Optimizing within-event nitrogen fertigation timing enhances winter wheat productivity by coordinating soil N distribution with root development

Under drip fertigation, the timing of nitrogen (N) application within a single irrigation event is a key determinant of water and N distribution, yet how N timing regulates root distribution and the spatial coordination between roots and soil N remains unclear. A three‑year lysimeter experiment with winter wheat was conducted on the North China Plain from 2021 to 2024. Three within-event N application timings were evaluated: application at the beginning (T1), middle (T2), or end (T3) of the irrigation cycle, under two irrigation regimes: full irrigation (W1) and deficit irrigation (W2). The study assessed how fertigation timing affected N distribution, root growth, canopy development, grain yield, and resource-use efficiency under contrasting water regimes. The results showed that T2 and T3 promoted the accumulation of nitrate-N and ammonium-N in the 0–20 cm topsoil layer, whereas T1 promoted nitrate movement into the 40–100 cm deep layer, particularly under full irrigation. T2 and T3 significantly increased root length density and root surface area density in the topsoil, reduced mean root diameter, and produced a “fine-root, high-density” architecture that spatially coincided with the surface N enrichment. This spatial coordination promoted canopy development and nutrient uptake, increasing grain yield by 11.5–19.5%, water use efficiency by 8.9–11.7%, and the partial factor productivity of N and phosphorus (P) by 11.5–19.5% under full irrigation. Structural equation modeling further confirmed that aligning surface N enrichment with dense root distribution strengthened the pathways from soil N availability to root growth, nutrient uptake, and yield formation under full irrigation. Overall, W1T2 and W1T3 established strong spatial coordination between root distribution and soil N availability, improving crop acquisition of water, N, and P and thereby achieving high yield and efficient water‑fertilizer use under full irrigation. These findings provide a theoretical basis for optimizing drip fertigation management in winter wheat grown in water‑sufficient regions.