Photosynthetic characteristics of 20% reduced irrigation combined with 25% organic substitution for chemical fertilizer in increasing silage maize yield

In response to the issues of excessive irrigation and fertilizer use, resource waste, and low yield in silage maize production in the oasis irrigation regions of Northwest China, this study investigates the photosynthetic mechanisms underlying yield improvement through reduced irrigation combined with optimized organic substitution for mineral fertilizer. The objective is to provide a scientifically based irrigation and nutrient management strategy for efficient silage maize cultivation in arid irrigated areas. A two-factor split-plot field experiment was conducted in Wuwei city, Gansu province, beginning in 2020. The irrigation treatments included I1 (20% reduction in conventional irrigation, 324 mm) and I2 (conventional irrigation, 405 mm), while the fertilization treatments comprised five levels: F1 (100% chemical nitrogen fertilizer), F2 (25% organic substitution), F3 (50% organic substitution), F4 (75% organic substitution), and F5 (100% organic fertilizer). From 2022 to 2024, data were collected on leaf area index (LAI), leaf area duration (LAD), photosynthetic parameters, chlorophyll fluorescence, and yields of fresh forage (FY) and hay (HY) to evaluate treatment effects. Results showed that substituting 25% of chemical nitrogen with organic fertilizer (I1F2) significantly improved both fresh and hay yields, outperforming other substitution ratios. Compared with I2F1, I1F2 increased fresh and hay yields by 5.0% and 13.8%, respectively. This treatment also enhanced mean LAI, total LAD, and PSII electron transport rate (ETR) by 11.5%, 40.8%, and 17.2%, respectively. Furthermore, net photosynthetic rate (<italic>P</italic>_n) increased by 15.9%, intercellular CO₂ concentration (<italic>C</italic>_i) decreased by 12.6%, stomatal conductance (<italic>G</italic>_s) improved by 7.8%, and non-regulated energy dissipation (Y(NO)) declined by 14.7%. The yield increase was primarily attributed to synergistic improvements in LAI, LAD, SPAD, ETR, <italic>P</italic>_n, and <italic>G</italic>_s, leading to enhanced leaf photosynthetic performance. Therefore, a 20% reduction in irrigation combined with 25% organic substitution for mineral nitrogen represents an effective strategy for improving silage maize yield and water use efficiency in northwest irrigated regions.