Increased C availability at elevated carbon dioxide concentration improves N assimilation in a legume

Plant growth is typically stimulated at elevated carbon dioxide concentration ([CO 2 ]), but a sustained and maximal stimulation of growth requires acquisition of additional N in proportion to the additional C fixed at elevated [CO 2 ]. We hypothesized that legumes would be able to avoid N limitation at elevated [CO 2 ]. Soybean was grown without N fertilizer from germination to final senescence at elevated [CO 2 ] over two growing seasons under fully open‐air conditions, providing a model legume system. Measurements of photosynthesis and foliar carbohydrate content showed that plants growing at elevated [CO 2 ] had a c . 25% increase in the daily integral of photosynthesis and c . 58% increase in foliar carbohydrate content, suggesting that plants at elevated [CO 2 ] had a surplus of photosynthate. Soybeans had a low leaf N content at the beginning of the season, which was a further c . 17% lower at elevated [CO 2 ]. In the middle of the season, ureide, total amino acid and N content increased markedly, and the effect of elevated [CO 2 ] on leaf N content disappeared. Analysis of individual amino acid levels supported the conclusion that plants at elevated [CO 2 ] overcame an early‐season N limitation. These soybean plants showed a c . 16% increase in dry mass at final harvest and showed no significant effect of elevated [CO 2 ] on leaf N, protein or total amino acid content in the latter part of the season. One possible explanation for these findings is that N fixation had increased, and that these plants had acclimated to the increased N demand at elevated [CO 2 ].