Boosting Maize Yield and Mitigating Greenhouse Gas Emissions Through Synergistic Nitrogen and Chemical Regulation by Optimizing Roots and Developing Grains Under High-Density Planting in Northeast China

Increasing planting density is an effective strategy for enhancing maize (<i>Zea mays</i> L.) yield. However, high density often inhibits plant growth and dry matter accumulation. Synergistic nitrogen management and chemical regulation offer an effective approach to overcoming yield limitations under high-density conditions. A two-year field experiment with two maize cultivars under high density (90,000 plants ha^-1), involving four nitrogen rates combined with PGR, explored their effects on root growth, yield formation, and greenhouse gas emissions. Results showed that 240 kg N ha^-1 significantly improved root morphological characteristics (root dry weight, root volume, root surface, root length) and physiological traits (bleeding sap rate, etc.), with chemical regulation providing additional enhancements. Additionally, nitrogen application increased the maximum grain-filling rate (V_max) and enzyme activity in grains, thereby enhancing grain weight; chemical regulation increased dry matter accumulation and its contribution to grains. Reduced nitrogen application combined with chemical regulation effectively decreased greenhouse gas emission. The highest maize yield was obtained under the application of 240 kg N ha^-1 combined with chemical regulation, which promoted root growth and grain formation, thereby improving yield and reducing emissions. This study indicates that the cultivation practice combining nitrogen application with chemical regulation provides an optimized approach for environmentally friendly and high-yield maize cultivation under high planting density.