Sustainable soybean cultivation using nitrogen-fixing bacteria and humic products derived from agricultural waste: A review

Sustainable intensification of legume-based cropping systems requires innovative strategies that enhance nitrogen fixation and nutrient use efficiency while minimizing environmental impacts. This review examines the co-application of nitrogen-fixing bacteria and humic substances derived from agricultural waste as an integrated biotechnological approach to support sustainable soybean production. The review also summarizes key roles of microbial inoculants, such as Bradyrhizobium, Azospirillum, and Pseudomonas (Ps), and the agronomic functions of humic acids, fulvic acids, and humin compounds. When applied separately, these biostimulants improve nodulation, nutrient uptake, and soil health. When combined, they demonstrate synergistic effects—improving nitrogen-use efficiency, drought tolerance, and crop yield. Mechanisms driving these outcomes include enhanced microbial colonization, micronutrient chelation, hormonal modulation, and antioxidant activity. In addition, the review considers challenges including soil pH variability, native microbial competition, product standardization, and formulation compatibility. Recent advances in encapsulated inoculants and hydrothermal humification methods demonstrate promise for improving bioavailability and resilience. Environmental benefits include reduced nitrate leaching, increased soil organic matter, and alignment with circular bioeconomy principles through the valorization of organic waste. Despite barriers, such as formulation variability, limited precision delivery systems, and regulatory gaps, the integration of microbial and humic inputs offers a scalable, eco-friendly alternative to synthetic fertilizers. Future research should focus on molecular characterization, genotype-strain matching, and long-term field validation to ensure robust performance across agroecological zones. Finally, this review provides a comprehensive synthesis for researchers, agronomists, and policymakers seeking to improve the ecological and economic sustainability of soybean production through advanced biotechnological interventions.