Strategic nitrogen reduction with organic fertilization enhances Chili pepper growth, fruit quality, and soil fertility

Excessive nitrogen application threatens sustainable pepper production, yet optimal organic substitution thresholds remain poorly defined. Here, we evaluated how integrating organic fertilizer with graded nitrogen reduction (20%, 40%, and 60%)influences pepper growth, yield, and soil quality in a pot experiment. The findings showed that nitrogen reductions exceeding 40% significantly suppressed plant growth (height: −12.2%; stem diameter: −13.4%), yield (−25.8%), and key soil indicators including urease activity (−44.4%) and available nitrogen content (−14.4%) relative to the conventional fertilization (CK), with sole organic application exacerbating these declines. Strategic organic co-application effectively counteracted these adverse effects of nitrogen shortfalls. Specifically, substituting 40% of chemical nitrogen with organic fertilizer (treatment N2M) emerged as the optimal threshold, activating enzymatic networks (sucrase: +93.0%; phosphatase: +42.5%), enhancing soil available phosphorus (+24.2%), improving fruit quality (soluble protein: +74.3%) and root architecture. This optimized treatment (N2M) was established as the ideal threshold, delivering a 46.8% increase in yield and a 96.3% improvement in SQI compared to CK. Our findings establish 40% organic substitution as an evidence-based strategy for reducing synthetic nitrogen use by 40% without compromising productivity, while cautioning that excessive substitution (>60%) or exclusive organic application risks decoupling nutrient supply and crop demand.