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.
Cover cropping combined with reduced tillage is increasingly being adopted to enhance carbon sequestration. A better understanding of the short-term response and carbon dynamics associated with cover cropping is essential for reducing uncertainties during its initial adoption. This study quantified net biome production under reduced-tillage cover cropping system and compared it with conventional tillage, tillage without fertilizer, and no-till without fertilizer in Hokkaido, Japan. Heterotrophic respiration, labile carbon fractions, and their linkages with residue inputs were also assessed during the fallow (in the presence of cover crop) and the subsequent soybean phase in the first-year adoption. Our results revealed that cumulative heterotrophic respiration was higher under reduced-tillage cover cropping than under treatments with bare fallow during the fallow phase, whereas no treatment differences were observed during the soybean phase. Although cover cropping increased water-extractable organic carbon at seeding time, neither microbial biomass carbon nor labile carbon fractions differed among treatments. Heterotrophic respiration was negatively associated with labile organic carbon in deeper soil layers during the fallow phase, whereas no linkage was detected among residue inputs, labile carbon fractions, and heterotrophic respiration during soybean phase. Nevertheless, reduced-tillage cover cropping resulted in moderate soil temperature sensitivity of heterotrophic respiration and basal respiration, falling between those of conventional tillage and reduced-tillage without cover cropping. Overall, the implementation of reduced-tillage cover cropping increased net biome production to 0.85 Mg C ha-1 in the first-year, shifting the system from a carbon source to a carbon sink by replacing the bare fallow phase with an additional period of carbon assimilation.
