Balanced Ammonium–Nitrate Supply During Tillering Stage by Drip Fertigation Improves Yield and Nitrogen Use Efficiency in Winter Wheat

Optimizing nitrogen form under drip fertigation may improve wheat productivity by regulating the root-zone inorganic N environment during early vegetative growth. A two-year field experiment evaluated nitrate-dominant (N1), balanced ammonium–nitrate (N2), and ammonium-enriched nitrogen strategies (N3) during GS13–GS31, with conventional farmer practice (CK) and a zero-N control (N0) for comparison. Nitrogen-form regulation markedly altered the soil NH4+-N ratios, especially in the 0–20 cm soil layer, with N3 highest, N1 lowest, and N2 intermediate. Compared with the nitrate- or ammonium-dominant strategy, the balanced treatment N2 improved spike formation rate and maintained relatively higher N accumulation at GS31 and GS65, and showed greater N translocation and contribution of translocated N to grain N than N1. Correlation analyses indicated that spike formation rate was closely related to spike number (R2 = 0.764) and N accumulation at GS31 was positively related to Ntrans (R2 = 0.588). N2 showed the most favorable overall performance, with the highest numerical values for grain yield, nitrogen recovery efficiency, irrigation water use efficiency, and net profit among the fertigation treatments. However, the advantages of N2 over N3 in grain yield and SPAD-AUC were modest and not consistently significant. These results indicate that balancing ammonium and nitrate supply during GS13–GS31 under drip fertigation can improve root-zone N conditions and support better overall agronomic performance in winter wheat under the alkaline soil conditions of the North China Plain.