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

Total protein and nitrogen concentrations in plants generally decline under elevated CO2 atmospheres(1,2). Explanations for this decline include that plants under elevated CO2 grow larger, diluting the protein within their tissues(3,4); that carbohydrates accumulate within leaves, downregulating the amount of the most prevalent protein Rubisco(2); that carbon enrichment of the rhizosphere leads to progressively greater limitations of the nitrogen available to plants(4); and that elevated CO2 directly inhibits plant nitrogen metabolism, especially the assimilation of nitrate into proteins in leaves of C-3 plants(5). Recently, several meta-analyses have indicated that CO2 inhibition of nitrate assimilation is the explanation most consistent with observations(6-8). 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.