Optimized Nitrogen Application Enhances Grain-Filling and Yield Formation of Spring Maize Through Improved Source-Sink Coordination in a Black Soil Region

Nitrogen application rate (N rate) is a key factor affecting maize yield, but the physiological mechanisms by which it improves yield through the coordination of the source-sink relationship remain unclear. In this study, field experiments were conducted in the black soil region of Northeast China for three consecutive years (2022–2024). Five N rates were tested: 0 kg ha−1 (N0), 90 kg ha−1 (N90), 135 kg ha−1 (N135), 180 kg ha−1 (N180), and 225 kg ha−1 (N225). The results showed that the N180 treatment achieved the highest yield across the three growing seasons, with yields 7.23–51.40% higher than those of the other treatments. This was mainly attributable to the synergistic improvement in grain number per ear and 100-grain weight. After anthesis, the N180 and N225 treatments significantly increased chlorophyll content (SPAD), PSII photochemical efficiency-related parameters (Fv/Fm, ΦPSII, and qP), and the activities of carbon assimilation-related enzymes (RuBPCase and PEPCase) in the ear leaf, indicating improved photosynthesis-related physiological status. Meanwhile, N180 significantly promoted the translocation of stored dry matter from pre-anthesis vegetative organs to the grains, resulting in a 5.21–51.41% increase in the proportion of dry matter allocated to the grains at maturity compared with the other treatments. Logistic equation fitting showed that an increased grain-filling rate and an extended grain-filling period were the main reasons for the higher grain weight under N180. Although N225 maintained high post-anthesis dry matter accumulation, more assimilates were retained in vegetative organs, which limited further improvement in grain-filling and grain weight. These findings provide a theoretical basis and practical guidance for the rational application of nitrogen fertilizer in this region.