Midgley et al. (1999) collected seed from different genotypes of four closely related Leucadendron species growing in different nutrient-poor environments of South Africa and grew their seedlings for approximately one year in open-top chambers maintained at atmospheric CO2 concentrations of 350 and 700 ppm while regularly irrigating them with solutions of low and high N.  No significant interactions were found between CO2 and N levels, or between CO2 and species, as elevated CO2 significantly increased leaf starch concentrations in all genotypes and N treatments by 19 to 24% while increasing photosynthesis in every species and fertility regime by an average of 40%.

Much the same thing was found by Ziska (2003), who grew Canada thistle (Cirsium arvense) in pots that were watered to the drip point daily with one of three complete nutrient solutions that differed only in N concentration (3.0, 6.0 or 14.5 mM).  This study was conducted in controlled environment chambers maintained at 287 and 373 ppm CO2 from seeding until flowering; and over this entire period, he observed that "N supply did not affect the relative response to CO2 for any measured vegetative parameter."

A somewhat different response was observed by Arp et al. (1998), who grew six perennial plants common to The Netherlands under well-watered conditions in giant steel pots containing soil of either high or low N content within greenhouse compartments maintained at atmospheric CO2 concentrations of either 354 or 566 ppm for one year, after which two levels of water stress were imposed upon the plants throughout the second year of the study.  They found that elevated CO2 increased biomass production in all six species under conditions of optimal water supply, but only in the high soil N treatment.  Under conditions of water stress, however, even the low-soil-N plants responded; but their CO2-induced increase in water use efficiency was only half as large as that exhibited by the high-soil-N plants.

In one final study of this "General" category, Ishizaki et al. (2003) grew specimens of the perennial herb Polygonum cuspidatum (Japanese knotweed) from seeds collected from a single clone growing on Mt. Fuji, Japan, in open-top chambers at three soil N concentrations in ambient-CO2 air (370 ppm) and elevated-CO2 air (700 ppm), measuring a number of plant parameters at various times throughout the growing season.  They found that elevated CO2 significantly increased leaf mass per unit leaf area at all N levels and ages by an average of 29% and that leaf net assimilation rate rose by an average of 27%.  The elevated CO2 also significantly increased plant dry mass at the end of the study: by 24.4% at low N, by 45.5% at medium N, and by 91.5% at high N.

The results of these experiments mirror those observed in the other subgroups of this Subject Index entry.  Some plants sometimes will not respond at all to atmospheric CO2 enrichment at low levels of soil N, while some will.  In fact, some plants respond equally well to increases in the air's CO2 content when growing in soils exhibiting a whole range of N concentrations.  Most common of all, however, is the observation that plants respond ever better to rising atmospheric CO2 concentrations as soil N concentrations rise.  Interestingly, the current state of earth's atmosphere and land surface is one of jointly increasing CO2 and N concentrations, respectively.  Hence, the outlook is good for continually increasing terrestrial vegetative productivity in the years and decades ahead, as these trends continue.

References
Arp, W.J., Van Mierlo, J.E.M., Berendse, F. and Snijders, W.  1998.  Interactions between elevated CO2 concentration, nitrogen and water: effects on growth and water use of six perennial plant species.  Plant, Cell and Environment 21: 1-11.

Ishizaki, S., Hikosaka, K. and Hirose, T.  2003.  Increase in leaf mass per area benefits plant growth at elevated CO2 concentration.  Annals of Botany 91: 905-914.

Midgley, G.F., Wand, S.J.E. and Pammenter, N.W.  1999.  Nutrient and genotypic effects on CO2-responsiveness: photosynthetic regulation in Leucadendron species of a nutrient-poor environment.  Journal of Experimental Botany 50: 533-542.

Ziska, L.H.  2003.  The impact of nitrogen supply on the potential response of a noxious, invasive weed, Canada thistle (Cirsium arvense) to recent increases in atmospheric carbon dioxide.  Physiologia Plantarum 119: 105-112.