Linking Soil Health, Microbial Function, Nutrient Use Efficiency, and Crop Productivity under Biochar–NPK Fertilizer Integration: A Systematic Review of Experimental Evidence

This systematic review aims to synthesize current evidence on the effects of biochar–NPK fertilizer integration on soil health, microbial function, nutrient use efficiency, and crop productivity compared with sole fertilization practices. The review will follow PRISMA principles, utilize predefined eligibility criteria, employ structured searches across major academic databases, and conduct a narrative synthesis of evidence without meta-analysis. The review seeks to identify mechanistic linkages between soil physical, chemical, and biological processes, nutrient cycling, nutrient use efficiency, and crop productivity under biochar–fertilizer integration. Additionally, the review will identify major knowledge gaps and provide recommendations for future research and sustainable nutrient management.