Decreased wheat grain yield stimulation by free air CO₂ enrichment under N deficiency is strongly related to decreased radiation use efficiency enhancement

Uncertainty still exists about the extent and mechanisms of yield stimulation by elevated atmospheric CO₂ (e [CO₂]) in wheat. Particularly, data of the e[CO₂] effect under severe N deficiency from field experiments are scarce. To investigate the interaction of e[CO₂] and N fertilization on important variables that determine grain yield and are often used in crop simulation models, e.g. radiation absorption by the canopy (AR), radiation use efficiency (RUE) and specific leaf N weight (SLNW), a two-year Free Air CO₂ Enrichment (FACE) experiment was conducted with two [CO₂] (393 and 600 ppm) and three N levels (severe N deficiency (Nd) with 40 (1st year) and 35 kg N ha⁻¹ (2nd year); adequate N supply with 180 (1st year) and 200 kg N ha⁻¹ (2nd year); and excess N supply with 320 kg N ha⁻¹ (1st and 2nd year). Final aboveground biomass ranged from 816 to 2012 g m⁻² and grain yield from 417 to 973 g m⁻². e[CO₂] increased aboveground biomass by 13, 18 and 14% and grain yield by 10, 17 and 17% under Nd, Nad and Nex, respectively. Yield stimulation was primarily due to enhanced grain number. With increasing N supply, peak values of green area index were increased under e[CO₂] by 4 up to 22%, while AR was unaffected. RUE was increased by both rising SLNW, which depended on N supply, and e[CO₂] and the RUE increase was larger under Nad (+20%) and Nex (+18%) than under Nd (+6%). SLNW was decreased by e[CO₂] and this decrease was very similar among N levels (∼−6%). However, if leaf area index was included as covariable, then a e[CO₂ induced decrease of SLNW was only found under Nd. The present study demonstrates that yield stimulation by e[CO₂] is smaller under severe N deficiency compared to high N supply in wheat. In contrast to the results of another FACE study, this decrease was not due to reduced AR but reduced RUE, which might be attributed to both restrictions on source activity, i.e. photosynthetic capacity and sink size, i.e. ear growth.