Moderate substitution with organic fertilizer and crushed straw increases maize yield and water productivity by improving soil water-nitrogen coordination

Although newly reclaimed farmland through “Gully Reclamation” projects can expand arable land resources on the Loess Plateau of China, its limited soil water retention and nitrogen (N) supply due to the mixing of raw and mature soils severely constrain the sustainable productivity of dryland agriculture. Organic fertilizer application and straw incorporation are widely used to address these constraints, yet their synergistic effects on soil water and nitrogen conditions and underlying mechanisms remain unclear. Therefore, a two-year field experiment was conducted to evaluate soil water and N conditions and spring maize productivity under seven treatments: chemical N fertilizer only (CK); 25% and 50% substitution of chemical N fertilizer with organic fertilizer (OF1 and OF2); 25% and 50% substitution with crushed straw (OS1 and OS2); and 25% and 50% substitution with the combination of organic fertilizer and crushed straw (FS1 and FS2). The results showed that FS treatments (FS1 and FS2) increased soil water content and soil NO₃⁻-N content by 3.38–6.61% and 2.13–10.48%, respectively, compared with CK. Averaged across the two growing seasons, FS1 achieved the highest soil water-nitrogen coordination among all treatments, increased leaf area index and aboveground biomass, and produced 2.68–82.32% higher maize yield than the other treatments. FS1 also showed superior water productivity of 23.05 kg ha–1 mm–1 and economic benefits of 6347.99 CNY ha−1, whereas FS2 had the greatest N utilization efficiency of 62.05 kg kg–1. Structural equation modeling revealed that soil water-nitrogen coordination indirectly increased maize yield by promoting crop biomass accumulation. Furthermore, FS1 was identified as the optimal strategy for maximizing the overall benefits of spring maize production based on the comprehensive evaluation model. Overall, FS1 provides a viable approach for improving soil water-nitrogen coordination and maize productivity in newly reclaimed farmland on the Loess Plateau of China.