The elevated atmospheric CO₂ concentration (eCO₂) is expected to increase the labile C input to the soil, which may stimulate microbial activity and soil N₂O emissions derived from nitrification and denitrification. However, few studies studied the effect of eCO₂ on N₂O emissions from maize field under the free-air CO₂ enrichment (FACE) conditions in the warm temperate zone. Here, we report a study conducted during the 12th summer maize season under long-term eCO₂, aiming to investigate the effect of eCO₂ on N₂O emissions. Moreover, we tested zero and conventional N fertilization treatments, with maize being grown under either eCO₂ or ambient CO₂ (aCO₂). We hypothesized that N₂O emissions would be increased under eCO₂ due to changes in soil labile C and mineral N derived from C-deposition, and that the increase would be larger when eCO₂ was combined with conventional N fertilization. We also measured the activities of some soil extracellular enzymes, which could reflect soil C status. The results showed that, under eCO₂, seasonal N₂O and CO₂ emissions increased by 12.4–15.6% (p < 0.1) and 13.8–18.5% (p < 0.05), respectively. N fertilization significantly increased the seasonal emissions of N₂O and CO₂ by 33.1–36.9% and 17.1–21.8%, respectively. Furthermore, the combination of eCO₂ and N fertilization increased the intensity of soil N₂O and CO₂ emissions. The marginal significant increase in N₂O emissions under eCO₂ was mostly due to the lower soil water regime after fertilization in the study year. Dissolved organic C (DOC) and microbial biomass C (MBC) concentration showed a significant increase at most major stages, particularly at the tasseling stage during the summer maize growth period under eCO₂. In contrast, soil mineral N showed a significant decrease under eCO₂ particularly in the rhizospheric soils. The activities of C-related soil extracellular enzymes were significantly higher under eCO₂, particularly at the tasseling stage, which coincided with concurrent increased DOC and MBC under eCO₂. We conclude that eCO₂ increases N₂O emissions, and causes a higher increase when combined with N fertilization, but the increase extent of N₂O emissions was influenced by environmental factors, especially by soil water, to a great extent. We highlighted the urgent need to monitor long-term N₂O emissions and N₂O production pathways in various hydrothermal regimes under eCO₂.

Elevated atmospheric CO₂ concentrations (eCO₂) has become the main feature and cause of global change that could affect crop growth in many aspects, including physiological processes and morphological development in plants and nutrient cycling and nutrient uptake from the soil. Studying the responses of crop growth to different nitrogen (N) supply forms under elevated atmospheric CO₂ concentrations can guide nutrient management strategies for agricultural production under future climate change scenarios. Few studies addressed the effect of eCO₂ on N uptake and morphological development for plants. This study was conducted in the CO₂-controlled light incubators based on the sand-pot incubation using wheat and maize as experimental plants. Six treatments were set with two different environmental CO₂ concentrations (aCO₂, 390 µmol mol⁻¹; eCO₂, 690 µmol mol⁻¹) and three different N supply forms, including ammonium−N, nitrate−N and ammonium-nitrate with 1:1 ratio. The following results were obtained: (i) Wheat and maize seedlings, as nitrate-preferring crops, grew better under mixed N forms than under single N forms. For the single N supply treatment, seedlings with nitrate−N supply showed better growth than with ammonium. (ii) For wheat plants, seedlings with a single ammonium−N supply showed slender height and fewer tillers; seedlings with a single nitrate−N supply were characterized by slightly shorter plant height, more tillers, and higher aboveground biomass. (iii) Compared to the aCO₂ group, wheat seedlings with the ammonium−N supply showed an increased maximum root length and a decreased carbon concentration in root exudates; wheat seedlings with a single nitrate−N supply under eCO₂ showed a significant increase in biomass and a decreased carbon concentration in root exudates; wheat seedlings with a mixed N supply under eCO₂ explored a significant increase in carbon concentration in root exudate and a relatively lower N concentration. (iv) For maize plants, seedlings with either single ammonium−N or nitrate−N supply did not show significant differences in most growing indices. Maize seedlings with a mixed N supply exhibited an increase in aboveground biomass and N concentration in root exudates compared to those with a single N supply. (v) Compared with the aCO₂ group, maize seedlings with mixed N supply under eCO₂ conditions exhibited significant increases in plant height, aboveground biomass, and N concentration in root exudates. Single ammonium−N supply was toxic to wheat and maize plants under eCO₂ conditions. We recommend raising the ratio of ammonium-to-nitrate under mixed N supply to improve the coordination of carbon and N metabolism for efficient crop growth under climatic change conditions of elevated atmospheric CO₂ concentration.