Coupling Effects of Straw Return and Fertilization Regime on the Photosynthesis-Soil-Yield Continuum of Spring Maize in Cold Regions

Long-term straw return combined with optimized fertilization represents a effective strategy to enhance soil quality and crop productivity in cold regions, yet its integrated effects on the photosynthesis–soil–yield continuum of spring maize remain unclear, particularly under conditions of low accumulated temperature and slow straw decomposition. Based on a 9-year field experiment (2017–2025) conducted in the cold spring maize zone of Northeast China, this study investigated five treatments: CK, CF, CK + S, CF + S, and OPT + S. Photosynthetic parameters at four growth stages, soil nutrients, and grain yield were systematically measured. The results showed that OPT + S achieved the highest grain yield (15,016.11 kg·ha−1 in 2025) and maintained superior photosynthetic performance throughout the growing season, with a photosynthetic decline rate of only 37.21% during the grain-filling stage–significantly lower than that of CK (48.20%). Soil available phosphorus (AP) and available potassium (AK) were significantly increased under straw return treatments (CK + S, CF + S, OPT + S). Correlation and stepwise regression analyses identified AP and net photosynthetic rate at the jointing stage (PnJ) as the key drivers of yield, jointly explaining 84% of yield variation. These findings demonstrate that the OPT + S treatment optimizes the coupling among early-stage photosynthesis, soil nutrient availability, and grain yield, providing a practical and high-yielding nutrient management strategy for spring maize in cold regions.