Sustainable nitrogen placement depth under different rainfall levels can enhance crop productivity and maintain the nitrogen balance in winter wheat fields

The deep placement of nitrogen (N) fertilizer is considered as an effective strategy for crop growth and yield formation. However, it is unclear how deep N fertilizer placement under different rainfall levels might affect the nitrogen balance of winter wheat fields. Thus, we conducted a two-year field experiment in a semi-humid region to assess the effects of different N fertilizer placement depths of 5 cm (D5), 15 cm (D15), 25 cm (D25), and 35 cm (D35) under three rainfall levels during winter wheat growth period (dry year = 125 mm, T1; normal year = 200 mm, T2; wet year = 275 mm, T3) on the N balance and crop productivity in winter wheat fields. Our results indicated that compared to D5, deeper N placement treatments increased the crop yield by 0.1-1.1 t ha-1 and N uptake by 3.0-18.0%, and reduced gaseous N losses by 0.2-1.1 kg N ha-1 under T1 and T2 (p < 0.05). T3D15 decreased the gaseous N losses by 1.2 kg N ha-1 compared with T3D5, and increased the yield and crop N uptake by 0.7-0.9 t ha-1 and 10.0-20.1 kg N ha-1 compared with T3D25 and T3D35, respectively (p < 0.05). T1 and T2 helped to maintain the N balance under deeper N placement (p < 0.05). Although T3 significantly increased NO3-N concentration in the 60-100 cm soil layer and decreased the crop N uptake under D25 and D35, which was not conducive to maintaining the N balance, while D15 was beneficial in maintaining N balance (p < 0.05). Placement of N fertilizer at depths of 13.5 cm, 17.5 cm, and 21.6 cm in the wet year (T3), normal year (T2), and dry year (T1), respectively, could maintain the N balance in winter wheat fields through increasing crop N uptake, reducing gaseous N losses, and preserving the soil N pool.