ABSTRACT In arid regions, relay‐intercropped soybean often experiences concurrent drought and nitrogen stress during reproduction, which disrupts carbon‐nitrogen metabolism. However, the physiological mechanisms underlying enhanced stress resilience through optimized water‐nitrogen management remain unclear. We hypothesized that coupling moderate deficit irrigation with reduced nitrogen fertilization would stabilize yield by coordinately optimizing plant water status, stomatal conductance, and nitrogen allocation to seeds. A two‐year field experiment tested three irrigation (W1: 3360; W2: 4200; W3: 5040 m 3 ha −1 ) and four nitrogen (N1: 0; N2: 105; N3: 150; N4: 195 kg N ha −1 ) regimes. The W2N3 regime (4200 m 3 ha −1 + 150 kg N ha −1 ) achieved yields equivalent to the high‐input regime (W3N4) but 35% greater inter‐annual stability (as indicated by a lower coefficient of variation, CV: 5.3% vs. 8.2% for W3N4). Crucially, W2N3 maintained favorable leaf water status, as indicated by leaf water content (WC), at the critical R5 stage. This favorable water status was associated with a coordinated physiological response: it optimized stomatal regulation, maintaining net photosynthesis (Pn) despite a 15% reduction in stomatal conductance (Gs), thereby significantly increasing intrinsic water use efficiency (iWUE). This was coupled with an 11.2% increase in nitrogen allocation efficiency to seeds, indicating enhanced source‐sink coordination. Consequently, W2N3 improved irrigation water productivity by 22.3% and nitrogen recovery efficiency by 17.3%, while reducing nitrate leaching risk. We conclude that moderate water‐nitrogen coupling enhances drought resilience by synchronously improving plant water status, which co‐optimizes stomatal behaviour and nitrogen partitioning, thereby stabilizing carbon gain and nitrogen use under stress. This physiological synergy provides a mechanistic foundation for sustainable intensification of soybean in arid relay‐intercropping systems.
He et al. (2026) studied this question.