This study aimed to investigate the regulatory mechanisms of partial substitution of chemical fertilizer with organic fertilizer and plastic film mulching on yield and water use efficiency (WUE) of spring wheat in dryland, and to provide a theoretical basis for high-yield and efficient production of spring wheat in the rainfed farming areas of the Loess Plateau.

A field experiment was conducted in Dingxi city, Gansu province, from 2023 to 2025, with four treatments established: chemical fertilizer only (NP, supplying 105 kg·N hm^-2 and 105 kg P₂O₅·hm^-2), partial substitution of chemical fertilizer with organic fertilizer(MNP, supplying 80 kg N·hm^-2 and 88 kg P₂O₅·hm^-2, plus 8500 kg·hm^-2 of composted sheep manure, corresponding to approximately 23.8% and 16.2% substitution of chemical N and P₂O₅, respectively), chemical fertilizer combined with plastic film mulching (FNP), and the combined application of organic substitution and film mulching (FMNP). The effects of these treatments on root traits, stage-specific water use, dry matter accumulation, yield, and water use efficiency (WUE) of spring wheat were investigated.

Film mulching treatments (FNP and FMNP) significantly enhanced soil water storage at sowing compared to non-mulched treatments (NP and MNP), with an average increase of 23.7 mm. In contrast, organic substitution without film mulching (MNP) did not exert a significant effect on soil water storage. Relative to NP treatment, FNP and FMNP significantly increased root length density in the 0-20 cm soil layer at maturity by 25.7% and 22.0%, and in the 60-140 cm layer by 33.7% and 45.9%, respectively. Water consumption during the jointing-flowering stage was significantly increased by 16.8% and 24.5%, and the stage-specific water use efficiency was correspondingly enhanced by 34.3% and 35.0% for FNP and FMNP compared to NP. Leaf area index (LAI) was significantly elevated by 12.5%, 30.2%, and 38.7% for MNP, FNP, and FMNP, respectively, compared with NP. Moreover, aboveground dry matter accumulation at maturity was significantly increased by 35.1% and 40.5% for FNP and FMNP, respectively, averaged over the two-year period. Compared with the NP treatment, the FNP and FMNP treatments significantly increased grain yield by 47.0% and 58.7%, respectively, and water use efficiency for grain yield (WUEg) by 31.3% and 41.7% on average, respectively. In contrast, the MNP treatment did not significantly improve grain yield or WUEg relative to NP. Random forest and structural equation modeling analyses indicated that the combined application of organic substitution and film mulching affected root traits, which in turn regulated stage-specific water use and dry matter partitioning, thereby significantly influencing yield and WUEg.

These results demonstrate that the integration of partial organic fertilizer substitution with plastic film mulching optimizes root architecture, enhances water use during the jointing-flowering stage, increases biomass accumulation, reduces the root-to-shoot ratio, and promotes the allocation of photosynthetic assimilates to grains, ultimately improving both grain yield and water use efficiency of spring wheat in dryland.