Positive effects of free air CO₂ enrichment on N remobilization and post-anthesis N uptake in winter wheat

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 twoyear Free Air CO₂ Enrichment (FACE) experiment was conducted with winter wheat grown under two CO₂ (˜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 (r²=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.