Effects of Nitrogen Application Rate on Yield, Nitrogen Use Efficiency of Spring Maize and Farmland Ecological Environment in the Black Soil Region of Northeast China

The black soil region of Northeast China is a major grain-producing area, where excessive nitrogen (N) fertilizer application is common in maize production. Optimizing N application rates and developing fertilization strategies that balance high yield, N-use efficiency, and environmental sustainability are therefore critical for green maize production. Based on a two-year stationary field experiment, this study systematically evaluated the effects of different N application rates on maize yield, N-use efficiency, N cycling and balance within the soil–crop system, and greenhouse gas emissions from farmland. Four treatments were established: no N control (N0, 0 kg N ha−1), low N (N1, 120 kg N ha−1), medium N (N2, 180 kg N ha−1), and high N (N3, 240 kg N ha−1). Among the N-fertilized treatments, N2 provided the best overall balance between maize yield, N-use efficiency, and environmental performance; N1 failed to achieve the high-yield target, whereas N3 increased potential environmental risks despite further yield gains. Compared with N1 and N3, N2 increased maize yield by 14.6% and 4.1%, N recovery efficiency (NRE) by 2.6% and 26.8%, and N agronomic efficiency (NAE) by 9.8% and 51.8%, respectively. Although N1 exhibited the highest partial factor productivity of applied N (PFPN), its PFPN was 27.2% and 68.7% higher than those of N2 and N3, respectively. Increasing N application significantly increased soil N residues and greenhouse gas emissions. After the 2025 autumn harvest, mineral N residues in the 0–80 cm soil profile were 82.7, 138.9, 172.5, and 207.9 kg N ha−1 under N0, N1, N2, and N3, respectively, and declined with soil depth. Compared with N3, N2 reduced apparent N loss, soil mineral N residue, apparent N surplus, nitrous oxide (N2O) and carbon dioxide (CO2) efflux by 47.8%, 17.0%, 26.2%, 25.5%, and 23.1%, respectively. Collectively, these results provide a theoretical and technical basis for high-yield, high-efficiency, and sustainable maize production in the black soil region of Heilongjiang Province.