Consider, for example, the study of Spinnler et al. (2003), who planted eight beech (Fagus sylvatica) and eight spruce (Picea abies) seedlings, along with five to six individuals from each of several typical understory species, in each of 32 lysimeters that were enclosed in pairs in 16 open-top chambers (OTCs) in the field at Birmensdorf, Switzerland.  One lysimeter in each OTC was filled with a nutrient-poor acidic soil, while the other lysimeter was filled with a fertile calcareous soil.  Adding yet another variable to the mix, the eight beech seedlings were obtained from four different provenances (populations), while the eight spruce seedlings were also selected from eight different provenances, but in this case from two populations and six clones.  Under these conditions, the trees were grown for four full years, half of them in ambient air having a mean CO2 concentration of 370 ppm, and half of them in CO2-enriched air having a mean concentration of 570 ppm.  At the end of the study, it was determined that the trees that were rooted in the fertile calcareous soil experienced CO2-induced growth responses that ranged from -4% to +40% for beech and from +10% to +74% for spruce, indicative of a primarily positive response among the vast majority of the several populations studied.  For spruce trees rooted in the nutrient-poor acidic soil, the responses were also positive, ranging from +9% to +38%; but for beech trees the responses were negative, ranging from -7% to -26% and indicative of the climate-alarmist view described in the preceding paragraph.

In another study from this extreme end of the CO2-response spectrum, where low soil fertility sometimes thwarts the stimulation of growth that is typically provided by increases in the air's CO2 content, Sigurdsson et al. (2001) grew black cottonwood (Populus trichocarpa) seedlings for three consecutive growing seasons in closed-top chambers maintained at atmospheric CO2 concentrations of 350 and 700 ppm in combination with low and high soil nutrition.  In this southern Iceland experiment, atmospheric CO2 enrichment boosted tree biomass production by nearly 50% in the high soil fertility treatment; but it had no effect upon the growth of the trees in the low soil fertility treatment.

Another study that saw low soil fertility negate the usual benefits of atmospheric CO2 enrichment was conducted by Hoffmann et al. (2000), who grew seedlings of the Brazilian savannah tree Keilmeyera coriacea in soil containers irrigated with low- and high-strength nutrient solutions in controlled environment chambers maintained at ambient (350 ppm) and elevated (700 ppm) atmospheric CO2 concentrations.  In addition, ten weeks after germination, half of the seedlings in each treatment were clipped to the ground to simulate burning.  In the first stage of the study, elevated CO2 and soil nutrition did not significantly interact with each other.  After cutting the seedlings to the ground, however, the extra CO2 stimulated their regrowth by fully 300%, but only under conditions of high soil fertility, providing a third example of a situation where the positive effects of elevated CO2 on plant growth were totally negated by a lack of sufficient soil fertility.

A fourth and final example of low soil nutrient availability negating the positive effects of atmospheric CO2 enrichment is provided by the study of Nagashima et al. (2003), who established even-aged stands of Chenopodium album (a weed commonly found in abandoned fields and flood plains) that were maintained at ambient and twice-ambient atmospheric CO2 concentrations at low and high levels of soil nutrient availability in open-top chambers located within the experimental garden of Tohoku University, Sendai, Japan, after which the growth of individual plants was monitored weekly until flowering.  At the conclusion of the experiment, the aboveground biomass of the plants in the high nutrient regime was found to have been enhanced by fully 50% by the doubling of the air's CO2 content, whereas there was no significant effect of elevated CO2 on aboveground biomass in the low nutrient regime.

Much more common are experiments that show a reduced, but still positive, growth response to atmospheric CO2 enrichment in low-fertility soil, a case in point being the study of Matthies and Egli (1999), who grew perennial ryegrass (Lolium perenne) and alfalfa (Medicago sativa) for two months within open-top chambers maintained at atmospheric CO2 concentrations of 375 and 590 ppm in pots containing either unfertilized soil or soil that was fertilized to produce an optimal nutrient regime.  In addition, they infected both species with Rhinanthus alectorolophus, a widely distributed parasitic plant of central Europe.  Under these conditions, the extra CO2 increased the biomass of the alfalfa and ryegrass by an average of 29% in the high soil-nutrient treatment and by 18% in the low soil-nutrient treatment.

