What was done
Seedlings of the nitrogen-fixing black locust (Robinia pseudoacacia L.) tree were grown for 100 days in controlled environments fumigated with atmospheric CO2 concentrations of 350 and 700 ppm.  In addition, seedlings were exposed to ambient and elevated air temperatures (26 and 30°C) and none or some additional nitrogen fertilization.  Thus, the authors studied the effects of elevated CO2, air temperature, and nitrogen fertilization on the growth and root exudation of this specialized N-fixing tree.

What was learned
Although elevated CO2 did not significantly increase total seedling biomass, it did increase it by 14% when averaged across all temperature and fertilization regimes.  However, elevated temperature did significantly increase seedling biomass by 55%, when averaged across all CO2 and fertilization treatments, as did fertilization, by 157%, when averaging across all CO2 and temperature levels.

With respect to root exudation, a similar pattern emerged.  Elevated CO2 did not significantly increase total dissolved organic carbon compounds exuded from seedling roots over a 24-hour period, yet plants grown in elevated CO2 exuded 20% more such compounds than ambiently-grown plants did, when averaged across all temperature and fertilization treatments.  And once again, elevated temperature and fertilization significantly increased root exudation by 71 and 55%, respectively, when averaged across the other main effect variables.

What it means
As the air's CO2 content rises, it is likely that this nitrogen-fixing tree will exhibit enhanced rates of biomass production and exudation of dissolved organic compounds from its roots.  Moreover, if the ambient air temperature rises, even by as much as 4°C, the positive effects of this phenomenon on biomass production and root exudation will likely be even greater than that resulting from increasing atmospheric CO2 concentration.  Similarly, if anthropogenic nitrogen deposition raises soil nitrogen content, black locust seedling growth and root exudation rates will likely be greater than the rate increase resulting from increasing atmospheric CO2 concentration alone.

Although it was undetectable in this short-term experiment, the results of this study suggest that rising atmospheric CO2 concentrations, in combination with rising air temperature and/or increasing nitrogen deposition, will increase seedling biomass and root exudation to a greater extent than what typically results from elevated CO2 alone.  This phenomenon, in turn, should enhance the life-supporting environment of the tree's rhizosphere and lead to greater populations of soil organisms.