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Effects of Elevated CO2 on Nitrogen Fixation in Soybeans
Prevost, D., Bertrand, A., Juge, C. and Chalifour, F.P. 2010. Elevated CO2 induces differences in nodulation of soybean depending on bradyrhizobial strain and method of inoculation. Plant and Soil 331: 115-127.

What was done
The authors grew soybean (Glycine max [L.] Merr. cv. Lotus) plants from seed in 24-cm-deep pots filled with a sandy loam soil that was watered and fertilized as per standard agricultural procedures for a period of six weeks within controlled-environment chambers maintained at atmospheric CO2 concentrations of either 400 or 800 ppm, after inoculating either the soil or the seeds with either a reference strain (532c) of the nitrogen-fixing bacteria Bradyrhizobium japonicum, which is widely used in commercial operations in Canada, or with one of two strains of B. japonicum that are indigenous to the soils of Quebec (5Sc2 or 12NS14).

What was learned
Prevost et al. report that "elevated CO2 increased mass (+63%) and number (+50%) of soybean nodules, particularly medium and large, allowed a deeper nodule development, and increased shoot dry weight (+30%), shoot carbon uptake (+33%) and shoot nitrogen uptake (+78%), compared to ambient CO2."

What it means
The four Canadian scientists say their results "constitute the first report showing that elevated CO2 affects nodule size by allowing a greater production of large nodules, and influences nodule localization by favoring deeper nodule development on roots." With respect to the significance of these findings, they write that "medium and/or large nodules may confer advantages to legumes," since "they have been shown to improve drought tolerance of soybean (King and Purcell, 2001) and to exhibit higher nitrogenase activity in peanut (Tajima et al., 2007)." Also, they say their finding that "both shoot nitrogen and carbon uptakes are stimulated by elevated CO2 agrees with Rogers et al. (2009), who stated that photosynthetic activity in legumes under elevated CO2 does not acclimate [decrease with time] under optimal growing conditions, since the additional photosynthates produced are allocated to root nodules for N2 fixation." And they note that similar increases in nodule mass and number have been observed "with other legume species (Schortemeyer et al., 2002; Cabrerizo et al., 2001; Haase et al., 2007)," as well as "with soybean under drought (Serraj et al., 1998)," all of which findings bode well for legume farmers of the future, as well as for the people and livestock that will consume their produce.

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.

Haase, S., Neumann, G., Kania, A., Kuzyakov, Y., Romheld, V. and Kandeler, E. 2007. Elevation of atmospheric CO2 and N-nutritional status modify nodulation, nodule-carbon supply, and root exudation of Phaseolus vulgaris L. Soil Biology & Biochemistry 39: 2208-2221.

King, C.A. and Purcell, L.C. 2001. Soybean nodule size and relationship to nitrogen fixation response to water deficit. Crop Science 41: 1099-1107.

Rogers, A., Ainsworth, E.A. and Leakey, A.D.B. 2009. Will elevated carbon dioxide concentration amplify the benefits of nitrogen fixation in legumes? Plant Physiology 151: 1009-1016.

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.

Serraj, R., Sinclair, T.R. and Allen, L.H. 1998. Soybean nodulation and N2 fixation response to drought under carbon dioxide enrichment. Plant, Cell and Environment 21: 491-500.

Tajima, R., Lee, O.N., Abe, J., Lux, A. and Morita, S. 2007. Nitrogen-fixing activity of root nodules in relation to their size in peanut (Arachis hypogaea L.). Plant Production Science 10: 423-429.

Reviewed 20 October 2010