What was done
Black locust (Robinia pseudoacacia) seedlings were grown for 100 days post-germination in controlled environments receiving atmospheric CO2 concentrations of 350 and 700 ppm.  In addition, seedlings were grown with and without supplemental nitrogen at a daytime air temperature of 26 or 30°C, to study the effects of elevated CO2, fertilization, and temperature on root nitrogen exudation in this nitrogen-fixing tree species.

What was learned
Elevated CO2 did not significantly affect the rate of organic nitrogen exudation in this species.  The 4°C rise in air temperature, however, did increase it; and resulted in 37% more nitrogen exudation than that observed in the lower temperature regime.  Similarly, nitrogen fertilization increased organic nitrogen exudation by 57% relative to that measured in unfertilized control seedlings.  There were no significant interactions found between the three tested variables.

Averaged across all treatments, the amount of organic nitrogen exuded from seedling roots amounted to between 1 and 2% of the total symbiotically-fixed nitrogen in this species, suggesting that a small amount of this nitrogen becomes available for uptake by neighboring vegetation upon its exudation.

What it means
As the atmospheric CO2 concentration rises, a small amount of organic nitrogen, which is exuded from root systems of nitrogen-fixing tree species (like black locust), will still become available for direct uptake by neighboring non-nitrogen-fixing species.  This phenomenon can increase the nitrogen content within such plants, thereby enhancing their absolute growth response to the increasing CO2 content of the air.

In addition, if air temperatures rise in the future, regardless of the cause, significantly greater rates and amounts of nitrogen exudation can be expected from nitrogen-fixing species, which should increase the amount of nitrogen available to neighboring species, thus amplifying their CO2-induced growth responses.