In a review of eleven of their previously published papers dealing with both loblolly pine (Pinus taeda L.) and ponderosa pine (Pinus ponderosa Dougl.), Johnson et al. (1998) report that when soil nitrogen levels were so low as to be extremely deficient, or so high as to be toxic, growth responses to atmospheric CO2 enrichment in both species were negligible.  For moderate soil nitrogen deficiencies, however, a doubling of the air's CO2 content sometimes boosted growth by as much as 1,000%.  In addition, atmospheric CO2 enrichment mitigated the negative growth response of ponderosa pine to extremely high soil nitrogen concentrations.

In a second paper published by some of the same scientists in the same year, Walker et al. (1998) describe how they raised ponderosa pine tree seedlings for two growing seasons in open-top chambers having CO2 concentrations of 350, 525 and 700 ppm on soils of low, medium and high nitrogen content.  They report that elevated CO2 had little effect on most growth parameters after the first growing season, the one exception being belowground biomass, which increased with both CO2 and soil nitrogen.  After two growing seasons, however, elevated CO2 significantly increased all growth parameters, including tree height, stem diameter, shoot weight, stem volume and root volume, with the greatest responses typically occurring at the highest CO2 concentration in the highest soil nitrogen treatment.  Root volume at 700 ppm CO2 and high soil nitrogen, for example, exceeded that of all other treatments by at least 45%, as did shoot volume by 42%.  Similarly, at high CO2 and soil nitrogen, coarse root and shoot weights exceeded those at ambient CO2 and high nitrogen by 80 and 88%, respectively.

Walker et al. (2000) published another paper on the same trees and treatments after five years of growth.  At this time, the trees exposed to the twice-ambient levels of atmospheric CO2 had heights that were 43, 64 and 25% greater than those of the trees exposed to ambient air and conditions of high, medium and low levels of soil nitrogen, respectively.  Similarly, the trunk diameters of the 700-ppm-trees were 24, 73 and 20% greater than the trunk diameters of the ambiently-grown trees exposed to high, medium and low levels of soil nitrogen.

Switching to a different species, Entry et al. (1998) grew one-year-old longleaf pine seedlings for 20 months in pots of high and low soil nitrogen content within open-top chambers maintained at atmospheric CO2 concentrations of 365 or 720 ppm, finding that the elevated CO2 caused no overall change in whole-plant biomass at low soil nitrogen, but that at high soil nitrogen, it increased it by 42%.  After two years of these treatments, Runion et al. (1999) also reported that rates of net photosynthesis were about 50% greater in the high CO2 treatment, irrespective of soil nitrogen content … and water content too.

Last of all, Finzi and Schlesinger (2003) measured and analyzed the pool sizes and fluxes of inorganic and organic nitrogen (N) in the floor and top 30 cm of mineral soil of the Duke Forest at the five-year point of a long-term FACE study, where half of the experimental plots are enriched with an extra 200 ppm of CO2.  In commencing this study, they had originally hypothesized that "the increase in carbon fluxes to the microbial community under elevated CO2 would increase the rate of N immobilization over mineralization," leading to a decline in the significant CO2-induced stimulation of forest net primary production that developed over the first two years of the experiment (DeLucia et al., 1999; Hamilton et al., 2002).  Quite to the contrary, however, they discovered "there was no statistically significant change in the cycling rate of N derived from soil organic matter under elevated CO2."  Neither was the rate of net N mineralization significantly altered by elevated CO2, nor was there any statistically significant difference in the concentration or net flux of organic and inorganic N in the forest floor and top 30-cm of mineral soil after 5 years of CO2 fumigation.  Hence, at this stage of the study, they could find no support for their original hypothesis, which suggests that the growth stimulation provided by elevated levels of atmospheric CO2 would gradually dwindle away to something rather insignificant before the stand reached its equilibrium biomass, although they continue to cling to this unsubstantiated belief.

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 pine trees by promoting a greater CO2-induced growth enhancement in soils of adequate, as opposed to insufficient, nitrogen content.  As in the case of aspen, however, there is evidence to suggest that at some point the response to increasing soil nitrogen saturates, and that higher N concentrations may sometimes even reduce the forest growth response to elevated CO2.

References
DeLucia, E.H., Hamilton, J.G., Naidu, S.L., Thomas, R.B., Andrews, J.A., Finzi, A., Lavine, M., Matamala, R., Mohan, J.E., Hendrey, G.R. and Schlesinger, W.H.  1999.  Net primary production of a forest ecosystem with experimental CO2 enrichment.  Science 284: 1177-1179.

Entry, J.A., Runion, G.B., Prior, S.A., Mitchell, R.J. and Rogers, H.H.  1998.  Influence of CO2 enrichment and nitrogen fertilization on tissue chemistry and carbon allocation in longleaf pine seedlings.  Plant and Soil 200: 3-11.

Hamilton, J.G., DeLucia, E.H., George, K., Naidu, S.L., Finzi, A.C. and Schlesinger, W.H.  2002.  Forest carbon balance under elevated CO2.  Oecologia 131: 250-260.

Johnson, D.W., Thomas, R.B., Griffin, K.L., Tissue, D.T., Ball, J.T., Strain, B.R. and Walker, R.F.  1998.  Effects of carbon dioxide and nitrogen on growth and nitrogen uptake in ponderosa and loblolly pine.  Journal of Environmental Quality 27: 414-425.

Runion, G.B., Mitchell, R.J., Green, T.H., Prior, S.A., Rogers, H.H. and Gjerstad, D.H.  1999.  Longleaf pine photosynthetic response to soil resource availability and elevated atmospheric carbon dioxide.  Journal of Environmental Quality 28: 880-887.

Walker, R.F., Geisinger, D.R., Johnson, D.W. and Ball, J.T.  1998.  Atmospheric CO2 enrichment and soil N fertility effects on juvenile ponderosa pine: Growth, ectomycorrhizal development, and xylem water potential.  Forest Ecology and Management 102: 33-44.

Walker, R.F., Johnson, D.W., Geisinger, D.R. and Ball, J.T.  2000.  Growth, nutrition, and water relations of ponderosa pine in a field soil as influenced by long-term exposure to elevated atmospheric CO2.  Forest Ecology and Management 137: 1-11.