Organic Amendment Enhances Maize Yield Through Improved Photosynthesis, Endogenous Hormones, and Defense Enzymes

Enhancing photosynthesis is considered one of the most rational strategies to increase crop yield. However, the dominant physiological driving processes of photosynthesis at different maize growth stages under long-term fertilization remain unclear. Based on a 45-years field experiment, four treatments were selected—namely, CK (no fertilizers), N (276 kg N ha−1), M (60,000 kg manure ha−1), and MN (60,000 kg manure + 276 kg N ha−1)—to study the response of maize physiological characteristics to fertilization. The results showed that long-term fertilization significantly changed the photosynthetic characteristics and endogenous hormones content. Among them, MN increased the photosynthetic rate (Pn) by 41.9–152.3% at different maize growth stages compared to CK. Organic fertilizer addition (M and MN) significantly increased superoxide dismutase (SOD), peroxidase (POD), and soluble sugar (SS) by 26.7–64.3%, 42.5–155.4%, and 16.4–89.5%, respectively. However, proline (Pro) and malondialdehyde (MDA) content decreased by 19.1–44.3% and 0.6-21.3%, respectively. Under these treatments, indole-3-acetic acid (IAA), gibberellins (GA), salicylic acid (SA), zeatin riboside (ZR), abscisic acid (ABA), SS, SOD, and POD were the main physiological driving indicators of photosynthesis. The result of PLS-SEM indicated that the main physiological driving pathways of photosynthesis were “IAA or ZR or SA-SOD-POD-Pn” and “GA or ABA-SOD or POD-SS-Pn”. Additionally, the maize grain yield of N, M, and MN increased by 111.2%, 218.1%, and 297.2% compared to CK, respectively. Overall, organic combined with nitrogen fertilizer could maintain a higher level of photosynthetic capacity by balancing endogenous hormones and enhancing the activity of antioxidant enzymes to support maize production.