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Elevated atmospheric CO2 concentration (e[CO2]) often increases cereal yield, but can also decrease vegetative and grain tissue nitrogen (N) concentration that might affect future food and feed quality. However, data about CO2 x N interactions on key processes determining grain N yield and concentration, which are remobilization of vegetative N taken up before anthesis (Nrem) and post-anthesis N uptake (Nabs), are scarce. Therefore, a two-year Free Air CO2 Enrichment (FACE) experiment was conducted with winter wheat grown under two CO2 ((similar to)393 and 600 ppm) and three N levels (severe deficiency with N nutrition index (NNI) of 0.4, adequate with NNI of 0.8 and excess with NNI of 1.1).

e[CO2] did not influence the allometric relation between aboveground N concentration and biomass up to anthesis. At anthesis, e[CO2] increased N acquisition of stem and ear, but not of leaf. Correspondingly, e[CO2] increased Nrem of stem and chaff. Moreover, e[CO2] enhanced the efficiency of Nrem of stem and aboveground plant in the first year, indicating increased N mobilization from vegetative tissue. Nabs tended to be increased by e[CO2], especially in the second year. Finally, e[CO2] increased grain N yield (8 to 12%), N use efficiency (13 to 18%) and N uptake efficiency (10 to 12%). Grain N concentration was slightly decreased by e[CO2] in both years (-1 to -6%), while grain N concentration was considerably larger (9 to 19%) in the second year compared to the first year. There was a strong linear relation between grain N yield and grain number (r(2) = 0.98) that was not influenced by e[CO2], suggesting grain number as important factor determining the grain N yield increase under e[CO2]. Grain N concentration was more strongly affected by e[CO2] than mean N content per grain.