Abstract: 【Objective】Excessive applications of nitrogen and irrigation water in greenhouse vegetable production lead to lower nitrogen (N) use efficiency and unsustainable use of land. The aim of this study was to investigate the potential of a saving on N and water in vegetable production and the best combination of N and irrigation rates.【Method】An experiment was conducted in a 4-year cultivated greenhouse with winter-spring cucumber and autumn-winter tomato double cropping system. Two irrigation levels as conventional irrigation (W₁, ＞100% field capacity) and reduced irrigation (W₂, 75% field capacity) and 4 N levels as conventional N rate (N₁), saving 25% N compared to conventional management (N₂), saving 50% N compared to conventional management (N₃) and no nitrogen control (N₀) was designed. The corresponding N rates were 1200, 900, 600 and 0 kg·hm^-2 for cucumber season and 900, 675, 450 and 0 kg·hm^-2 for tomato season. Five combination treatments were designed as W₁N₁, W₂N₂, W₂N₃, W₁N₀, and W₂N₀ in this experiment.【Result】(1) Severe accumulation of NO₃^--N was observed in the conventional management, and the NO₃^--N was leached away from the surface soil layer. No significant NO₃^--N accumulation was observed in W₂N₃ treatment. The average NO₃^--N content in W₂N₃ treatment at 0-60 cm depth was at the proper level, and the average NO₃^--N content at 0-100 cm depth decreased by 13.9%-31.1% compared to W₁N₁ treatment. Moreover, the apparent N loss decreased by 56% and the N use efficiency increased by 2.4-3.3 percentage point in W₂N₃ with relatively higher economic benefit. (2) Based on the significant correlations between the NO₃^--N content and the yield, the optimal NO₃^--N contents were 37.4-72.9 mg·kg^-1 for cucumber growing, and the corresponding values were lower than 90 mg kg^-1 for tomato growing. (3) In order to keep the proper NO₃^--N value for uptaking by plants at 0-20 cm depth, 600 kg·hm^-2of N with 450-550 mm of irrigation were recommended for cucumber with the yield level of 160-180 t hm^-2, and 250 kg·hm^-2 of N with 170-200 mm of irrigation were recommended for tomato of the yield level of 70-80 t·hm^-2. (4) The mechanism for improving N use efficiency by reducing N and irrigation applications is as follows: more NO₃^--N could distribute around rootzone as saving 20%-30% of water, and less NO₃^--N could be accumulated in the soil profile as saving 50% of N supply. 20%-30% of water saving combined with 50% of N reducing could keep the NO₃^--N supply at a proper level at rootzone and decrease the loss amount of N, which become an effective way to improve N use efficiency.【Conclusion】The potential of a saving on water and nitrogen fertilizer is huge in North China Plain. Reasonable irrigation amount is the base to reduce N application and improve N use. Recommending reasonable N rate under the proper irrigation amount is an effective way to improve N use efficiency. Saving 20%-30% irrigation water combined with reducing 50% N fertilizer could effectively decrease the NO₃^--N accumulation in the soil and the apparent N loss and improve the N use efficiency.

Key words: greenhouse vegetable, water and irrigation rates, soil NO₃^--N content, N balance, economic benefit