Optimizing Nitrogen Source Management to Improve Millet Yield and Nitrogen Accumulation: A Field Experiment on the North China Plain

Foxtail millet (Setaria italica (L.) P. Beauv.) exhibits varying efficiency in utilizing different nitrogen (N) forms. While selecting the appropriate N form is a recognized strategy for enhancing yield and reducing N losses, the integrated responses of millet productivity and soil N dynamics to specific N forms remain poorly understood. To address this, a three-year field experiment integrated with 15N isotopic tracing was conducted on the North China Plain. We systematically evaluated six fertilization treatments: control (CK), organic fertilizer (M), ammonium sulfate (AF), potassium nitrate (NF), ammonium nitrate (ANF), and urea (UR). The results demonstrated that M showed the greatest yield stability but a lower mean grain yield. In contrast, AF treatment achieved the highest grain yield (increasing by 0.90–27.68%) and N accumulation (increasing by 1.65–41.45%), along with the second-highest yield stability. During the growing season, the composition of soil inorganic nitrogen changed significantly. Across all treatments, the dominant form shifted from NH4+-N at the heading stage to NO3−-N at the flowering and maturation stages. As demonstrated by the 15N-labeling experiments, foxtail millet presented a stage-dependent shift in nitrogen uptake preference from NO3− to NH4+. An in-depth analysis identified that sustaining soil inorganic N within 30–38 kg·ha−1 and optimizing the NO3−:NH4+ ratio (4.5–5.3 at flowering; 1.5–1.8 at maturity) were critical for achieving high productivity. In conclusion, AF enhances yield by synchronizing N availability with crop demand, thereby optimizing N accumulation and reducing losses. These findings provide critical insights for designing sustainable millet production systems through tailored N source selection.