The effect of elevated carbon dioxide concentration [CO₂] on symbiotic nitrogen (N₂) fixation by soybean under open-air conditions has not been reported. Two soybean cultivars (Glycine max (L.) Merr. cv. Zhonghuang 13 and cv. Zhonghuang 35) were grown to maturity under ambient (415 ± 16 mmol mol^–1) and elevated (550 ± 17 mmol mol^–1) [CO₂] at the Free-Air Carbon dioxide Enrichment (FACE) experimental facility in a semi-arid region in northern China. Elevated [CO₂] increased above- and below-ground biomass by 17% and 16%, respectively, but had no significant effect on the tissue total N and total C concentration at maturity. Elevated [CO₂] marginally increased the percentage of N derived from the atmosphere (%Ndfa, estimated by natural abundance) from 59 to 79% for the above-ground parts of Zhonghuang 13, and the amount of N fixed from 1.1 to 1.9 g N plant^–1 (16.6 to 27.5 g N m^–2), but had no significant effect on either parameter for Zhonghuang 35. It appears that impact of elevated [CO₂] on N₂ fixation of soybean in this semi-arid region is cultivar dependent and that these differences could be exploited to improve N supply in these cropping systems in future high carbon dioxide atmospheres.

 

Keywords

Free-air carbon dioxide enrichment (FACE), symbiotic nitrogen fixation, nodulation, natural abundance, soybean cultivar