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Nitrate assimilation is inhibited by elevated CO2 in field-grown wheat

Abstract

Total protein and nitrogen concentrations in plants generally decline under elevated CO2 atmospheres1,2. Explanations for this decline include that plants under elevated CO2 grow larger, diluting the protein within their tissues3,4; that carbohydrates accumulate within leaves, downregulating the amount of the most prevalent protein Rubisco2; that carbon enrichment of the rhizosphere leads to progressively greater limitations of the nitrogen available to plants4; and that elevated CO2 directly inhibits plant nitrogen metabolism, especially the assimilation of nitrate into proteins in leaves of C3 plants5. Recently, several meta-analyses have indicated that CO2 inhibition of nitrate assimilation is the explanation most consistent with observations6,7,8. Here, we present the first direct field test of this explanation. We analysed wheat (Triticum aestivum L.) grown under elevated and ambient CO2 concentrations in the free-air CO2 enrichment experiment at Maricopa, Arizona. In leaf tissue, the ratio of nitrate to total nitrogen concentration and the stable isotope ratios of organic nitrogen and free nitrate showed that nitrate assimilation was slower under elevated than ambient CO2. These findings imply that food quality will suffer under the CO2 levels anticipated during this century unless more sophisticated approaches to nitrogen fertilization are employed.

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Figure 1: Total nitrogen (percentage of dry matter) in wheat leaves as a function of time after emergence (weeks).
Figure 2: Nitrate as a percentage of total N in wheat leaves as a function of time after emergence (weeks).
Figure 3: Isotopic signature of organic N (δ15Norg) in wheat leaves as a function of time after emergence (weeks).
Figure 4: Isotopic signature of nitrate (δ15Nnitrate) in wheat leaves as a function of time after emergence (weeks).

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Acknowledgements

This work was supported by NSF IOS-08-18435 and the National Research Initiative Competitive Grant no. 2008-35100-04459 from the USDA National Institute of Food and Agriculture. We thank A. Torbert and S. Prior, USDA–ARS National Soil Dynamics Laboratory, Auburn, Alabama for sharing unpublished data from their Arizona FACE soil analyses.

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All authors contributed to the data set, discussed the results and commented on the manuscript. A.J.B. and M.B. designed the study. M.B. conducted the chemical analyses. A.J.B. carried out the statistical analysis and wrote the paper.

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Correspondence to Arnold J. Bloom.

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The authors declare no competing financial interests.

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J. Bloom, A., Burger, M., A. Kimball, B. et al. Nitrate assimilation is inhibited by elevated CO2 in field-grown wheat. Nature Clim Change 4, 477–480 (2014). https://doi.org/10.1038/nclimate2183

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