Combining optimized irrigation with reduced N fertilization increases wheat N use efficiency by increasing soil N cycling and plant N uptake

With the aim of maximizing nitrogen use efficiency (NUE) of wheat in the North China Plain by optimizing irrigation and nitrogen application, a field experiment with a split-plot design was conducted. The main plots were subjected to three irrigation levels: bringing soil water content in the 0–40 cm profile to 65% (I1), 75% (I2) and 85% (I3) of field water capacity. The subplots were subjected to three nitrogen application rates: 150 (N150), 210 (N210) and 270 (N270) kg N ha⁻¹. Compared with the N270, N210 treatment enhanced grain yield, NUE, and net income by 4.5%, 6.2%, and 5.8%, respectively (two-year averages). Additionally, it reduced soil nitrate reductase activity, the abundance of denitrification-related bacteria, and loss rate of fertilizer nitrogen by 12.9%, 53.3%, and 16.3%, respectively. Compared with the N150, N210 treatment increased grain yield, grain nitrogen accumulation, and net income by 15.9%, 14.2%, and 26.3%. Relative to I1 and I3, I2 treatment increased root length density in the 20–60 cm soil layer, uptake rate of fertilizer nitrogen, grain yield, and net income. Overall, the combination of irrigation to 75% of field capacity with nitrogen application at 210 kg N ha⁻¹ increased wheat’s capacity for nitrogen uptake and remobilization and thereby grain nitrogen accumulation, and increased NUE by reducing nitrogen loss rate.