Will Elevated Carbon Dioxide Concentration Amplify the Benefits of Nitrogen Fixation in Legumes?

Growth at elevated [CO2] stimulates photosynthesis and increases carbon (C) supply in all C-3 species. A sustained and maximal stimulation in productivity at elevated [CO2] requires an enhanced nutrient supply to match the increase in C acquisition. The ability of legumes to exchange C for nitrogen (N) with their N-2-fixing symbionts has led to the hypothesis that legumes will have a competitive advantage over non-leguminous species when grown at elevated [CO2]. On balance, evidence suggests that in managed systems, legumes are more responsive to elevated [CO2] than other plants (e.g. Ainsworth and Long, 2005); however, in natural ecosystems, nutrient availability can limit the response of legumes to elevated [CO2] (Hungate et al., 2004; van Groenigen et al., 2006). Here, we consider these observations, outline the mechanisms that underlie them, and examine recent work that advances our understanding of how legumes respond to growth at elevated [CO2]. First we highlight the global importance of legumes and provide a brief overview of the symbiotic relationship.