In an experiment that addresses this question, Egli et al. (1998) rooted saplings of different genotypes of Norway spruce (Picea abies L. Karst.) directly into calcareous or acidic soils in open-top chambers and exposed them to atmospheric CO2 concentrations of 370 or 570 ppm and low or high soil nitrogen contents.  They found that elevated CO2 generally stimulated light-saturated rates of photosynthesis under all conditions by as much as 35%, regardless of genotype, which consistently led to increased aboveground biomass production, also regardless of genotype, as well as without respect to soil type or nitrogen content.

Murray et al. (2000) grew Sitka spruce (Picea sitchensis (Bong.) Carr.) seedlings for two years in pots within open-top chambers maintained at atmospheric CO2 concentrations of 355 and 700 ppm.  In the last year of the study, half of the seedlings received one-tenth of the optimal soil nitrogen supply recommended for this species, while the other half received twice the optimal amount.  Under this protocol, the extra CO2 increased the seedlings' light-saturated rates of net photosynthesis by 19% and 33% in the low- and high-nitrogen treatments, respectively, while it increased their total biomass by 0% and 37% in these same treatments.  Nevertheless, Murray et al. note there was a reallocation of biomass from aboveground organs (leaves and stems) into roots in the low-nitrogen treatment; and they remark that this phenomenon "may provide a long-term mechanism by which Sitka spruce could utilize limited resources both more efficiently and effectively," which suggests that although low soil nitrogen precluded a short-term CO2-induced growth response in this tree species, it is possible that the negative impact of nitrogen deficiency could be overcome in the course of much longer-term atmospheric CO2 enrichment.

In a related experiment, Liu et al. (2002) grew Sitka spruce seedlings in well-watered and fertilized pots within open-top chambers that were maintained for three years at atmospheric CO2 concentrations of either 350 or 700 ppm, after which the seedlings were planted directly into native nutrient-deficient forest soil and maintained at the same atmospheric CO2 concentrations for two more years in larger open-top chambers either with or without extra nitrogen being supplied to the soil.  After the first three years of the study, they determined that the CO2-enriched trees possessed 11.6% more total biomass than the ambient-treatment trees.  At the end of the next two years, however, the CO2-enriched trees supplied with extra nitrogen had 15.6% more total biomass than their similarly-treated ambient-air counterparts, while the CO2-enriched trees receiving no extra nitrogen had 20.5% more biomass than their ambient-treatment counterparts.

In light of these several observations, it would appear that the degree of soil nitrogen availability can indeed alter the aerial fertilization effect of atmospheric CO2 enrichment on the growth of spruce trees by promoting a greater CO2-induced growth enhancement in soils of adequate, as opposed to insufficient, nitrogen content.  As in the cases of aspen and pine, however, there is evidence to suggest that at some point the response of spruce trees to increasing soil nitrogen also saturates, and that higher nitrogen concentrations may possibly even reduce the growth response to elevated CO2 below that observed at optimal or low soil nitrogen concentrations.

References
Egli, P., Maurer, S., Gunthardt-Goerg, M.S. and Korner, C.  1998.  Effects of elevated CO2 and soil quality on leaf gas exchange and aboveground growth in beech-spruce model ecosystems.  New Phytologist 140: 185-196.

Liu, S.R., Barton, C., Lee, H., Jarvis, P.G. and Durrant, D.  2002.  Long-term response of Sitka spruce (Picea sitchensis (Bong.) Carr.) to CO2 enrichment and nitrogen supply.  I.  Growth, biomass allocation and physiology.  Plant Biosystems 136: 189-198.

Murray, M.B., Smith, R.I., Friend, A. and Jarvis, P.G.  2000.  Effect of elevated [CO2] and varying nutrient application rates on physiology and biomass accumulation of Sitka spruce (Picea sitchensis).  Tree Physiology 20: 421-434.