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

Abstract

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.