Simulation of critical nitrogen concentration and nitrogen nutrition index of tomato under different water and nitrogen conditions

【Objectives】 The critical nitrogen(N)concentration in plant aboveground biomass is defined as the minimum N concentration required for maximum plant growth. This study investigated the effects of different water and nitrogen supply on tomato aboveground biomass, nitrogen accumulation, and drew a critical N concentration dilution curve. The N status of tomato plant was analyzed based on a model of N uptake and nitrogen nutrition index(NNI), which provided a theoretical basis for optimal water and nitrogen management. 【Methods】A pot experiment was conducted in greenhouse of the Key Laboratory of Agricultural Soil and Water Engineering in Arid and Semiarid Area of Ministry of Education in Northwest Agriculture and Forestry University in 2013. Cultivar of tomato(M6088)was used in this experiment. The treatment comprised three levels of irrigation(W1, 60%-70% θf; W2, 70%-80% θf; W3, 80%-90% θf), θf is the field capacity, and three levels of nitrogen(N1, N 0.24 g/kg; N2, N 0.36 g/kg; N3, N 0.48 g/kg). For determining the critical N concentration dilution curves under different water conditions, the treatments were replicated fifteen times in random complete block designs to examine the dynamic changes of tomato aboveground biomass and nitrogen accumulation under different water and nitrogen conditions. 【Results】 The aboveground biomass and N accumulations presented a Logistic curve over time. Different water and nitrogen supply had different effects on maximum theoretical value of tomato aboveground biomass: the maximum theoretical value of tomato aboveground biomass increased firstly and decreased with the increase of nitrogen rate under two levels of irrigation(W2, W3). It also increased with the increase of nitrogen rate under the level of irrigation(W1), which indicated that moderate nitrogen supply could enhance the inhibiting effect of drought on aboveground biomass accumulation of tomato. The beginning time of fast accumulation period for nitrogen was 8-17 days earlier than those for biomass, the maximum accumulation rates of tomato aboveground biomass and nitrogen were both found in W2N2 treatment. Under the same water supply condition, the nitrogen concentration of tomato aboveground biomass increased with the improving of applied N rates, and decreased in the growing process. The relationship between the aboveground biomass and N concentration could be described by the power equation, appropriate irrigation could improve the capacity of plant for nitrogen absorption and relieved the decline of nitrogen concentration with the aboveground biomass growth to ensure a steady and orderly growth of tomato. The yield was significantly affected by water and nitrogen supply, appropriate condition of water and nitrogen achieved maximum yield. 【Conclusions】 Based on the model of nitrogen nutrition(NNI)and the model of N uptake, the W2N2 treatment was the optimal option with irrigation amount of 62.1 L/plant, nitrogen rate of 15.1 g/plant, and the highest yield was 1602 g/plant. Thus, the models built in this study were reasonable and feasible for the research objectives.