Long-Term Biochar Application Enhances Peanut Yield by Delaying Leaf Senescence and Optimizing Nutrient Balance

Premature leaf senescence during the late growth stage of Arachis hypogaea (peanut) reduces photosynthetic capacity and restricts pod filling, thereby limiting yield. Although biochar has been widely used for soil improvement due to its porous structure and stability, its long-term effects on sustaining photosynthetic performance and improving yield through nutrient regulation remain insufficiently understood. Based on a long-term field micro-plot experiment established in 2009, four fertilization treatments were evaluated: maize straw return + NPK (CS), pig manure compost + NPK (PMC), biochar + NPK (BIO), and biochar-based fertilizer (BF), with three replicates. Peanut was used as the test crop in 2024. Photosynthetic parameters, SPAD values, leaf N, P, and K contents, nutrient use efficiencies (PNUE, PPUE, PKUE), nutrient ratios, carbon allocation characteristics, and yield were measured at key growth stages. Results showed that the BIO treatment maintained higher net photosynthetic rate and SPAD value at the full-pod maturity stage compared with CS, PMC, and BF. Leaf K content was significantly increased under BIO, while the N:K ratio remained below 2.1, indicating reduced risk of potassium-related nutrient imbalance. The N:P ratio also remained relatively stable, suggesting improved nutrient balance with respect to phosphorus during the late growth stage. BIO showed the highest estimated contribution of photosynthetic carbon to pod dry weight, and yield increased by 19.2–28.6% compared with other treatments. These results indicate that long-term biochar application was associated with sustained late-stage photosynthetic performance and improved leaf nutrient balance, which may contribute to higher peanut yield. This study provides new evidence supporting the potential role of long-term biochar application in sustainable peanut production.