Effects of free air carbon dioxide enrichment (FACE) on nitrogen assimilation and growth of winter wheat under nitrate and ammonium fertilization

A 2-year Free Air CO₂ Enrichment (FACE) experiment was conducted with winter wheat. It was investigated whether elevated atmospheric CO₂ concentration (e[CO₂]) inhibit nitrate assimilation and whether better growth and nitrogen acquisition under e[CO₂] can be achieved with an ammonium-based fertilization as it was observed in hydroponic culture with wheat. Under e[CO₂] a decrease in nitrate assimilation has been discussed as the cause for observed declines in protein concentration in C₃ cereals. Wheat was grown under ambient [CO₂] and e[CO₂] (600 ppm) with three levels (deficiency, optimal, and excessive) of nitrate-based fertilization (calcium ammonium nitrate; CAN) or with optimal ammonium-based fertilization. Ammonium fertilization was applied via injection of an ammonium solution into the soil in the 1st year and by surface application of urea combined with nitrification inhibitors (UNI) in the 2nd year. Results showed that ammonium-based fertilization was successfully achieved in the 2nd year with respect to nitrification control, as soil ammonium concentration was considerably higher over the growing season for UNI fertilized plots compared to optimal CAN plots. Also, stem nitrate concentration, flag leaf nitrate reductase activity, and transcript levels were lower in UNI fertilized plants compared to optimal CAN. Regarding the e[CO₂] effect on nitrate reductase activity and transcript levels, no alteration could be observed for any nitrogen fertilizer treatment. Flag leaf growth was stimulated under e[CO₂] leading to an enhanced nitrate reductase activity referred to m² ground area at late flowering being in line with a higher nitrogen acquisition under e[CO₂]. Moreover, nitrogen acquisition was considerably higher in nitrate fertilized plants compared to ammonium fertilized plants under e[CO₂]. Our results obtained under field conditions show that a change from nitrate- to ammonium-based fertilization will not lead to a better growth and nitrogen acquisition of winter wheat under future e[CO₂].