Abstract: Abstract: The nitrogen nutrition index (NNI), derived from the critical N concentration dilution curve, allows the discrimination between crop N status under shortage and surplus N supply. The on-farm survey results shows that nitrogen fertilizer is excessively used in maize production in Guanzhong Plain. However, the famers are lacked of an efficient and simple nitrogen status diagnostic tool to guide the nitrogen application both for increasing crop yield and reducing widespread environmental pollution as well. The objectives of this study were to build and validate the applicability of the critical N concentration dilution curves for summer maize in Guanzhong irrigated area (with the whole growing period of about 105 d) and spring maize in Weibei dryland (with the whole growing period of about 158 d), respectively. The modeling procedure was as following: 1) to collect and analyze 8-site-year (2004-2012) field experiments data under different N application rates in the Guanzhong Plain according; and 2) to determine key points of the critical N concentration curve using data for modeling and to validate the curve using other independent data. Results showed that there existed a negative power function between aboveground maximum biomass (W) and critical N concentration (Nc) (summer maize: Nc=22.5W-0.27; spring maize: Nc=25.3W-0.26) when aboveground biomass was >1 t/hm2. When aboveground biomass was <1 t/hm2, the critical value was 23.4 and 32.2 g/kg for summer and spring maize, respectively. The critical N dilution curves were lower than that of spring maize in France and close to that of summer maize in North China Plain. The critical N dilution curves were validated through the NNI defined as the ratio between measured N concentration and critical Nc. The results showed that the nitrogen nutrition index model could be used for accurate diagnosis of maize plant nitrogen status and that plant nitrogen nutrition could be affected by both N application rates and dates, i.e., NNI value would generally increase with increasing N rates under a certain fertilization mode. For example, the average NNI value over summer maize growing season improved from 0.81 to 1.16 as the N rates increased from 0 to 375 kg/hm2 in Guanzhong irrigated area, while the average NNI value over spring maize growing season increased from 0.53 to 1.22 for N rates from 0 to 400 kg/hm2 in Weibei rainfed area. Maize would get nutrition imbalance when nitrogen was applied only as the basal fertilizer or with inappropriate fertilizer ratio of basal and topdressing. For example, for summer maize with 300 kg N/hm2 applied only before planting, the NNI value were 1.18 and 0.91 in vegetative and reproductive growth period, respectively, indicating that extravagant and deficient N uptake in early and later maize-growing season. For spring maize with 70% N applied prior to anthesis date, over-supply of nitrogen was found in vegetative growth stage for all treatments of N≥200 kg/hm2. The present critical N dilution curves could be used for the diagnosis of maize nitrogen nutrition, providing a key technical approach to precise N fertilization management in maize production in Guanzhong Plain.