CO₂ Effects on Nitrogen Fixation in Soybeans

Lam et al. (2012) 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).

To further explore this topic and 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. And what did they learn?

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."

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 it should be noted, 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.

Additional 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.