Interactive Effects of Deficit Irrigation and Organic Fertilizer Substitution on Winter Wheat Yield, WUE, and Soil Fertility

Although integrating organic amendments with chemical fertilizers is widely advocated to maintain crop productivity and soil health under reduced synthetic inputs, the optimal substitution ratio within such combined systems is still undetermined. Nevertheless, the optimal substitution ratio and the mechanisms underlying its effects in the North China Plain remain poorly understood. A three-year field trial (2021–2024) was therefore conducted to evaluate how deficit irrigation combined with organic fertilizer substitution influences grain yield, resource use efficiency, and soil quality. A split-plot design was employed. The main plots received two water-limited treatments (I1 = 30 mm and I2 = 60 mm), and the subplots included three fertilization regimes (T1, T2, T3). These regimes utilized organic materials to replace 20%, 40%, and 60% of the inorganic fertilizer inputs. According to the results, the maximum levels of soil Available N, Available P, and Available K in the 0–20 cm layer were recorded under I2T2. This treatment also produced grains with the highest N, P, and K concentrations. Compared with I1T1, I1T2 gave 43.1%, 39.8%, and 47.9% greater grain yield. Conversely, the evapotranspiration of I1T1 was the lowest. Relative to I2T2, the WUE under I1T1 declined by 27.1%, 26.5%, and 25.3% across the three seasons. This indicates that a moderate organic substitution rate can partially offset the WUE penalty induced by restricted irrigation. The structural equation model further revealed the driving mechanism of yield improvement: water is the fundamental regulatory factor for yield formation, having a significant positive impact on soil nutrient status (0.89) and soil water use efficiency (0.58); among them, soil water use efficiency had the strongest direct contribution to yield (0.59), and soil nutrients also showed a small but significant positive impact on yield (0.16*). This study proves that partially substituting traditional chemical fertilizers with organic fertilizers can increase wheat yield and improve the effectiveness of soil nutrients. Among the rates tested, 40% organic substitution gave the highest yield under both deficit irrigation levels. The results of this study provide an experimental basis for achieving water-saving and yield-increasing through optimizing the proportion of organic fertilizer substitution under conditions of insufficient irrigation.