Objectives Water and nitrogen are important factors limiting crop yield and quality. Water and fertilizer management is studied with isotope techniques to search the appropriate rate of water and nitrogen coupling for achieving the highest nitrogen use efficiency and water use efficiency (WUE) and good taste quality of tomato fruits.

Methods A cherry tomato cultivar‘Qianxi’was used as the test crop in a pot experiment in greenhouse. The water treatments included fully, medium and stress levels, which was 90% (WH), 70% (WM) and 50% (WL) of the field water capacity, respectively. The three nitrogen levels tested were high (N 0.23 g/kg), medium (N 0.16 g/kg) and low (N 0.08 g/kg). The stable isotope technique was employed to determine the biomass and C, N and O isotope compositions in different parts of plants. The growing period of tomato was 60 days before harvest for sampling.

Results Under sufficient (WH) and medium water stress (WM) treatments, the increase of nitrogen level increased the above-ground dry matter and leaf area significantly by 19.8%–45.4% and 29.4%–106.8%, respectively. Under the same nitrogen conditions, the above-ground dry matter weight and leaf area in the WH and WM treatments increased by 24.7%–83.4% and 17.6%–90.4%, respectively, compared with the WL treatment. WHNH treatment had the highest dry matter weight and leaf area, followed by WMNH treatment, but the plants in the latter treatment consumed less water and thus had the highest WUE. Under the sufficient (WH) and medium water stress (WM) treatments, the increase of N fertilization improved WUE and N content. Reduction of soil water level increased water use efficiency and plant δ¹³C and δ¹⁸O, indicating that the improved WUE was mainly due to the regulation of stomatal aperture, and leaf δ¹³C and δ¹⁸O could be used as time-integrated indicators for WUE under fertigation. WLNM treatment increased sugar contents in tomatoes, while WHNM treatment decreased organic acid contents in the tomatoes, which improved the taste and quality of tomatoes.

Conclusions Moderate water stress and high nitrogen treatment (WMNH) can reduce the water use, while increase dry biomass accumulation and nitrogen uptake and use efficiency with the highest WUE, which is the optimal water and nitrogen coupling treatment for tomato production.