What was done
Using controlled-environment growth chambers, wherein they grew well-watered and adequately-fertilized alfalfa (Medicago sativa L.) inoculated with two different strains (A2 and NRG34) of rhizobia (Sinorhizobium meliloti), the authors studied the effects of a 400- to 800-ppm doubling of the atmosphere's CO2 concentration on a number of plant physiological parameters, as well as on plant nutritive value and digestibility, which they assessed at the end of the 56-day experiment.

What was learned
By the end of the study, plant shoot dry weight had been increased by approximately 70% and 50% in the 800-ppm-CO2 air (enriched) compared to the 400-ppm-CO2 air (ambient) in the A2 and NRG34 strains, respectively, while corresponding results for plant root dry weight were 60% and 0%. Similarly, nitrogenase activity, indicative of the degree of symbiotic nitrogen fixation, was stimulated by about 55% and 25%, respectively, in the A2 and NRG34 strains. Last of all, the CO2-enriched air resulted in what the researchers describe as "a slight increase" in the in vitro true digestibility of the A2 strain, while it resulted in the opposite in the NRG34 strain.

What it means
Bertrand et al. write in the concluding sentence of their paper's abstract that their results "show that it is possible to identify rhizobial strains to improve plant performance under predicted future CO2 concentrations with no negative effect on nutritive value." This ability -- when utilized -- should enable mankind to greatly benefit from the ongoing rise in the air's CO2 content.