Establishment of critical nitrogen-concentration dilution curves based on leaf area index and aboveground biomass for drip-irrigated spring maize in Northeast China

The unreasonable application of nitrogen fertilizer poses a threat to agricultural productivity and the environment protection in Northeast China. Therefore, accurately assessing crop nitrogen requirements and optimizing fertilization are crucial for sustainable agricultural production. A three-year field experiment was conducted to evaluate the effects of planting density on the critical nitrogen concentration dilution curve (CNDC) for spring maize under drip irrigation and fertilization integration, incorporating two planting densities: D1 (60,000 plants ha −1 ) and D2 (90,000 plants ha −1 ) and six nitrogen levels: no nitrogen (N0), 90 (N90), 180 (N180), 270 (N270), 360 (N360), and 450 (N450) kg ha −1 . A Bayesian hierarchical model was used to develop CNDC models based on dry matter (DM) and leaf area index (LAI). The results revealed that the critical nitrogen concentration exhibited a power function relationship with both DM and LAI, while planting density had no significant impact on the CNDC parameters. Based on these findings, we propose unified CNDC equations for maize under drip irrigation and fertilization integration: Nc = 4.505DM −0.384 (based on DM) and Nc = 3.793LAI −0.327 (based on LAI). Additionally, the nitrogen nutrition index (NNI), derived from the CNDC, increased with higher nitrogen application rates. The nitrogen nutrition index (NNI) approached 1 with a nitrogen application rate of 180 kg ha −1 under the D1 planting density, while it reached 1 at 270 kg ha −1 under the D2 planting density. The relationship between NNI and relative yield (RY) followed a “linear + plateau” model, with maximum RY observed when the NNI approached 1. Thus, under the condition of drip irrigation and fertilization integration in Northeast China’s spring maize production, the optimal nitrogen application rates for achieving the highest yields were 180 kg ha −1 at a planting density of 60,000 plants ha −1 , and 270 kg ha −1 at a density of 90,000 plants ha −1 . The CNDC and NNI models developed in this study are valuable tools for diagnosing nitrogen nutrition and guiding precise fertilization practices in maize production under integrated drip irrigation and fertilization systems in Northeast China.