What was done
The authors grew well watered garden bean (Phaseolus vulgaris L.) seedlings for 21 days in rhizoboxes maintained at two levels of soil nitrogen supply (limited and sufficient) within controlled-environment growth chambers maintained at atmospheric CO2 concentrations of either 400 or 800 ppm, while making a number of measurements pertinent to the development of the symbiosis responsible for the fixation of nitrogen in this particular leguminous species.

What was learned
Haase et al. report that "the release of nod-gene-inducing flavonoids (genistein, daidzein and coumestrol) was stimulated under elevated CO2, independent of the N supply, and was already detectable at early stages of seedling development," as was "a CO2-induced stimulation in root exudation of sugars and malate as a chemo-attractant for rhizobia," which "was detected in 0.5-1.5 cm apical root zones as potential sites." In addition, they found, in the nodules, that "elevated CO2 increased the accumulation of malate as a major carbon source for the microsymbiont and of malonate with essential functions for nodule development." Hence, it was no surprise (except, perhaps, for the huge size of the CO2-induced enhancements observed) that the number of root nodules was found to be 340% greater in the CO2-enriched microcosms and that nodule biomass was fully 400% greater.

What it means
Noting that similar results have been reported by Zanetti et al. (1996), Cabrerizo et al. (2001), Schortemeyer et al. (2002) and Feng et al. (2004), the six German researchers say their results are suggestive of a "selective stimulation of factors involved in establishing and maintaining the rhizobium symbiosis at elevated CO2," which bodes well indeed for the acquisition of much-needed nitrogen by earth's natural and agro-ecosystems in the years and decades ahead, as the atmosphere's CO2 concentration continues to climb ever higher.

References
Cabrerizo, P.M., Gonzalez, E.M., Aparico-Tejo, P.M. and Aresse-Igor, C. 2001. Continuous CO2 enrichment leads to increased nodule biomass, carbon availability to nodules and activity of carbon-metabolizing enzymes but does not enhance specific nitrogen fixation in pea. Physiologia Plantarum 113: 33-40.

Feng, Z., Dyckmans, J. and Flessa, H. 2004. Effects of elevated carbon dioxide concentration on growth and N2 fixation of young Robinia pseudoacacia. Tree Physiology 24: 323-330.

Shortemeyer, M., Atkin, O.K., McFarlane, N. and Evans, J.R. 2002. N2 fixation by Acacia species increases under elevated atmospheric CO2. Plant, Cell and Environment 25: 567-579.

Zanetti, S., Hartwig, U.A., Luscher, A., Hebeisen, T., Frehner, M., Fischer, B.U., Hendrey, G.R., Blum,. H. and Nosberger, J. 1996. Stimulation of symbiotic N2 fixation in Trifolium repens L. under elevated atmospheric pCO2 in a grassland ecosystem. Plant Physiology 112: 575-583.