Biofertilizers in Sustainable Crop Production: Linking Rhizosphere Mechanisms, Formulation, and Context-Dependent Field Performance

Biofertilizers are increasingly considered components of resource-efficient nutrient management, but their agronomic performance remains strongly dependent on crop genotype, soil properties, microbial strain, formulation, and management context. This structured critical narrative review evaluates molecular, agronomic, environmental, and implementation evidence on plant growth-promoting rhizobacteria, arbuscular mycorrhizal fungi, nitrogen-fixing bacteria, phosphate-solubilizing microorganisms, and emerging nano-enabled microbial formulations. Molecular and omics studies have clarified mechanisms of nutrient cycling, colonization, plant signalling, and rhizosphere restructuring, but these indicators alone do not predict field efficacy. A global meta-analysis reported an average yield increase of 16.2% relative to non-inoculated controls, but responses vary across crops and environments, and reliable mineral-fertilizer substitution remains context-dependent. The evidence favors site-specific deployment within integrated nutrient-management systems and assessment based on yield, nutrient-use efficiency, crop quality, resource-use efficiency, and resilience. Waste-derived substrates, biodegradable carriers, and nano-enabled formulations may improve resource circularity and delivery, but their environmental advantages require life-cycle, stability, safety, and efficacy-based evaluation. Future progress depends on multi-environment validation, robust formulations, predictive multi-omics linked to agronomic outcomes, quality standards, and proportionate regulation.