Abstract: The fully mulched double ridge-furrow planting of maize is a typical high-yield model in the Loess Pla-teau's semi-arid region. Exploring the optimal substitution ratio of organic fertilizer for chemical fertilizer is important to enhance crop yield of forage maize and reduce CO₂ emissions. We conducted an experiment on organic fertili-zer substitution for chemical fertilizer in forage maize in the semi-arid region of the Loess Plateau between 2023 and 2024. There were four treatments, including no nitrogen application (S₁), chemical fertilizer alone (S₂, inorganic nitrogen 200 kg·hm^-2), 25% organic fertilizer substitution for chemical fertilizer (S₃, organic nitrogen 25% + inorganic nitrogen 75%), and 50% organic fertilizer substitution for chemical fertilizer (S₄, organic nitrogen 50% + inorganic nitrogen 50%). We measured the dry matter accumulation of forage maize, soil respiration, yield, and farmland CO₂ emissions, as well as carbon emission efficiency and carbon balance. The results showed that substituting chemical fertilizers with organic fertilizers increased the dry matter accumulation of forage maize during the filling and maturity stages compared to the application of chemical fertilizer alone. Specifically, the average dry matter accumulation during the filling stage under treatments S₃ and S₄ increased by 88.0% and 79.1%, respectively, compared to S₁, and by 79.1% and 6.4% compared to S₂. At harvest, the increases were 81.3% and 78.7% compared to S₁, and 15.3% and 13.6% compared to S₂. The rational substitution of chemical fertilizers with organic fertilizers significantly increased the yield of forage maize. In 2023 and 2024, the yield of treatment S₃ increased by 84.9%, 9.9% and 140.1%, 17.9% compared to S₁ and S₂ respectively, and by 18.1% and 14.5% compared to S₄. Soil respiration rate and the differences among treatments during the growth period of forage maize were greater than those during the fallow period. The substitution of chemical fertilizer with organic fertilizer increased soil respiration rate compared to the application of chemical fertilizer alone. The higher the proportion of organic fertilizer substitution, the greater the soil respiration rate. Specifically, the annual average respiration rates of treatments S₃ and S₄ increased by 46.3% and 53.9% compared to S₁, and by 5.4% and 10.9% compared to S₂, respectively. The total CO₂ emissions increased by 43.9% and 51.9% compared to S₁, and by 5.7% and 11.5% compared to S₂, respectively. The rational substitution of chemical fertilizer with organic fertilizer improved carbon emission efficiency, with the highest value under treatment S₃, which was 42.7%, 7.7%, and 18.8% higher than that of S₁, S₂, and S₄, respectively. Collectively, the partial substitution of 25% chemical fertilizer with organic fertilizer significantly enhanced dry matter accumulation in forage maize during the grain-filling and maturation stages, synergistically increased both yield and farmland carbon emission efficiency, and demonstrated a strong carbon sequestration effect. This approach offered a carbon-friendly fertilization strategy for forage maize production under whole plastic film mulching and double ridge-furrow planting in the semi-arid regions of the Loess Plateau.

Key words: organic fertilizer substitution for chemical fertilizer, forage maize, yield, CO₂ emission