AbstractWith the rapid expansion of agriculture on saline–alkaline soils, environmental problems such as increased greenhouse gas (GHG) emissions, eutrophication and soil degradation are becoming increasingly serious. To clarify the characteristics of carbon (C) and nitrogen (N) cycling and their loss mechanisms in cultivated saline–alkaline soils, an undisturbed soil column experiment was conducted to analyse C and N leaching and GHG emissions by applying different fertilizer rates. The experiment had six treatments using N‐(NH₄)SO₄ over a 40‐day seedling stage, with and without maize. Treatments were: no N with maize (0N_maize: 0 kg N ha⁻¹), reduced N with maize (RN_maize: 63 kg N ha⁻¹), conventional N with maize (CN_maize: 160 kg N ha⁻¹) and their equivalents without maize (0N_soil: 0 kg N ha⁻¹; RN_soil: 63 kg N ha⁻¹; CN_soil: 160 kg N ha⁻¹). The results indicated that reduced N with maize reduced the N₂O emission by 21%, with N leaching (TN: 41%, NO₃⁻–N: 19%, NH₄⁺ − N: 63%) within 15 days after fertilization, but had no significant effect on CH₄ emission compared to conventional N with maize. Therefore, reduced N with maize had the smallest N loss, which accounted for 1.5% of the relative percentage of N flow including N₂O (0.3%), N leaching (2%), aboveground biomass N (76%) and root biomass N (22%). Compared to conventional N with maize, reduced N with maize significantly reduced N leaching by 40% because conventional N with maize greatly exceeded the crop N uptake when maize root length was only within 20 cm. Reduced N without maize reduced CO₂ emission (19%) compared to conventional N without maize. Uncultivated saline–alkaline soils face greater N overuse and leaching risk because higher NO₃⁻–N leaching (6.9 mg L⁻¹) that occurred in bare soils without fertilization, which increased by 2.6–3.6 times when the N input increased from 63 to 160 kg N ha⁻¹ compared to control. In conclusion, reducing conventional N fertilizer inputs by 60% is not only an effective strategy to reduce CO₂ and N₂O emission and N leaching but also effectively absorbs C, and the N retained in the soil tillage layer can help to meet maize seedling growth requirements in Solonchaks.