In a somewhat similar study, Hartz-Rubin and DeLucia (2001) established model herbaceous communities - representative of the early stages of abandoned agricultural fields in process of returning to their native state - in plastic tubs placed within growth chambers that were maintained at atmospheric CO2 concentrations of either 370 or 800 ppm, while the plant assemblages within the tubs were either fertilized with a soil nutrient solution or left unfertilized.  In both situations, atmospheric CO2 enrichment caused faster canopy development, independent of soil fertilization, as indicated by greater canopy heights (relative to the plants in the ambient-air treatment) at every point in time throughout the two-month study.  By the end of the experiment, however, there was a significant difference in the CO2-induced biomass responses of the plants in the two soil nutrient treatments, with total biomass being enhanced by 26% in the fertilized treatment and by a lesser-but-still-respectable 20% in the non-fertilized treatment.

Even the results of these latter two experiments may be misleading, however, for in a review of a much larger body of the pertinent scientific literature, Lloyd and Farquhar (1996) found that relative (percentage) plant growth enhancements due to atmospheric CO2 enrichment were, in their words, "nearly as often as not greater under low nutrient conditions."  And in an expanded review published four years later, Lloyd and Farquhar (2000) found much the same thing, stating that "the most frequent observation is that there is actually no significant difference in the growth responses of low- vs. high-nutrient plants and quite often low-nutrient plants have greater responses."

The very next year, Poorter and Perez-Soba (2001) performed yet another meta-analysis of published experimental data; and their work indicated that inadequate levels of soil fertility may in fact slightly reduce the percentage plant growth enhancement caused by atmospheric CO2 enrichment.  A similar result had been obtained several years earlier by Idso and Idso (1994) in their review of the scientific literature, but only for plants exposed to a 300 ppm increase in atmospheric CO2 concentration.  For an increase of 600 ppm or greater, their results were even more positive than those described by Lloyd and Farqhuar, with plant growth enhancements due to atmospheric CO2 enrichment being more often than not "greater under low nutrient conditions."

In conclusion, these many observations suggest that, in the mean, earth's vegetation will likely exhibit significant increases in relative growth rates in response to increases in the air's CO2 content, independent of soil fertility levels, for as long as humanity continues to pump CO2 into the atmosphere.

References
Hartz-Rubin, J.S. and DeLucia, E.H.  2001.  Canopy development of a model herbaceous community exposed to elevated atmospheric CO2 and soil nutrients.  Physiologia Plantarum 113: 258-266.

Hoffmann, W.A., Bazzaz, F.A., Chatterton, N.J., Harrison, P.A. and Jackson, R.B.  2000.  Elevated CO2 enhances resprouting of a tropical savanna tree.  Oecologia 123: 312-317.

Idso, K.E. and Idso, S.B.  1994.  Plant responses to atmospheric CO2 enrichment in the face of environmental constraints: a review of the past 10 years' research.  Agricultural and Forest Meteorology 69: 153-203.

Lloyd, J. and Farquhar, G.D.  1996.  The CO2 dependence of photosynthesis, plant growth responses to elevated CO2 concentrations and their interactions with soil nutrient status. I. General principles and forest ecosystems.  Functional Ecology 10: 4-32.

Lloyd, J. and Farquhar, G.D.  2000.  Do slow-growing species and nutrient-stressed plants consistently respond less to elevated CO2?  A clarification of some issues raised by Poorter (1998).  Global Change Biology 6: 871-876.

Matthies, D. and Egli, P.  1999.  Response of a root hemiparasite to elevated CO2 depends on host type and soil nutrients.  Oecologia 120: 156-161.

Schutz, M. and Fangmeier, A.  2001.  Growth and yield responses of spring wheat (Triticum aestivum L. cv. Minaret) to elevated CO2 and water limitation.  Environmental Pollution 114: 187-194.

Nagashima, H., Yamano, T., Hikosaka, K. and Hirose, T.  2003.  Effects of elevated CO2 on the size structure in even-aged monospecific stands of Chenopodium album.  Global Change Biology 9: 619-629.

Poorter, H. and Perez-Soba, M.  2001.  The growth response of plants to elevated CO2 under non-optimal environmental conditions.  Oecologia 129: 1-20.

Sigurdsson, B.D., Thorgeirsson, H. and Linder, S.  2001.  Growth and dry-matter partitioning of young Populus trichocarpa in response to carbon dioxide concentration and mineral nutrient availability.  Tree Physiology 21: 941-950.

Spinnler, D., Egli, P. and Korner, C.  2003.  Provenance effects and allometry in beech and spruce under elevated CO2 and nitrogen on two different forest soils.  Basic and Applied Ecology 4: 467-478.