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Effects of Elevated CO2 and Mycorrhizal Colonization on a Nitrogen-Fixing Tree
Olesniewicz, K.S. and Thomas, R.B.  1999.  Effects of mycorrhizal colonization on biomass production and nitrogen fixation of black locust (Robinia pseudoacacia) seedlings grown under elevated atmospheric carbon dioxide.  New Phytologist 142: 133-140.

What was done
Black locust (Robinia pseudoacacia) seedlings were grown for approximately two months in controlled environment chambers receiving atmospheric CO2 concentrations of 350 and 710 ppm.  Additionally, half of the seedlings in each CO2 treatment were previously inoculated with an arbuscular mycorrhizal fungal species, or were left un-inoculated to serve as controls.  Thus, the authors studied the effects of elevated CO2 and fungal inoculation on growth in this nitrogen-fixing tree species.

What was learned
Non-inoculated seedlings showed no signs of fungal colonization on any of their roots.  In inoculated seedlings, elevated CO2 increased the proportion of fine-root fungal colonization by 39% relative to that observed in ambiently-grown inoculated control seedlings.  Although inoculated plants had biomass values that were 2- to 3-fold larger than those displayed by non-inoculated plants, elevated CO2 caused the greatest percentage increase in total plant biomass in the non-inoculated plants (180% vs. 51%).  In addition, elevated CO2 increased rates of nitrogen-fixation in non-inoculated plants by 69%, while it had no effect on this parameter in inoculated plants.  It also increased nodule mass in non-inoculated and inoculated seedlings by 92 and 135%, respectively.  Furthermore, atmospheric CO2 enrichment increased the amount of seedling nitrogen derived from nitrogen-fixation by 212% in non-inoculated seedlings and by 90% in inoculated seedlings.

What it means
As the air's CO2 concentration maintains its upward course, black locust seedlings will likely respond by enhancing their rates of photosynthesis, biomass production and carbon sequestration under all degrees of mycorrhizal fungal colonization, including non-colonization, of their roots.