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Effects of Elevated CO2 on Nitrogen Assimilation by Soybeans
Rogers, A., Gibon, Y., Stitt, M., Morgan, P.B., Bernacchi, C.J., Ort, D.R. and Long, S.P. 2006. Increased C availability at elevated carbon dioxide concentration improves N assimilation in a legume. Plant, Cell and Environment 29: 1651-1658.

What was done
Can legumes overcome the potential constraints of a limiting N supply during growth at elevated CO2? This is the question the authors sought to answer in a two-year study of soybeans (Glycine max (L.) Merr.) grown without N fertilization from germination to senescence at the SoyFACE facility in Champaign, Illinois, USA, under fully open-air conditions.

What was learned
Rogers et al. report that the plants growing at elevated CO2 (552 vs. 370 ppm) "had a c. 25% increase in the daily integral of photosynthesis and c. 58% increase in foliar carbohydrate content." They also say that plants of both treatments "had a low leaf N content at the beginning of the season, which was a further c. 17% lower at elevated CO2." At mid-season, however, they found that "ureide, total amino acid and N content increased markedly, and the effect of elevated CO2 on leaf N content disappeared," as the CO2-enriched plants "overcame an early-season N limitation." Last of all, the seven scientists report that the plants exposed to the extra 49% of atmospheric CO2 "showed a c. 16% increase in dry mass at final harvest and showed no significant effect of elevated CO2 on leaf N, protein or total amino acid content in the latter part of the season."

What it means
Rogers et al. remark that "one possible explanation for these findings is that N fixation had increased, and that [the] plants had acclimated to the increased N demand at elevated CO2." In addition, they say their results suggest that "in N-poor ecosystems, growth at elevated CO2 favors legumes that tend to avoid limitations of photosynthetic capacity and dominate non-leguminous species (Hanley, Trofimov and Taylor, 2004; Winkler and Herbst, 2004)," and they further suggest that "in the absence of other major limitations, field-grown soybean, and legumes in general, will show a continued increase in productivity with rising CO2."

Hanley, M.E., Trofimov, S. and Taylor, G. 2004. Species-level effects more important than functional group-level responses to elevated CO2: evidence from simulated turves. Functional Ecology 18: 304-313.

Winkler, J.B. and Herbst, M. 2004. Do plants of a semi-natural grassland community benefit from long-term CO2 enrichment? Basic and Applied Ecology 5: 131-143.

Reviewed 8 November 2006