Effects of interaction between elevated atmospheric CO₂ concentration and nitrogen fertilizer on photosynthetic characteristic and yield of maize

Since the industrial revolution, the concentration of atmospheric CO₂ has increased from 280 μmol·mol^-1 to 400 μmol·mol^-1. Nitrogen is a necessary element for many important enzyme-mediated processes in plant growth and is the primary nutrient needed for plant growth. Among different C₄ crops grown worldwide, including China, maize is the most widely planted crop. Clear answers regarding the effect of elevated atmospheric CO₂ concentration (eCO₂) on corn growth and the interaction between eCO₂ and nitrogen fertilizers (N) are yet not to be attained. Studying the impact of eCO₂ on maize growth under different nitrogen supply conditions is important to assess the role of climate change in the C₄ crop growth. A Free Air CO₂ Enrichment (FACE) facility was used in this experiment. The FACE facility has six octagon loops for eCO₂, (550±15) μmol·mol^-1, and six additional octagon loops for ambient CO₂ concentration of (400±15) μmol·mol^-1 (aCO₂); three of which are eCO₂ experimental loops and the other three are aCO₂ experimental loops applied with conventional nitrogen fertilizer, 180 kg(N)·hm^-2 (CN). The rest are low nitrogen, 72 kg(N)·hm^-2 (LN), application treatments. Twelve experimental loops were arranged randomly in the maize field, with the plants spacing of 25 cm and a rows spacing of 60 cm. Results showed that under eCO₂, the chlorophyll concentration of maize seedling leaves increased significantly by 9.5%, and the net photosynthetic rate increased by 9.0% at the tasseling stage. During the maize growth period, eCO₂ significantly enhanced the intercellular CO₂ concentration by 34.8%-48.5% and 40.0%-49.4% under low nitrogen and conventional nitrogen application conditions, respectively. In addition, the stomatal conductance decreased by 21.6% and 22.1% at the 12-leaf and the tasseling stages, respectively. As a consequence of decreased stomatal conductance, the efficiency of water consumption in maize leaves increased by 12.9%, 9.8%, and 18.8% at the 12-leaf stage, tasseling stage and filling stage, respectively. eCO₂ also decreased Non-Photochemical Quenching (NPQ), and increased PSⅡ effective photochemical quantum yield (F_v'/F_m') value. At the same nitrogen fertilizer (N) level, eCO₂ had no significant effect on the maize yield. Secondly, the interaction of eCO₂ and a reasonable increase of N application rate promoted the chlorophyll content, net photosynthetic rate, and F_v'/F_m' of maize functional leaves. For instance, the chlorophyll content of functional leaves for CN-eCO₂ against LN-aCO₂ increased by 17.3% and 10.7%, respectively, at the 12-leaf and tasseling stages. The combination of eCO₂ and a reasonable increase in the N application achieved the maximized maize yield, indicating the promotional effect of N application under the eCO₂ conditions. Appropriate application of nitrogen fertilizer has the potential to promote the growth and development of maize crop under eCO₂ conditions in future.