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CO2 Effects on Nitrogen Fixation in Soybeans
Reference
Lam, S.K., Hao, X., Lin, E., Han, X., Norton, R., Mosier, AR.., Seneweera, S. and Chen, D. 2012. Effect of elevated carbon dioxide on growth and nitrogen fixation of two soybean cultivars in northern China. Biology and Fertility of Soils 48: 603-606.

Background
The authors write that "the response of symbiotic N2 fixation in legumes to elevated CO2 is crucial to carbon sequestration in future CO2 environments because continued C sequestration is contingent on additional N input (Hungate et al., 2003)," and they state, in this regard, that "a recent review suggests that symbiotic N2 fixation at elevated CO2 could be enhanced as a result of CO2-induced increase in nodule size, nodule number per plant, or specific nitrogenase activity," citing Rogers et al. (2009).

What was done
Working at a free-air carbon dioxide enrichment (FACE) facility in Changping, Beijing, China, Lam et al. grew two cultivars (Zhonghuang 13, a high-protein cultivar, and Zhonghuang 35, a high-oil cultivar) of soybean (Glycine max L.) from seed to maturity at either ambient or elevated atmospheric CO2 concentrations (415 or 550 ppm, respectively) under conditions of adequate fertility and water supply.

What was learned
The seven scientists report that the elevated CO2 treatment increased the above- and below-ground biomass of Zhonghuang 13 by 18.3 and 11.1%, respectively, while it increased the above- and below-ground biomass of Zhonghuang 35 by 15.6 and 20.0%, respectively. They additionally note that the high-CO2 treatment also boosted the percentage of N derived from the atmosphere (%Ndfa) for Zhonghuang 13 from 59% to 79%, corresponding to an amount of N fixed ranging from 166 to 275 kg N ha-1; but they say that the elevated CO2 treatment "had no significant effect on either parameter for Zhonghuang 35."

What it means
The Chinese researchers say their results suggest that "variation in N2 fixation ability in response to elevated CO2 should be used as a key trait for selecting cultivars for future climate with respect to meeting the higher N demand driven by a carbon-rich atmosphere." And we wish to note, in this regard, that it is most interesting that it was the high-protein (nitrogen-needing) cultivar that experienced the greatest increase in %Ndfa (34%) as a result of the nearly identical 33% increase in the atmosphere's CO2 concentration.

References
Hungate, B.A., Dukes, J.S., Shaw, M.R., Luo, Y. and Field, C.B. 2003. Nitrogen and climate change. Science 302: 1512-1513.

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.

Reviewed 28 November 2012