A Harpin Protein-Based Enzyme Complex Sustains Maize Yield Under Reduced Fertilization by Enhancing Soil Nutrient Availability

Excessive chemical fertilization in maize production has reduced fertilizer-use efficiency and increased pressure on soil quality, whereas reducing fertilizer input without yield loss remains challenging. This challenge has shifted attention toward strategies that improve crop nutrient acquisition and utilization under lower fertilizer supply. Harpin protein-based enzyme complexes may provide a regulatory approach, but their field performance under reduced fertilization remains unclear. A two-year field experiment was conducted from 2023 to 2024 using two maize cultivars, Heyu236 and Fuyuan2. In 2023, the harpin protein-based enzyme complex was applied at 200-fold and 300-fold dilutions under conventional fertilization to identify effective spraying concentrations. In 2024, the same two concentrations were evaluated under conventional fertilization and 15%, 30%, and 45% fertilizer reductions. In the 2023 concentration screening trial under conventional fertilization, the enzyme complex increased kernels per ear by 5.6–9.7% and tended to increase the yield by 0.4–17.2% (not significant). In 2024, under reduced fertilization, enzyme application combined with 30% fertilizer reduction produced a stable yield response. In particular, the 300-fold dilution combined with 30% fertilizer reduction increased kernels per ear by 18.1% and 13.2% and grain yield by 16.9% and 9.5% in Fuyuan2 and Heyu 236, respectively. Soil analyses showed that the enzyme treatment mainly improved nutrient availability, as reflected by higher available P, available K, alkali-hydrolyzable N, organic matter, and available Cu, Zn, Fe, and Mn in the soil. These findings suggest that the harpin protein-based enzyme complex helped maintain maize yield under moderate fertilizer reduction by improving kernel formation and soil nutrient availability. Among the tested treatments, foliar application at 300-fold dilution combined with 30% fertilizer reduction showed the greatest potential for reducing fertilizer input while sustaining maize productivity.