Background
Plants growing in CO2-enriched air sometimes experience less-than-expected increases in their rates of photosynthesis if the soil in which they grow has an insufficient supply of nitrogen; and this phenomenon has led some scientists to suggest that low concentrations of soil nitrogen will reduce, or even eliminate, the productivity-enhancing effect of atmospheric CO2 enrichment in various tree species, thereby also reducing their ability to sequester carbon in their tissues and transfer additional carbon to the soils in which they grow.

What was done
In a study that sheds further light on this complex topic, Watanabe et al. grew three-year-old seedlings of hybrid larch F1 (Larix gmelinii var. japonica x Larix kaempferi) for 139 days under two different concentrations of atmospheric CO2 (360 and 720 ppm) together with two levels of soil nitrogen (N) supply -- low and high (0 and 30 kg/ha, respectively) -- in 7-liter pots placed within phytotrons located in Sapporo, Japan. This they did under natural daylight conditions (~90% of full sunlight) and at day/night temperatures of 25/16°C; and on the 118th day of the experiment, between 0900 and 1500 hours, they made numerous net photosynthesis rate measurements, while at the end of the 139-day period they harvested the trees and determined their increase in dry-weight biomass.

What was learned
The six scientists report that there was no significant difference between the net photosynthetic rates of the trees grown at the two different CO2 concentrations in either the low or high soil N treatments at the 118th day of the study, indicative of an essentially complete down-regulation of this important plant process. Nevertheless, they state that the biomass gained by the trees over the full 139-day experiment was significantly enhanced in the CO2-enriched air: by fully 44% in the low soil N treatment and 35% in the high soil N treatment.

As for how these large growth stimulations could have occurred, Watanabe et al. surmise that net photosynthesis rates under elevated CO2 would have been higher than those under ambient CO2 "during the early period of the experiment (short-term response to elevated CO2), in turn causing the seedlings under elevated CO2 to use more carbohydrates to produce needles." And they thus conclude that the greater number of needles in the CO2-enriched trees led to "an increase in the assimilation of the whole-plant scale even after down-regulation of photosynthesis," which would obviously have led to a greater production of biomass in the CO2-enriched trees.

What it means
The implications of these observations are rather straightforward. In the words of the Japanese researchers, "these results suggest that elevated CO2 may increase the growth of hybrid larch F1 even under low nutrient availability," such that "the growth of hybrid larch F1 may be stimulated under future elevated CO2," irrespective of soil nutrient conditions.