this question, Kubiske et al. (1998) grew cuttings of four quaking aspen genotypes for five months at CO2 concentrations of 380 or 720 ppm and low or high soil nitrogen in open-top chambers in the field in Michigan, USA.  They found that the elevated CO2 treatment significantly increased net photosynthesis, regardless of soil nitrogen content, although there were no discernible increases in aboveground growth within the five-month study period.  Belowground, however, elevated CO2 significantly increased fine root production, but only in the high soil nitrogen treatment.

Working at the same site, Zak et al. (2000) and Curtis et al. (2000) grew six aspen genotypes from cuttings in open-top chambers for 2.5 growing seasons at atmospheric CO2 concentrations of 350 and 700 ppm on soils containing either adequate or inadequate supplies of nitrogen.  Curtis et al. report that at the end of this period the trees growing in the doubled-CO2 treatment exhibited rates of net photosynthesis that were 128% and 31% greater than those of the trees growing in the ambient-air treatment on the high- and low-nitrogen soils, respectively, while Zak et al. determined the CO2-induced biomass increases of the trees in the high- and low-nitrogen soils to be 38% and 16%, respectively.

In yet another study from the Michigan site, Mikan et al. (2000) grew aspen cuttings for two years in open-top chambers receiving atmospheric CO2 concentrations of 367 and 715 ppm in soils of low and high soil nitrogen concentrations.  They report finding that elevated CO2 increased the total biomass of the aspen cuttings by 50% and 26% in the high and low soil nitrogen treatments, respectively, and that it increased coarse root biomass by 78% and 24% in the same respective treatments.

Last of all, but again at the same site, Wang and Curtis (2001) grew cuttings of two male and two female aspen trees for about five months in open-top chambers maintained at atmospheric CO2 concentrations of 380 and 765 ppm on soils of high and low nitrogen content.  In the male cuttings, there was a modest difference in the CO2-induced increase in total biomass (58% and 66% in the high- and low-nitrogen soils, respectively), while in the female cuttings the difference was much greater (82% and 22% in the same respective treatments).

Considering the totality of these several observations, it would appear that the degree of soil nitrogen availability does indeed impact the aerial fertilization effect of atmospheric CO2 enrichment on the growth of aspen trees by promoting a greater CO2-induced growth enhancement in soils of adequate, as opposed to insufficient, nitrogen content.

References
Curtis, P.S., Vogel, C.S., Wang, X.Z., Pregitzer, K.S., Zak, D.R., Lussenhop, J., Kubiske, M. and Teeri, J.A.  2000.  Gas exchange, leaf nitrogen, and growth efficiency of Populus tremuloides in a CO2-enriched atmosphere.  Ecological Applications 10: 3-17.

Kubiske, M.E., Pregitzer, K.S., Zak, D.R. and Mikan, C.J.  1998.  Growth and C allocation of Populus tremuloides genotypes in response to atmospheric CO2 and soil N availability.  New Phytologist 140: 251-260.

Mikan, C.J., Zak, D.R., Kubiske, M.E. and Pregitzer, K.S.  2000.  Combined effects of atmospheric CO2 and N availability on the belowground carbon and nitrogen dynamics of aspen mesocosms.  Oecologia 124: 432-445.

Wang, X. and Curtis, P.S.  2001.  Gender-specific responses of Populus tremuloides to atmospheric CO2 enrichment.  New Phytologist 150: 675-684.

Zak, D.R., Pregitzer, K.S., Curtis, P.S., Vogel, C.S., Holmes, W.E. and Lussenhop, J.  2000.  Atmospheric CO2, soil-N availability, and allocation of biomass and nitrogen by Populus tremuloides.  Ecological Applications 10: 34-46.