Biochar effects on rhizobium–alfalfa interactions and biological nitrogen fixation

Alfalfa (Medicago sativa L.) is an important perennial forage crop with a high capacity for biological nitrogen fixation (BNF) through symbiosis with Sinorhizobium (Ensifer) meliloti. However, the efficiency of this symbiosis is often constrained by adverse soil conditions and environmental stress. Biochar has emerged as a soil amendment capable of modifying soil physical, chemical, and biological properties and influencing plant–microbe interactions. This review synthesizes current knowledge on the effects of biochar on rhizobium–alfalfa interactions, with emphasis on soil-mediated mechanisms regulating rhizobial survival, nodulation, and nitrogenase activity. Biochar properties such as porosity, pH, nutrient retention, and water-holding capacity are discussed in relation to their roles in shaping rhizosphere processes and enhancing BNF. The potential of biochar as a carrier for rhizobial inoculants is also considered. Although direct studies on alfalfa are limited, evidence from legume systems suggests that biochar can improve symbiotic performance by modifying soil-plant–microbe interactions. Key knowledge gaps include the need for long-term field studies, optimization of biochar characteristics, and improved understanding of molecular mechanisms underlying symbiosis. Overall, biochar application represents a promising strategy to enhance nitrogen fixation and sustainability in alfalfa-based systems.