Optimizing irrigation and nitrogen rates for sustainable wheat production: Balancing yield and nitrate leaching in a 7-year field study

Integrated irrigation and nitrogen management strategies enhance wheat productivity while mitigating soil nitrate-nitrogen accumulation through optimized water-N synergies. This study conducted a 7-year split-plot experiment to evaluate three irrigation regimes—no irrigation (W0), 60 mm at jointing (W1), and 60 mm at jointing plus anthesis (W2)—under three N rates: 0 (N0), 120 (N1), and 240 kg ha⁻¹ (N2). The results showed that continuous N0 application significantly reduced soil total nitrogen by 18.9–20.3 % and organic matter by 10.4–13.1 % at 0–20 cm depth compared to the baseline levels, whereas N1 and N2 maintained soil fertility over time. Yield gains under the high-input W2N2 treatment exceeded those under W2N0 by 4.8–71.2 % over the seven seasons, primarily driven by nitrogen-induced increases in spike numbers. However, delayed sowing in 2017–2018 nullified the advantages of high irrigation and nitrogen inputs. While W2N2 achieved the highest yield in most seasons, its superiority over W2N1 was statistically significant in only three seasons. Water productivity (WP) increased with irrigation intensity, peaking at 19.00 kg hm⁻² mm⁻¹ under W2N2, with W1 and W2 consistently outperforming W0. Similarly, nitrogen application improved WP, though nitrogen use efficiency (NUE) declined with higher N rates—a decline partially mitigated by irrigation. Environmental assessments revealed that irrigation accelerated nitrate-nitrogen leaching to deeper soil layers (140–200 cm). The N2 rate exceeded crop nitrogen uptake capacity, causing substantial nitrate-nitrogen accumulation throughout the 0–200 cm profile. Crucially, W2N1 achieved yield stability comparable to W2N2 in multiple seasons but reduced deep-layer (140–200 cm) nitrate-nitrogen accumulation by 37 % compared to N2. Therefore, moderate nitrogen application at 120 kg ha⁻¹ (N1) combined with dual-stage irrigation (W2) emerges as the optimal strategy for North China plain wheat systems. This approach sustains high productivity and water-nitrogen use efficiency while significantly curbing environmental risks associated with nitrate leaching and accumulation.