Organic-chemical fertilizer combination enhances maize yield through coordinated regulation of leaf C-N metabolism, photosynthesis and dry matter partitioning in northeast black soil, China

Aiming at how organic fertilizer regulates the growth and development of maize in the Northeast black soil region, and to explore sustainable nutrient management models, this study aims to investigate how it regulates the photosynthetic physiological characteristics, leaf senescence dynamics, dry matter accumulation and translocation, grain filling characteristics, and final yield formation of maize. A long-term field trial was conducted using maize cultivar Jingnongke 728, with five treatments: conventional fertilization (CK), full substitution of chemical fertilizer with organic fertilizer, and organic fertilizer substituting 10%, 20%, and 30% of chemical fertilizer. Results showed that the 10% organic substitution treatment significantly improved leaf photosynthetic capacity, including higher relative chlorophyll content (SPAD), actual PSII photochemical efficiency ( Y(II) ), photosynthetic characteristics, and activities of carbon-nitrogen metabolism enzymes (1,5-bisphosphate (RuBP) carboxylase, phosphoenolpyruvate carboxylase (PEPC), nitrate reductase (NR), glutamine synthetase (GS), glutamate oxoglutarate aminotransferase (GOGAT), glutamic oxaloacetic transaminase (GOT), glutamic pyruvic transaminase (GPT), glutamate dehydrogenase (GDH). The 20% organic substitution treatment achieved optimal performance at the silking (R1) and milk (R3) stages. In terms of leaf area, the 10% and 20% substitution treatments increased the maximum leaf area by 4.75%-22.22% (2024) and 2.10%-7.41% (2025) respectively, and all organic substitution treatments delayed the maximum leaf senescence rate. For dry matter accumulation, the 10% substitution promoted early vegetative growth, while the 20% substitution benefited late reproductive growth; both modulated dry matter contribution to grain formation and extended the active grain filling period (up to 51.65 days in 2024 and 52.19 days in 2025 for 20% substitution). Regarding yield, all organic-chemical combination treatments increased yield, with the 10% and 20% substitution treatments showing the best effects replacing 10% of chemical fertilizer with organic fertilizer increased yield by 3.58% in 2024 and 3.30% in 2025, while replacing 20% of chemical fertilizer with organic fertilizer increased yield by 1.87% in 2024 and 5.25% in 2025. Full organic substitution reduced kernels per ear and yield. In conclusion, organic fertilizer slows leaf senescence, but sole organic substitution impairs early nutrient uptake and yield. Substituting 10%-20% of chemical fertilizer with organic fertilizer balances early and late nutrient supply, enhances photosynthetic capacity