Abstract: In order to investigate the effect of water regulation on nitrogen utilization in greenhouse Chinese cabbage, ¹⁵N stable isotope tracer technique was used and different nitrogen application rates （115 kg/hm² and 230 kg/hm²） and different lower water control limits （60%, 70% and 80% of field water holding capacity,respectively） were designed. Biomass accumulation, nitrogen accumulation in different organs of Chinese cabbage under different water and nitrogen treatments were observed. The relationship between nitrogen utilization and its influencing factors was analyzed. The results showed that there was a synergistic effect between water regulation and nitrogen application, especially in root biomass. At the same time, under 230 kg/hm² nitrogen application rate, the synergistic effect of water and nitrogen on leaf biomass accumulation was significant. Under 115 kg/hm² nitrogen application, nitrogen use efficiency of Chinese cabbage was significantly higher than the same water control condition with 230 kg/hm² N application rate treatment; 115 kg/hm² and 230 kg/hm² N application rate with 70% water control lower limit were the highest in nitrogen use efficiency, which reached 60.1% and 49.9%, respectively; Nitrogen use efficiency was negatively correlated with ¹⁵N input amount（r=-0.988）, and was positively correlated with root biomass（r=0.948）. In practice, the lower limit of 70% is recommended as the water control index for high-efficient use of nitrogen fertilizer（urea） by Chinese cabbage.

Key words: water, nitrogen, isotope, biomass, ¹⁵N