Unraveling the CO₂—Microbe Nexus: A Conceptual Framework for Nitrogen Fixation Efficiency in Legume—Rhizobium Systems

Elevated atmospheric CO₂, a principal driver of anthropogenic climate change, imposes emerging constraints on symbiotic nitrogen fixation (SNF), particularly within legume—Rhizobium systems. This study critically assesses the impact of CO₂ enrichment on nitrogen fixation efficiency (NFER) in Rhizobium spp., a microbial process essential for sustainable crop productivity. Literature search across Scopus and ScienceDirect identified 2,066 peer-reviewed articles, of which 39 met rigorous inclusion criteria focused on plant—microbe interactions under elevated CO₂ conditions. To mechanistically synthesize current understanding, we propose a conceptual framework defining NFER as a function of three core parameters: nodule formation and development rate (N), symbiotic connection efficiency (S), and genetic expression and regulation (G). Evidence from the reviewed studies indicates that elevated CO₂ can reduce nodulation efficiency by up to 30%, disrupt symbiotic signaling pathways, and impair transcriptional regulation critical for effective nitrogen fixation. These interlinked perturbations contribute to a systemic decline in NFER, threatening agroecosystem resilience and global food security under projected climate scenarios. This synthesis highlights key vulnerabilities in legume—Rhizobium symbioses and delineates strategic knowledge gaps, providing a foundation for targeted quantitative research into microbial adaptation under atmospheric CO₂ enrichment.