Abstract: Abstract: Rizosphere nutrient regulation for vegetable seedling is the basis to cultivate high quality seedlings. Among them, nitrogen regulation in the rhizosphere is of great importance for the growth and development of tomato seedling. Previous studies on the nitrogen regulation in the rhizosphere were mostly focused on the physiological development of tomato seedlings. The absorption and utilization of nitrogen are closely related to the root system architecture at the seedling stage. Researches have shown that root length density, root depth, root radius related to root architecture parameters can limit uptake, transport and utilization of nitrogen and phosphorus in the root. Furthermore, the interactions of root three-dimensional (3D) architecture with different nitrogen concentrations will affect root nitrogen uptake, transfer and use efficiency. However, these interactions and the responses of the 3D architecture of root system for tomato seedling to different nitrogen levels are not well reported yet. Therefore, the aim of current study was using 15N tracer method and 3D digitizing technology to investigate the efficiency of nitrogen absorption and transfer, and root 3D architecture at seeding stage. A greenhouse experiment of tomato seedling under different nitrogen concentration treatments was conducted by sand cultivation in the Chinese Academy of Agricultural Sciences. Greenhouse temperature was controlled at 25-28℃ for the day and at 15-20℃ for the night. Tomato seeding of 'Zhong Za 109' was used as the study material. After cultured under 3 nitrogen levels, the seedlings were transplanted in the plastic pots (diameter 35 cm, height 10 cm, volume 35 dm3) with quartz sand matrix. The 3 nitrogen levels were 4, 12 and 20 mmol/L (N4, N12 and N20). Hoagland complete nutrient solution was used to water the tomato seedlings with 700 mL per day, according to the requirements of normal growth of tomato seedling. Moisture content within quartz sand remained at about 60%. Nine days after transplanting, isotope test was carried out, in which 15N labeled nutrient solution was used to water the plant and this lasted till 18 days after transplanting. Then, nitrogen nutrient solution without the labeled 15N was continuously used to water the plant till the end of experiment, 30 days after transplanting. Experiments were designed by using randomized block design with totally 120 plants for each treatment and 3 repetitions per treatment. Four plants were sampled for each treatment at each time. Measurements of plant biomass, leaf area and nitrogen content for each organ, and root architecture information were done every 9 days after transplanting. Isotope 15N for each type of organ was measured 18 and 27 days after transplanting respectively. ScanMaker i800 Plus (Microtek) was used to scan the root system with the precision of 400 dpi. The spatial coordinates of the root system were measured with Fastrak (Polhemus, 3Space, USA). The results showed that tomato seedling dry weight, leaf area, root-shoot ratio were significantly affected under different nitrogen treatments. The root total length, root surface area and root branching density under N4 treatment were 16.5%, 17.5% and 15.5% higher than those under N20 treatment. Root architecture under N4 treatment was with narrow radius and deep depth, compared with those under N20 treatment. High nitrogen concentration would promote root 15N uptake and distribution rate. 15N allocation rate sequence was leaf > stem > root, which showed the largest contribution of root nitrogen transfer for leaf. The transfer of 15N and utilization rate of nitrogen fertilizer were the highest for 12 mmol/L treatment, with 508.3 mg transfer per plant and 8.9% nitrogen use efficiency. The transfer of 15N for 20 mmol/L treatment was 128 mg per plant higher than that for 4 mmol/L treatment; however, the utilization rate of nitrogen fertilizer was reduced by 2%. The results indicate that the nitrogen concentrations can be decreased in tomato production, and the tomato seedlings will take the initiative to change the root 3D configurations to improve nitrogen use efficiency.