This study investigated the effects of different nitrogen and phosphorus application rates on summer maize aboveground biomass, nitrogen accumulation, and drew a critical nitrogen concentration dilution curve. The nitrogen status of maize plant was diagnosed and evaluated based on a model of nitrogen nutrition index (NNI) under different nitrogen and phosphorus interaction conditions, which provided a theoretical basis for the rational application of nitrogen and phosphorus fertilizers in summer maize.

By using Zhengdan958 (ZD958) and Yuyu22 (YY22) as tested materials, the field experiments in Guanzhong Plain, Shaanxi included four phosphorus application rates and five nitrogen application rates, such as 0 (P0), 60 (P1), 120 (P2), 180 (P3) kg P₂O₅·hm^-2 and 0 (N0), 75 (N1), 150 (N2), 225 (N3), 300 (N4) kg N·hm^-2 during 2019-2020. The aboveground samples were taken during the jointing, tasseling, filling, and maturity stages of summer maize to analyze the effects of nitrogen and phosphorus application rate on maize dry matter accumulation, dynamic changes of nitrogen concentration and grain yield. The field test data was used to construct and verify the critical nitrogen dilution curve model of summer maize.

The results showed that nitrogen and phosphorus application rate significantly increased aboveground biomass, plant nitrogen concentrations and grain yield of summer maize. The grain yield and aboveground biomass of summer maize increased as the nitrogen application rate increased within the same phosphorus application condition. The nitrogen concentration of maize plants showed a decreasing trend with the extension of growth period and the increase of aboveground dry matter weight. There was a power exponential relationship between nitrogen concentration and aboveground biomass. In addition, the phosphorus application could promote maize plant nitrogen absorption and aboveground dry matter accumulation. The overall performance of the phosphorus application treatments was P2>P3≈P1>P0 under the same nitrogen application conditions, appropriate phosphorus application could improve the capacity of maize plant for nitrogen absorption and relieved the decline of nitrogen concentration. The critical nitrogen concentration (N_c) curves of maize (P0, N_c=27.98DM^-0.249; P1, N_c=29.77DM^-0.182; P2, N_c= 30.81DM^-0.138; P3, N_c=30.06DM^-0.187) were constructed according to the aboveground dry matter (DM) weight and its nitrogen concentration under different phosphorus application conditions; the relatively stable model had a linear correlation between the fitted and actual plant nitrogen concentrations, which showed that the n-RMSE were 10.23%, 6.67%, 6.95% and 7.19%, respectively. The NNI values were calculated based on the critical nitrogen concentration curves. NNI increased with the increase of nitrogen application in each growth stages within the same phosphorus application conditions, which was also positively correlated with relative aboveground biomass (RDW) and relative yield (RY).

Based on the model of nitrogen nutrition(NNI) in this study, N2-N3 and P1-P2 were the best conditions. Based on the fitting curve of comprehensive nitrogen application rate and grain yield, the nitrogen rate of 187.5-205.7 kg·hm^-2 and phosphorus rate of 60-120 kg·hm^-2 was the optimal fertilization option for summer maize in Guanzhong Plain, Shaanxi.