Construction and validation of critical nitrogen concentration dilution model for maize in central and western Inner Mongolia

Objectives The critical nitrogen concentration dilution curve model of maize in the central and western regions of Inner Mongolia was established, and the corresponding nitrogen nutrition index was used to diagnose the nitrogen nutrition of maize, and the reliability of the curve was verified, to provide a theoretical basis for the rational application of nitrogen fertilizer to maize in central and western Inner Mongolia.

Methods Modeling field trials were conducted from 2019 to 2021 in three typical regions of central Inner Mongolia, Dalad Banner, western Wuyuan County, and Urad Front Banner, using Xinyu 12, Jindan 42, Xianyu 1225, Zeyu 19, Hongyu 203, Jindan 542, and Dongnong 258 as test materials, respectively. Six N fertilizer treatments included no N application control (CK), traditional N rate 400 kg/hm² (CON), optimum N application rate 180 kg/hm² (OPT), and 70% OPT, 130% OPT, 170% OPT. Plant sampling was at jointing stage (V6), eigh-leaf stage (V8), ten-leaf stage (V10), twelve-leaf stage (V12), silking stage (R1), milk stage (R3) and waxing stage (R5) of maize, respectively, to determine plant above-ground biomass and plant N concentration, and a critical N concentration dilution model was developed using above-ground biomass and plant N concentration. A validation trial was conducted in 2021 in Dalad Banner, where a recommended N application demonstration field and a traditional customary production field were set up, and above-ground biomass and plant N concentration of maize plants were measured to validate the critical N concentration dilution model using the nitrogen nutrient index (NNI).

Results The yield levels of different maize cultivars in central and western Inner Mongolia were comparable, ranging from 10.60–12.72 t/hm². The dilution curves of critical N concentration in the three typical areas of Dalad Banner, Wuyuan County, and Urad Front Banner were Nc = 3.09DM^–0.32, Nc = 3.30DM^–0.28 and Nc = 3.58DM^–0.35 respectively, and the determination coefficients (R²) of biomass and critical nitrogen concentrations were 0.98, 0.82, and 0.88, respectively. Integrating of critical N concentration data from the seven maize cultivars at three test sites, the critical N concentration dilution curve model was further fitted across sites and cultivars to establish a critical N concentration dilution curve for maize in central and western Inner Mongolia with Nc = 3.32DM^–0.305, R² of 0.89 and reaching a significant level, and the root mean square error RMSE for model validation was 2.39 g/kg. According to the newly constructed critical nitrogen concentration dilution curve model, the optimum N application rate for maize in central and western Inner Mongolia was N 180–220 kg/hm².

Conclusions Maize varieties with close yield levels in the same region share a single critical N concentration dilution curve. Maize yields in central and western Inner Mongolia ranged from 10.60 to 12.72 t/hm². The constructed critical N concentration dilution curve for spring maize in central and western Inner Mongolia was Nc = 3.32DM^–0.305, and model validation showed that the model was stable and could effectively diagnose the nitrogen nutrient status of maize plants in central and western Inner Mongolia. The model inferred that the reasonable N application rate for maize in central and western Inner Mongolia was N 180–220 kg/hm².