Alternative carrier materials for plant growth-promoting rhizobacteria: progress and perspectives

Abstract Purpose Plant growth-promoting rhizobacteria (PGPR) inoculants are eco-friendly microbial formulations that offer sustainable alternatives to chemical fertilizers and pesticides, playing an increasingly important role in modern agriculture. However, the success of solid inoculants depends on the choice of carrier material, with peat traditionally dominant, facing limitations due to its non-renewable nature. This review examines alternative carriers that can enhance microbial viability, functionality, and field performance. Methodology Publications from 2000 to 2025 (with emphasis on 2020–2025) were reviewed across mineral substrates, decayed organic materials, biochar, agro-industrial residues, biopolymers and nano-enabled carriers, focusing on how these materials influence microbial survival, shelf life, crop response, and stress tolerance in both controlled and field studies. Results Mineral-based carriers such as talc, kaolin, and vermiculite demonstrated improved storage stability, while organic residues, including lignite, mill mud, and composted manures, supported long-term survival and biological nitrogen fixation. Biochar emerged as a versatile carrier with high porosity and water-holding capacity, consistently enhancing root colonization and nodulation. Biopolymers such as alginate and starch improved microbial encapsulation and gradual release in soil environments. More recently, nanomaterials, including polymer nanoshells, silica, and metal organic frameworks, have shown promise for protecting PGPR against abiotic stress and improving delivery efficiency, though their scalability remains limited. Conclusion Alternative carriers offer viable, sustainable options to replace peat, with biochar and biopolymers demonstrating strong immediate potential and nanocarriers representing a promising frontier. Future research should focus on optimizing these carriers for cost-effectiveness, ecological safety, and large-scale agricultural applications.